Dayside and Polar Cap Aurora (Astrophysics and Space Science Library)

DAYSIDE AND POLAR CAP AURORA ASTROPHYSICS AND SPACE SCIENCE LIBRARY VOLUME 270 EDITORIAL BOARD Chairman W.B. BUR...

Author: Per Even Sandholt | H.C. Carlson | A. Egeland

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DAYSIDE AND POLAR CAP AURORA

ASTROPHYSICS AND

SPACE SCIENCE LIBRARY

VOLUME 270

EDITORIAL BOARD Chairman W.B. BURTON, National Radio Astronomy Observatory, Charlottesville, Virginia, U.S.A. ([email protected]); University of Leiden, the Netherlands ([email protected]) Executive Committee J. M. E. KUIJPERS, Faculty of Science, Nijmegen, The Netherlands

E. P. J. VAN DEN HEUVEL, Astronomical Institute, University of Amsterdam,

The Netherlands

H. VAN DER LAAN, Astronomical Institute, University of Utrecht,

The Netherlands

MEMBERS I. APPENZELLER, Landessternwarte Heidelberg-Königstuhl, Germany J. N. BAHCALL, The Institute for Advanced Study, Princeton, U.S.A. F. BERTOLA, Universitá di Padova, Italy J. P. CASSINELLI, University of Wisconsin, Madison, U.S.A. C. J. CESARSKY, Centre d'Etudes de Saclay, Gif-sur-Yvette Cedex, France O. ENGVOLD, Institute of Theoretical Astrophysics, University of Oslo, Norway R. McCRAY, University of Colorado, JILA, Boulder, U.S.A. P. G. MURDIN, Institute of Astronomy, Cambridge, U.K. F. PACINI, Istituto Astronomia Arcetri, Firenze, Italy V. RADHAKRISHNAN, Raman Research Institute, Bangalore, India K. SATO, School of Science, The University of Tokyo, Japan F. H. SHU, University of California, Berkeley, U.S.A. B. V. SOMOV, Astronomical Institute, Moscow State University, Russia R. A. SUNYAEV, Space Research Institute, Moscow, Russia Y. TANAKA, Institute of Space & Astronautical Science, Kanagawa, Japan S. TREMAINE, CITA, Princeton University, U.S.A. N. O. WEISS, University of Cambridge, U.K.

DAYSIDE AND

POLAR CAP

AURORA

by PER EVEN SANDHOLT Department of Physics, University of Oslo, Norway HERBERT C. CARLSON Air Force Office of Scientific Research, Arlington, Virginia, U.S.A. and

ALV EGELAND Department of Physics, University of Oslo, Norway

KLUWER ACADEMIC PUBLISHERS NEW YORK, BOSTON, DORDRECHT, LONDON, MOSCOW

eBook ISBN: Print ISBN:

0-306-47969-9 1-4020-0447-8

©2004 Kluwer Academic Publishers New York, Boston, Dordrecht, London, Moscow Print ©2002 Kluwer Academic Publishers Dordrecht All rights reserved No part of this eBook may be reproduced or transmitted in any form or by any means, electronic, mechanical, recording, or otherwise, without written consent from the Publisher Created in the United States of America Visit Kluwer Online at: and Kluwer's eBookstore at:

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This page intentionally left blank

Erratum to ISBN 1-4020-0447-8

Dear Reader, Kluwer Academic Publishers is the proud publisher of ASSL Volume 270 “Dayside and Polar Cap Aurora” written by Dr. Per Even Sandholt, Dr. Herbert Carlson, and Dr. Alv Egeland. Despite several rounds of proof reading the wrong version of Figure 4.77 was printed on page 159. Here, we present a correct replacement figure that could be glued into the book. In addition, a few other smaller mistakes were found. This erratum provides a list of corrections that should be taken into account. With apologies for the unfortunate mistakes. Dr. Harry (J. J.) Blom Publishing Editor for Astronomy, Kluwer Academic Publishers E-mail: [email protected]

Erratum sheet

Acknowledgements: Peter Stauning (Danish Meteorological Institute; Copenhagen) kindly provided magnetometer data from stations on Greenland. Page 46, line 6: Figure 3.6 instead of Figure 3.7. Page 68, line 25 in section 4.3.2: “corresponding to strongly negative conditions” instead of “corresponding to strongly northward IMF IMF orientation.” Page 132, Figure 4.59 (caption): MSP observations at 630.0 nm for the interval 0720-0800 UT. Page 138, Figure 4.65 (last sentence of caption): The F 13 trajectory is given in Figure 4.64. Page 143, line 17 from bottom: We suggest that the type 1a/b and 2 forms represent a substructure of the cusp region morphology during conditions.

strong IMF Page 159: New Figure 4.77.

Page 201, Figure 5.23 (caption): 427.8 nm instead of 557.7 nm.

Page 227, Figure 5.42 (caption): upper and lower panels instead of left

and right panels.

Page 229, Figure 5.43 (caption): 844.6 nm instead of 888.4 nm.

Page 283, line 11, Sandholt, P. E. and Farrugia, C. J.: 2002, Ann.

Geophys., 20, 629.

Page 283, bottom line, Sandholt et al.: 1996, Geophys. Res. Lett., 23,

1725.

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Preface

The optical auroral emissions in the upper atmosphere of the polar regions of the earth are evidence of the capture of energetic particles from the atmosphere of the sun, streaming by the earth as the solar wind. These auroral emissions, then, are a window to outer space, and can provide us with valuable information about electrodynamic coupling processes between the solar wind and the earth’s magnetosphcre. ionosphere, and upper atmosphere. Studying the physics of these phenomena extends our understanding of our plasma universe. Ground-based remote-sensing techniques, able to monitor continuously the variations in the signatures of aurora, in combination with in-situ satellite and rocket measurements, promise to advance dramatically our understanding of the physical processes taking place at the interface of the atmospheres of the earth and the sun. Decoding their complexity brings us closer to reliable prediction of communication and GPS navigation environments, especially at high latitudes. This understanding, in turn, will help us resolve problems of communication and navigation across polar regions. Aurora have been the object of wonder and scientific curiosity for centuries. Only since the early 1980s, however, have we been able to detect, with sensitive instrumentation, noontime aurora, and persistent aurora deep within the polar cap. This book is the first to provide a morphological and theoretical framework for understanding these dayside and polar-cap aurora. Writing such a book, focused on auroral phenomena known only since the early 1980s, was inspired by our interactions with colleagues at the NATO Advanced Study Institute on Polar-Cap Boundary Phenomena, held at Longyearbyen, Svalbard, in June of 1997. Chapter 1 is a historical overview, where we quote a 13th-century description of aurora, and then note some major milestones in auroral research, up to the first documented observations of dayside and polar-cap aurora. In Chapter 2, we give an introduction to some of the interaction processes of the solar wind, magnetosphere, and ionosphere that are relevant to subsequent presentation and discussion of observations and data. Chapter 2 also contains some representative data examples to illustrate central aspects of the morphology and dynamics of the dayside and polar-cap aurora. Chapter 3 summarizes the physics of optical emissions, within the context of the electro dynamics of the polar ionosphere. Chapters 4, 5 and 6 contain detailed descriptions of auroral observations, consisting of coordinated ground-based measurements by optical instrumentation, radio beacon receivers, HF and incoherent scatter radars, and magnetometers; as well as in-situ measurements of particles, fields, and waves, mostly from satellites in polar orbits. With our detailed analysis of these observations, we demonstrate that the interplanetary magnetic field controls the dayside and polar-cap aurora. The polar cap with its dayside boundary in the cusp ionosphere is a focal point in solar-terrestrial coupling. Experiments employing combined ground based and satellite observations such as shown here, focused on testing key issues of process, theory, and modeling, will produce major advances in the

[ vii ]

[ viii ] understanding of our space environment over the coming decades. We hope that our presentation will stimulate the student of nature and the scientific professional alike, and accelerate progress in this research area of growing interest and practical concern.

Oslo, May 2001 Per Even Sandholt

Herbert C. Carlson

Alv Egeland

Acknowledgements

The present work has benefited from a close scientific cooperation with many scientists over the last 20 years. We would like to express our special thanks to the following collaborat ors: Charles J. Farrugia (University of New Hampshire, NH, USA), Stanley W. H. Cowley (University of Leicester, UK), Michael Lockwood (Rutherford Appleton Laboratory, UK), Charles S. Deehr (University of Alaska, Fairbanks, USA), Nelson C. Maynard (Mission Re search Corporation, NH, USA), Bjørn Lybekk, ,Jøran Moen (University of Oslo, Norway), Marit Øieroset (University of California, Berkeley, USA), Santi Basu (Air Force Research Lab., USA), Sunanda Basu (National Science Foundation, USA), Jörgen Buchau (Air Force Research Lab., USA), Rod Heelis (University of Texas, Dallas, USA), Edward Weber (Air Force Research Lab., USA) and Cesar Valladares (Boston College, USA) Economic support to the present study has been provided by the following institutions: Department of Physics, University of Oslo, The Norwegian Research Council (Norges for skningsråd), The Norwegian Polar Research Institute, and AFOSR. We would like to thank the persons/institutions listed below for having contributed to this study with data, technical assistance, and/or via scientific cooperation. Bjørn Lybekk and Espen Trondsen (University of Oslo) for operating the optical auroral instruments in Ny Ålesund, Svalbard and for data processing. The Norwegian Polar Research Institute for operating the research station in Ny Ålesund, where most of the auroral observations were made. The Kings Bay Coal Company, Ny Ålesund, Svalbard for providing excellent living conditions during our auroral observation campaigns in Ny Ålesund, Svalbard. Charles J. Farrugia (University of New Hampshire, NH, USA) for providing interplanet ary magnetic field and solar wind plasma data from spacecraft Wind, ACE, and IMP-8, and magnctospheric plasma data from the Polar satellite. Mark Lester (University of Leicester, UK) and Jean-Claude Cerisier (CETP, Saint-Maur, France) for providing ionospheric ion drift data from the CUTLASS HF radar located in Hankasalmi, Finland. William F. Denig (Air Force Research Lab., USA) for providing particle precipitation data from instruments in the Defence Meteorological Spacecraft Program (DMSP). The German-Finnish-Polish-Norwegian project IMAGE (International Monitor for Au roral Geomagnetic Effects) conducted by the Technical University of Braunschweig, the Finnish Meteorological Institute, Helsinki, Finland as well as Truls Hansen and Børre Holmeslet, University of Tromsø, Norway for providing the ground magnetometer data used in this study.

[ ix]

[x] Runar Jørgensen for invaluable coordination work and the final preparation of this report

using

Ragnhild Holm for technical assistance with the manuscript.

Geir Holm for phototechnical work.

Eigil Whist for producing several illustrations used in this report.

Ann Carlson for editing.

Contents

vii

Preface Acknowledgements

ix

1 Auroral Physics: The Preamble 1.1 A Look at History 1.2 Twentieth-Century Pioneers 1.3 The Origins of Aurora 1.4 Observing Aurora 1.5 Concepts of the Auroral Oval and Sun-Aligned Arcs 1.6 New Discoveries

1 1 2 3 5 6 6

2 Near-Earth Space and Dayside Aurora 2.1 Introduction 2.2 Magnetospheric Boundary Layers 2.3 Dayside and Polar Cap Auroras 2.4 Convection Patterns 2.5 Responses to IMF Transitions: Aurora and Convection 2.6 Remarks on Major Results

9 9 11 13 16 17 28

3 Optical Aurora

33 33 34 37 38 39 39 43 43 44 46 47 48 49

3.1 Introduction 3.1.1 Altitude Profiles 3.1.2 Useful Relationships 3.1.3 Auroral Emissions from Dayside Aurora and Sun-Aligned Arcs 3.1.4 Atomic vs. Molecular Emission 3.2 Forbidden Atomic Lines in the Auroral Emissions 3.3 Permitted Atomic Lines 3.4 Thermal Excitation in General 3.5 Atmospheric Temperatures and Auroral Emissions 3.6 The Hydrogen Lines — Proton Auroras 3.7 Characteristics of Auroral Emissions 3.8 Units of Auroral Intensities 3.9 Summary 4 Dayside Auroral Forms and Activities

4.1 Introduction 4.2 Case 1: December 3, 1997 4.2.1 Auroral Observations

[ xi ]

53 53 56 56

[ xii ]

4.3

4.4 4.5

4.6 4.7

4.8

4.9

4.10

4.11

CONTENTS

4.2.2 Magnetic Observations 4.2.3 Case Review Case 2: November 30, 1997 4.3.1 Solar Wind and IMF Observations 4.3.2 Auroral Observations 4.3.3 Magnetic Observations 4.3.4 Combined Ground and Satellite Observations 4.3.5 Case Review Case 3: January 11, 1993 4.4.1 Case Review Case 4: January 3, 1995 4.5.1 IMF Observations 4.5.2 Auroral Observations 4.5.3 Magnetic Observations 4.5.4 Observations of Particle Precipitation Case Review Case 5: January 12, 1997 4.7.1 Solar Wind and IMF Observations 4.7.2 Auroral Observations 4.7.3 Case Review Case 6: November 22, 1995 4.8.1 Solar Wind and IMF Observations 4.8.2 Auroral and Magnetic Observations 4.8.3 Ionospheric Ion Drift Observations 4.8.4 Observations of Particle Precipitation 4.8.5 Case Review Case 7: November 20, 1995 4.9.1 Solar Wind and IMF Observations 4.9.2 Auroral Observations 4.9.3 Plasma Convection 4.9.4 Ground and Satellite Observations: Aurora and Particle Precipitation

4.9.5 Case Review Case 8: December 16, 1998 4.10.1 Solar Wind and IMF Observations 4.10.2 Auroral Observations 4.10.3 Radar Observations: Ionospheric Ion Drift 4.10.4 Satellite Observations: Particle Precipitation 4.10.5 Case Review Summary of Observations 4.11.1 Classification of Dayside Forms 4.11.2 Cusp Aurora and Plasma Convection in Relation to the IMF 4.11.3 Dayside Auroras in Relation to Particle Precipitation and Boundary

Layer Plasma Sources 4.11.4 Cusp Aurora in Relation to Solar Wind Density/Dynamic Pressure

5 Morphology of Polar-Cap Sun-Aligned Arcs 5.1 Introduction 5.1.1 Why this Interest in Sun-Aligned Arcs? 5.1.2 Presentation Plan 5.2 Historical Background and IMF Context

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5.4

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5.9

5.2.1 Historical Context 5.2.2 First Sighting of Sub-Visual Sun-Aligned Arc Aurora 5.2.3 The Two States of the Polar Cap 5.2.4 Ground-Based Signatures of Southward vs. Northward IMF 5.2.5 Improved Sensitivity 5.2.6 Improved Physical Insights Arc Electrodynamics 5.3.1 Ohm’s-Law Arcs 5.3.2 Verification and Calibration Using Satellite Data 5.3.3 Incoherent-Scatter Radar Verification and Calibration Studying Arc Events Using Satellite Overflights 5.4.1 Supporting ASIP Images 5.4.2 In Situ Measurements by Dynamics Explorer, DE-2, Satellite 5.4.3 In Situ Signatures 5.4.4 Geophysical Noise in Arc Detection 5.4.5 Small-Scale Flow Reversals 5.4.6 Small-Scale and Large-Scale Flow Studying Arcs with Incoherent-Scatter Radar (ISR)

5.5.1 Imaging 5.5.2 Mapping 5.5.3 Deriving Cross Sections 5.5.4 Mesoscale Dynamics 5.5.5 Directly-Observed ISR Parameters 5.5.6 Derived Parameters 5.5.7 Non-Particle Heating 5.5.8 Polar Thermospheric Thermal Balance 5.5.9 Variability Along Arcs 5.5.10 Summary of ISR studies Studying Arcs with Rockets 5.6.1 Ion Composition 5.6.2 Lower Hybrid Frequency 5.6.3 Plasma Instabilities Statistical Behavior of Sun-Aligned Arcs 5.7.1 Methodology for Statistical Studies 5.7.2 Coordinates for Sun-Aligned Arc Statistics 5.7.3 Detecting Arcs 5.7.4 Arc Orientation 5.7.5 Dawn-Dusk Velocity 5.7.6 Arc Motion vs. IMF 5.7.7 Arc Lifetime Particle and Emission Spectra 5.8.1 Satellite Study of Electron Precipitation Energy Spectra 5.8.2 Spectroscopic Measurements Response Time and Dynamics Reversals 5.9.1 Response Time

5.9.2 Simultaneous Arcs and Patches 5.9.3 Arcs at the Auroral Oval Boundary

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[ xiv ]

CONTENTS

6 Theory of Polar-Cap Sun-Aligned Arcs 6.1 Introduction 6.1.1 Elements of Theory and Processes 6.1.2 Energy-Flow Framework 6.2 Anatomy of a Polar-Cap Sun-Aligned Arc 6.2.1 Ionospheric Altitudes 6.2.2 Near-Earth Altitudes 6.2.3 Out to the Solar Wind Interface 6.3 Electrodynamics of Sun-Aligned Arcs 6.3.1 Ohm’s-Law Arcs 6.3.2 Neutral Gas Rest Frame 6.3.3 Electric Fields, Plasma Drift, and Currents 6.3.4 Self-Consistent Calculation of Arc Conductivities 6.3.5 A New Research Tool 6.3.6 Chemical Lifetime 6.4 Current Sheets 6.4.1 Optical Emission in and Near Sun-Aligned Arcs 6.4.2 Thermal Excitation of Arc Emission 6.4.3 IR and UV Auroral Emissions 6.4.4 Hydrogen Emissions 6.5 The Polar Ionosphere 6.5.1 Polar lonization 6.5.2 Polar Plasma Motion 6.5.3 The Thermosphere as a Rest Frame for Electrodynamics 6.5.4 Plasma Instabilities 6.5.5 General Instability Theory for F-Region Ionosphere 6.5.6 Ground-Based Scintillation Studies 6.5.7 Satellite Direct Measurement of Plasma Irregularities 6.6 Energy Estimates 6.6.1 Particle-Energy Deposition 6.6.2 Poynting Flux 6.7 Electron and Ion-Gas Thermal Balance 6.7.1 General Considerations for Plasma Thermal Balance 6.7.2 Electron Gas Thermal Balance 6.7.3 Ion Gas Thermal Balance 6.7.4 Overall Energy Flow Within the Arc 6.8 Thermospheric Heating and Momentum Transfer 6.8.1 Thermospheric Heating 6.8.2 Thermospheric Winds 6.9 Composite Essential Arc Features, Properties, and Processes 6.9.1 Signatures of Sun-Aligned Arcs 6.9.2 Near-Earth Physics 6.9.3 Far-Earth Physics 6.10 Further Physical Insights From Statistical Studies

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Chapter 1

Auroral Physics: The Preamble

The development of auroral physics has been a fascinating drama and an intellectually chal lenging puzzle, and today it represents an area for important international cooperation both scientifically and geopolitically. Its story is all the more interesting for the manner in which it punctuates historical records tracing back not merely centuries, but millennia. Its story is also an intriguing subplot in the evolution of the role of science in our civilization and our lives. Evolving as an alternative to superstition, into an example of the acceptance of the scientific method, auroral physics now makes headline news in the everyday life of mankind.

1.1

A Look at History

Since earliest time, aurora have been appreciated as visually spectacular phenomena, and sparked scientific speculation, long before technology provided the means for quantitative observations. The ancient literature, particularly several passages in the Old Testament, as well as in classical Greek and Chinese literature, contain references to what can only be taken to be auroral phenomena (Eather 1980). The oldest detailed descriptions of auroral displays are found in Norse literature dating back to ~1200 AD. In the Konungs skuggsjá (“King’s Mirror”), a 13th-century Norwegian chronicle, we have a remarkable example. Weaving observable technical detail and human emotional response into a unified narration of the experience, the chronicler describes the northern light in vivid terms. Figure 1.1 provides a translated excerpt, and shows a fragment of the original Old Norse text, handwritten nearly 800 years ago. Note that three different interpretations of the observed phenomena are suggested. Note also that the name “Northern Lights” (Nordurljos) is introduced in this Old Norse saga, more than 400 years before Galileo (1564–1642) proposed the name “Aurora Borealis” (literally translated, “Northern Dawn”). The 18th century may be said to mark the beginning of the application of the scientific approach to the study of aurora. The extraordinary auroral display of March 6, 1716, visible far south, from Ireland in the west to the Russian-Polish border in the east, inspired the English astronomer, Edmund Halley (1656–1742), to publish his personal observations, and to develop a theory on auroral origins that correlated for the first time the location of the aurora with the earth’s magnetic field. The French physicist, Jean de Mairan (1678–1771), who witnessed the display of northern lights over Paris on October 19, 1726, was likewise moved to speculation. His theory postulated an interplay between the atmospheres of the sun and the earth, a significant advance in understanding. In addition to these two remarkably prescient [1]

[2]

Auroral Physics: The Preamble

scientists, numerous 18th-century scientists and clergy, throughout Europe and particularly Scandinavia, wrote papers and books about aurora (Brekke and Egeland 1994). Significant progress was made in auroral research in the 19th century. Particularly notable was work by Christopher Hansteen (1784–1873), who, as early as 1825, was convinced that the auroral forms completed rings around the magnetic poles, and postulated a diameter of about 2000 to 4000 km (Figure 1.2). Equally important was the contemporary conclusion by Jean Baptiste Biot (1774–1862) that no trace of polarization was found in aurora. Aurora could therefore have nothing to do with, for example, scattered moon or sunlight. By the end of the 19th century, auroral research had emerged as a scientific discipline.

1.2

Twentieth-Century Pioneers

Significantly, it was in the middle of the 20th century that the study of aurora became a uni fying theme for international collaborative research, and became a worldwide discipline. This expanded interest was largely inspired by the much earlier research of Norwegian scientists Kristian Birkeland (1867–1917) and Carl Størmer (1874–1957). Størmer’s theoretical calcu lations of auroral particle trajectories, a landmark of rigor in building the foundation of auroral particle physics, gained renewed significance as a fundamental building block of modern space science with the discovery of earth’s trapped radiation belts. These “Van Allen belts”, which

1.3 The Origins of Aurora

[3]

are energetic charged particles, trapped in trajectories defined by closed-earth magnetic field lines, are periodically responsible for satellite failures due to radiation damage to electronics on spacecraft. The broad significance of Birkeland’s experimental and theoretical work on particle flow was not appreciated until even later, when the first evidence of field-aligned “Birkeland currents” was found by satellite observations in the mid 1960s (Potemra 1989). In the 1940s and 1950s, the Nobel Prize winner Hannes Alfvén (1908–1988) introduced several concepts and processes of lasting importance to auroral physics, and used the discipline as a testbed for understanding many areas of physics, from applied plasma physics to pure astrophysics. When the Space Age was ushered in by the launch of Sputnik I in October of 1957, aurora were recast as a key link in the still-wider discipline of solar-terrestrial physics, that is, the physics of earth-sun coupling.

1.3

The Origins of Aurora

The sun’s atmosphere is so hot that its gas-particle atomic bond is not strong enough to keep outer-shell electrons bound to the particle nucleus. Thus, the sun’s atmosphere is ionized, that is, it is a plasma. The sun's gravitational field cannot keep its atmosphere bound to the sun. This escaping atmosphere blows outward, away from the sun, and is called the ‘solar wind’. The solar wind particles are of low density. Since they are a plasma, they actually flow around earth’s magnetic field, coming no closer to earth on the sunward side than that distance (about ten earth radii) at which the mechanical gas pressure is just balanced by earth’s magnetic field pressure (Figure 1.3). The earth’s distorted magnetic field, and electrons confined within the long cylindrical magnetic cavity (extending downstream beyond the distance to the moon) bounded by the external solar wind, is called the ‘magnetosphere’. The near-earth, weakly ionized, upper atmosphere is called the ‘ionosphere’. The nonionized portion of the upper atmosphere, within which the ionosphere is immersed, is called the ‘thermosphere’, and is found above roughly 100km. The mechanical energy of the solar wind is electrically carried by currents, oriented along earth’s magnetic field lines, and then deposited as heat in earth’s upper atmosphere, at alti tudes near and above 100 km. Definition and understanding of Birkeland currents has been

[ 4 ]

Auroral Physics: The Preamble

greatly refined since the 1960s through measurement of their associated signature magneticfield variations, as detected by satellite-borne magnetometers. The importance of Birkeland currents to the coupling between the magnetosphere and the auroral atmosphere and iono sphere is great. Their total intensity can exceed one million amperes, and the energy they dissipate in the upper atmosphere can be ten or more times the energy deposited by electro magnetic radiations at these altitudes by an overhead sun. The three-dimensional magnetosphere and the current system that flows into and away from earth’s auroral regions is depicted in Figures 1.3 and 1.4. The currents are guided by the earth’s magnetic field, and form cone-shaped regions. These field-aligned currents are called Birkeland currents in honor of the man who first proposed their existence. The properties of the solar wind, the nature of its interactions and coupling through the magnetosphere to the thermosphere and ionosphere, and how it controls the structure and dynamics of the polar ionosphere/thermosphere is today’s most important area of research in this field. These mechanisms can drive the global character and dynamics of the ionosphere/thermosphere, and therefore affect, and can indeed control, the utility and reliability of satellites in our modern space age. The intellectual challenge to understand is enormous; the practical impact on communications, navigation, a host of commercial entertainment services, Landsat services, and space capital is compelling.

1.4 Observing Aurora

1.4

[5]

Observing Aurora

Before the advent of cameras, aurora could only be seen and studied with the unaided human eye. In modern terminology, the only sensor was the human eye, and the only recorder was the human mind and hand. Aurora away from population centers largely went unseen, except for dedicated efforts or expeditions by an intrepid few with keen, if not passionate, interest in understanding what fundamental processes nature could be using to produce such mercurially brilliant signatures in the night sky (Nansen 1897); (Tromholt 1885); (Størmer 1955). To these observers we owe knowledge about the secular changes, that is, the long-term trends and climatological behavior, including the 11-year solar cycle to century-scale variations in location and frequency of aurora occurrence. Such records are invaluable, but must be used scientifically and thoughtfully. An interesting anecdote is a lesson in the value of going to original source material, and using it with discrimination. One scientist corrected a few “anomalous” minima reported in the recent technical literature by going to original historical log books, which revealed that the scientific observer had to reduce or interrupt his observing nights because of economic hardship and war. The “minima” were therefore a sociological factor, not a solar or geophysical observation. With the advent of cameras on the ground, and in space, the eye of the human observer was aided by more uniformly and comprehensively collected observations in time and space. Furthermore, other in-situ sensors, of currents, particles, electric fields, and so on, measured

[6]

Auroral Physics: The Preamble

correlative physical parameters. This combined data collection has driven explosive growth in the understanding of aurora during the past 50 years. Yet, the dark-adapted human eye, as well as ground-based all-sky cameras, and satelliteborne imagery before the 1980s, are all limited in their sensitivity. They can only see aurora of brightnesses of at least a kilo-Rayleigh. Only in the past two decades have we developed and used more sensitive tools with which to see light 100 times more dim. These new “eyes” have now seen and studied aurora that have been occurring since before the dawn of mankind, yet have never before been seen by man, even though, as we know now, they are present overhead in the polar sky fully half of the time.

1.5

Concepts of the Auroral Oval and Sun-Aligned Arcs

As early as 1882, Sophus Tromholt (1851–1896) published a book (Tromholt 1885) in which he reported that the zone within which aurora were observed could be displayed over 24 hours following a boundary, for which the time of auroral maximum was later as one progressed fur ther poleward. He effectively discovered ‘the auroral oval’, though he never gave it that name. He also discussed the expansion of the oval with increasing sunspot number, or increasing solar activity, as we now call it. The first clear drawings of sun-aligned arcs, not called such at the time, seem to be in the detailed visual observations of aurora carried out by Carlheim Gyldensköld (1886) and colleagues at Svalbard, during the Swedish Expeditions of the First Polar Year 1882–83. These observers also confirmed Tromholt’s results, but were not the discoverers of the auroral ring/auroral oval, as has been claimed by some authors. Others claim that the first person who carefully recorded sun-aligned arcs was the Australian scientist, Sir Douglas Mawson, during the Antarctic expedition of 1907–1909 (Mawson 1916). It was Feldstein (1963) who finally demonstrated that the aurora appear oriented along an oval-shaped band, rather than oriented along the auroral zone. The auroral zone is an area bounded by two ovals and within which aurora are statistically most likely to be observed. His results were based on the network of all-sky cameras used since 1957. They showed that the instantaneous distribution of aurora is located near 67 degrees magnetic latitude (MLAT) at local magnetic midnight, but at about 78 degrees near magnetic local midday, for quiet magnetic conditions. Because the aurora are brightest, and thus most readily observed, during nighttime, the dayside aurora were neglected for decades, up to about 1980. Only with the advent of a new technological capability to image very weak aurora (tens to hundreds of Rayleighs, vs. one to ten kilo-Rayleighs), has major progress been possible in the study of polar sun-aligned aurora. This progress has followed from the polar network of images from ASIPs (All-Sky Intensified Photometers), which are the post-1980 equivalent of the ground breaking capability provided by the network of all-sky cameras in the post-1957 era. The much weaker and much more stable (slowly varying) auroral arcs amenable to study with these much more sensitive “eyes” (~100 times more sensitive) have opened the door to new understanding of solar-wind/ionosphere coupling.

1.6

New Discoveries

It is these dayside and polar cap aurora, carefully recorded only within the past two decades, that we write about in this book. Deep within what was previously thought to be a black, lifeless, polar cap sky, in the region inside the polar-most extreme of the auroral oval, we find polar cap aurora half the time. They extend thousands of km across the polar cap, appearing

1.6 New Discoveries

[7]

as narrow “sundials” in the night sky, and signaling the turning on and off of strong coupling by the solar wind to the earth’s polar ionosphere. At certain locations, in midwinter, we can see the noon auroral region in overhead darkness. Here, at the midday, or noon, of the polar cap, in sharp contrast to what was known before the 1980s as the “midday gap” in aurora, we now regularly see dramatic, dynamic, auroral displays. They streak dawnward, duskward, and straight into the polar cap at speeds far in excess of any supersonic jet. These cusp or midday aurora are, in fact, most dramatic during that half of the time when sun-aligned arcs are absent from the polar cap. New work on the midday aurora and the polar cap sun-aligned aurora is the focussed subject of this book. It is this excitement of discovery, and the nature of these newly-discovered auroral displays, that we treat in this book. We hope you, too, will find this a fascinating voyage of discovery, one that you may share with your friends, colleagues, and fellow students of nature wherever and whoever they may be.

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Chapter 2

Near-Earth Space and Dayside Aurora In this chapter we give a brief introduction to the field of solar wind-magnetosphere coupling and the resulting manifestations in dayside and polar cap auroral forms, which are discussed in greater detail in later chapters. The general introduction is rounded off with examples illustrating the responses in the dayside and polar cap auroras to rapid changes of direction of the interplanetary magnetic field and a brief summary of major results.

2.1

Introduction

The sun and its continuously outward-streaming outer atmosphere (the corona), in the form of the solar wind, is the source of the near-earth space plasma. Some fraction of this electron/proton plasma (the solar wind) is captured by the earth’s magnetic field, constituting the magnetosphere, before penetrating along the earth’s field down to the polar upper atmosphere. This latter component is called the auroral particles. The resulting excitation of the atomic and molecular constituents of the upper atmosphere gives rise to auroral light emissions, which can be continuously monitored by ground optical instrumentation. The solar wind kinetic en ergy which is continuously dissipated in magnetosphere-ionosphere system may be separated into the following three major components (Akasofu 1981): (1) high-energy particles trapped in the earth’s magnetic field (the ring current), (2) the precipitating auroral particles, and (3) heating of the upper atmospheric plasma by frictional coupling between the electric currents and the neutral gas of the ionosphere. The magnetic field embedded in the solar wind, the interplanetary magnetic field (IMF), plays a crucial role in the energy coupling from the solar wind to the near-earth space (Akasofu 1981). We shall see that the energy transfer takes different routes corresponding to different IMF orientations. Thus, the strong variabilities of the solar wind plasma and the IMF give rise to corresponding variabilities of the plasma populations in near-earth space. This variability, an important aspect of space weather, is manifested in the ring current, the particle precipit ation (aurora), and Joule heating in the polar upper atmosphere. The rate of energy transfer to near-earth space changes by three orders of magnitude, within associated with solar wind variability at the earth’s distance. This change is due to the variability of the solar wind bulk speed, taking values within ~300–1000 km/s, combined with the variable IMF intensity (within the range ~3–30 nT), and the multitude of IMF orientations. In this book we shall use dayside and polar cap auroral emissions as tracers of the dynamic [9]

[ 10 ]

Near-Earth Space and Dayside Aurora

interaction between the solar wind and the magnetosphere-ionosphere system. The dayside aurora is observed with high temporal resolution by sensitive ground-based instrumentation. Our strategy is then to extract information on the dynamic solar wind-magnetosphere coup ling processes by studying the responses in the dayside and polar cap auroras to specific solar wind/IMF variations. This inference from aurora to solar wind-magnetosphere coup ling is based on the presently available knowledge of various magnetospheric boundary layers/processes and the identification of their corresponding footprints in the polar upper at mosphere in the form of auroral forms and activities. In the next section we give a brief introduction to solar wind-magnetosphere-ionosphere coupling processes and the various mag netospheric boundary layers.

2.2 Magnetospheric Boundary Layers

2.2

[ 11 ]

Magnetospheric Boundary Layers

A schematic, three-dimensional illustration of magnetospheric morphology with the boundary layers toward the solar wind plasma is given in Figure 2.1. The figure illustrates how the solar wind and the earth’s field give rise to an ordering of the near-earth plasma in a cellular structure. Here we shall focus on the boundary regions in the vicinity of the solar windmagnetosphere interface, the magnetopause. Compared to a dipole field the earth’s field is compressed by the solar wind (dynamic pressure) on the sunward side and stretched into a long tail on the nightside. A cusp configuration, marked 0 in Figure 2.1, is formed at high latitudes on the dayside, and marks the demarcation line between field lines closing on the dayside and those that are open towards the magnetotail. Active auroral events, both on the day- and nightside, are related to disruption of the magnetospheric current sheets, marked by red arrows in Figure 2.1. In such cases a part of the magnetospheric sheet current is guided down to the polar upper atmosphere (the ionosphere), via Birkeland currents (green arrows in Figure 2.1). Of major interest to us are the boundary layers that form at the interface between the shocked solar wind (the magnetosheath) and the magnetosphere, in the vicinity of the polar cusp. Four such boundary layers have been marked in the figure. Boundary regions/layers marked 0, 1, 2 and 5 represent important channels for entry of solar wind plasma into the magnetosphere. Layers 1, 2 and 5 are sources of strong momentum coupling to the iono sphere via field-aligned currents. Low-altitude manifestations of these boundary layers and their associated field aligned-currents, in the form of specific auroral forms/activities, is a major topic in this book. Boundary layers 1, 2, and 5 represent so-called active space plasma elements, characterized by strong field-aligned current filaments and discrete auroral forms, as opposed to the passive plasmas occupying vast regions of space where the electric current intensity is weak (Alfvén 1981, Alfvén 1987). Active plasmas are characterized by inhomogen eity in space, variability in time, and by strong electric fields, including field-aligned electric fields (Fälthammar 1997). Boundary layers 1 and 2 are located respectively on the equatorward (low-latitude) and poleward (high-latitude) sides of the magnetic cusp in the midday sector. As we shall see, layer 1 and its auroral manifestations are most active during southward oriented interplanetary magnetic field. Layer 2 is strongly activated during northward IMF orientation. Plasma and momentum transfers through layers 1 and 2 are to a large extent due to magnetic interconnec tion (reconnection) processes (magnetic field diffusion in strong current sheets), taking place where the earth’s field and the IMF have antiparallel components. During southward IMF reconnection (current disruption) takes place near the subsolar magnetopause, while the highlatitude magnetopause, poleward of the cusp, is a favoured reconnection site during northward IMF. As we shall see later, when the IMF is oriented in the east-west direction, current layers 1 and 2 can be both activated, giving rise to a so-called cusp bifurcation, with corresponding auroral manifestation. The exterior cusp, marked 0 in Figure 2.1, is a site of direct plasma entry from the solar wind, mainly by so-called diffusive processes, which in terms of its importance as momentum transfer mechanism, is considered secondary relative to reconnection (Cowley 1984). The auroral manifestation of such secondary processes (diffusive entry) is a relatively weak cusp aurora, which can be observed when the interplanetary magnetic field is radially oriented (small north-south and east-west components). In this case momentum coupling via boundary layers 1 and 2 is weak or absent. In such cases, the magnetosphere is approaching a “ground state”, and the corresponding cusp aurora is often referred to as the “midday gap“ aurora, due to the low intensity of the auroral green line emission (Dandekar and Pike 1978). The low-latitude boundary layer on the postnoon flank, marked 5, in Figure 2.1, maps

[ 12 ]

Near-Earth Space and Dayside Aurora

down (along magnetic field lines) to the so-called multiple fan auroral arcs in the postnoon ionosphere, on the eastward side of the cusp. A similar boundary layer and corresponding auroral forms exist in the prenoon sector. The plasma population labelled 3 in the figure is the dayside extension of the central plasma sheet. Its low-altitude auroral manifestation is very different from those of layers 1, 2, and 4. This difference is due to the fact that solar wind plasma normally has no direct access to this region. The plasma here is of magnetospheric origin, drifting around from the nightside plasma sheet. Dynamic auroral events, both on the dayside and on the nightside, are related to unstable magnetospheric current sheets, marked by red arrows in Figure 2.1. In such cases the mag netospheric sheet current is disrupted and part of it is guided towards the ionosphere, by field-aligned currents, marked by green arrows in the figure. The strong field-aligned currents give rise to the excitation of active, discrete auroras. Figure 2.2 shows a schematic illustration of electric currents generated in the boundary layers along the prenoon and postnoon flanks, and their coupling to the polar upper atmo sphere (the ionosphere). The electric field coupling to the ionosphere, with its associated standard ion circulation pattern, is indicated in the figure. In the presence of a dawn-to-dusk electric field, the ions of the upper atmosphere are convected antisunward from the dayside cusp region across the polar cap to the nightside. The circulation is completed by return flow in the morning and evening auroral regions.

2.3 Day side and Polar Cap Auroras

[ 13 ]

In a later section we document by coordinated ground and satellite observations a corres pondence between a filamentation of the large-scale field-aligned current in this region and mid-morning multiple arcs. In such cases, the time-variability of the auroral emission, ob tained by the continuous ground observations, can be used to infer the corresponding time dependence of the boundary layer mechanism that is responsible for the current generation. Among the possible mechanisms of plasma transfer from the solar wind to the mag netosphere are the following: 1) quasi-steady reconnection, i.e., plasma entry by merging of interplanetary and magnetospheric magnetic field lines in a quasi-steady state/large scale process, either in the subsolar region or at high-latitudes, tailward of the cusp (Sonnerup, Paschmann, Papamastorakis, Sckopke, Haerendel, Bame, Asbridge, Gosling and Russell 1981, Cowley 1982, Paschmann, Papamastorakis, Baumjohann, Sckopke, Carlson, Sonnerup and Luhr 1986, Paschmann, Baumjohann, Sckopke, Phan and Luhr 1993, Gosling, Thomsen, Bame, Elphic and Russell 1990a, Gosling, Thomsen, Bame, Onsager and Russell 1990b, Gos ling, Thomsen, Bame, Elphic and Russel 1991, Song and Russell 1992, Woch and Lundin 1992, Phan and Paschmann 1996), 2) flux transfer events (FTEs), i.e., transient and smallscale merging processes (Russell and Elphic 1978, Rijnbeek, Cowley, Southwood and Russell 1984, Kawano and Russell 1997), 3) viscous diffusion, i.e., plasma entry into the magnetic ally closed magnetosphere due to viscous type interactions at the magnetopause and/or in the exterior cusp (Eastman, Hones Jr., Bame and Asbridge 1976, Mozer 1984), 4) gradient drift entry, i.e., plasma entry into a closed magnetosphere due to gradients in the magnetic field (Olson and Pfitzer 1985), 5) impulsive penetration events (IPEs), i.e., direct entry of plasma blobs/irregularities with enhanced density (Lemaire 1977, Lemaire and Roth 1978, Lundin and Aparicio 1982, Ma, Hawkins and Lee 1991, Woch and Lundin 1991, Lundin 1997). It is an important task for the magnetospheric physics community to identify the ionospheric signatures (particularly in aurora and plasma convection) of such plasma entry mechanisms. The identification of the different particle precipitation regimes on the dayside and their correspondence with the various magnetospheric plasma regimes, by Newell and coworkers, represented one step towards this goal (Newell and Meng 1988, Newell and Meng 1994). The ground-based auroral observations of dayside auroral forms provide complementary informa tion with respect to these statistical studies of particle precipitation from satellites in polar orbit. Whereas the statistical information gives averaged pictures of the large-scale features, the ground-based observations provide more detailed information on the dynamics of the dif ferent precipitation regimes. The ground-based observations are particularly useful as a source of information on the response to external (solar wind/IMF) variations.

2.3

Dayside and Polar Cap Auroras

Optical observations of dayside auroral forms and activities have been performed by meridian scanning photometer (MSP) and all-sky cameras from Ny Ålesund, Svalbard (76°MLAT) during the last 20 years. In the data reported here we focus on the auroral oxygen emissions at 630.0 and 557.7 nm. The red-to-green line intensity ratio is a good indicator of the energy spectrum of the precipitating electrons (Rees and Roble 1986). F-layer auroras in the cusp region, due to particles of magnetosheath (shocked solar wind) origin, are characterized by high red line intensities (typically 2-10 kR) and green line intensities below 2 kR. By contrast, much less red line emission and much lower red-to-green line ratios are observed in auroras that are excited by higher-energy particles of magnetospheric origin, such as those from the dayside extension of the central plasma sheet (our layer 3 above). Sporadic enhancements of the green line emission also occur in the cusp region, for example in the well known sequence of equatorward boundary intensifications (EBIs; footprint of boundary layer 1) and associated poleward moving auroral forms, and the brightening sequence observed at the cusp poleward

[ 14 ]

Near-Earth Space and Dayside Aurora

boundary (footprint of boundary layer 2). The enhanced green line emission in these cases, as well as in the prenoon and postnoon auroral arcs (footprint of boundary layers 4 and 5), is a signature of strong field-aligned current intensity (active plasmas). Figure 2.3 shows a schematic illustration of dayside auroral forms representing atmospheric manifestations of the plasma boundary layers discussed above. The midday sector is often dominated by one or both of the forms we call types 1 and 2. The type 1 aurora is generally located in the magnetic latitude (MLAT) region 70°–75°. It is characterized by a quasiperiodic sequence of equatorward boundary intensifications (EBIs), marked by a heavy curved line on the edge of the hatched region in the figure. EBIs are recurring at 1–2 min intervals. Thus this activity represents pulsations in the PC 4 regime. Many EBIs are followed by poleward moving auroral forms (PMAFs). This aurora is observed when the interplanetary magnetic field (IMF) has a southward component, or, when the IMF is dominated by the component. The east-west component of motion of the PMAFs is regulated by IMF polarity, as indicated in the figure. Multi-instrument observations from ground and space, reported in later sections, indicate that the type 1 aurora is a low-altitude manifestation of plasma entry from the solar wind in boundary layer 1 via magnetic reconnection processes. Impulsive injections of plasma (electrons and protons) and its subsequent penetration to low altitudes along newly-reconnected flux tubes convecting over the polar magnetosphere may give rise to the auroral signatures we refer to as poleward moving auroral forms (PMAFs). The identification of PMAFs as an ionospheric signature of transient magnetopause reconnection (flux transfer events) is based on the following observational characteristics: 1) proton and electron precipitation in PMAFs, 2) plasma convection in PMAFs, 3) occurrence and motion patterns of PMAFs in relation to IMF polarity, and 4) the lifetime of individual PMAFs (~10 min) and the recurrence period (~5 min) between events.

2.3 Dayside and Polar Cap Auroras

[ 15 ]

When the IMF is northward-directed, the midday aurora is shifted to higher latitudes, typ ically located within 75°–80°MLAT. It is characterized by a sequence of poleward boundary intensifications (indicated by a heavy curved line at the poleward boundary of the hatched area). The type 2 auroral forms also show east-west motions regulated by IMF polarity (not shown in Figure 2.3). Multi-instrument observations of this activity (particle precipitation and auroral emissions) strongly indicate that the type 2 forms represent an ionospheric signature of plasma and momentum transfer from the solar wind along the poleward boundary of the cusp, i.e., boundary layer 2 in Figure 2.1. The source mechanism is magnetic reconnection, occurring tailward of the cusp during northward IMF orientation (Gosling et al. 1991, Kessel, Chen, Green, Fung, Boardsen, Tan, Eastman, Craven and Frank 1996). Auroral forms of types 4 and 5 are the prenoon and postnoon multiple arcs, which are manifestations of plasma and momentum transfer from the solar wind via the flank boundary layers. These forms are also regulated by IMF orientation. The exact mechanism remains to be identified. A belt of diffuse aurora, called type 3, is located on the equatorward side of the type 1 and 4 forms in the prenoon sector. This aurora is very different from the previously described forms in morphology (diffuse glow) and optical spectral composition (dominated by the green line oxygen emission at 557.7 nm; very weak in the red line). It is a signature of precipitation from the central plasma sheet. The auroral form labelled 6 in Figure 2.3 marks the presence of polar cap arcs emanating from the poleward boundary of the type 2 cusp aurora during northward IMF conditions. Polar cap arcs are generally associated with flow lines of polar plasma flow and gradients of plasma flow (converging horizontal electric field) present during northward IMF orienta tion (Burke, Gussenhoven, Kelley, Hardy and Rich 1982, Gussenhoven 1982, Hardy, Burke and Gussenhoven 1982). Polar arcs are furthermore associated with paired sheets of field-aligned currents, the outflowing component being carried by incoming electrons, and an associated ionospheric flow channel (Chiu 1989). Initially thought a curiosity present a couple of percent of the time, polar cap arcs came to be recognized as present fully that half of the time the IMF is northward (Carlson, Heelis, Weber and Sharber 1988, Valladares, Carlson and Fukui 1994). In Figure 2.4 we give an example of auroral observations by a meridian scanning photo meter (MSP) in the ~1100-1130 MLT region, along the field of view indicated in Figure 2.3. Figure 2.4 shows MSP observations at 630.0 nm (upper panel) and 557.7 nm (lower panel) for the interval 0810-0840 UT on Jan. 8, 1999. In the early part (0800-0825 UT), three latitu dinally separated auroral forms/types were present. The southernmost form, called type 3 in Figure 2.3, is seen around 60° south of zenith in the green line (lower panel) during the interval 0810–0825 UT. This aurora is pulsating and totally absent in the red line. Four type 3 brightenings, with the intensity decreasing with time, are observed during 0810–0825 UT, within 1100–1130 MLT. The recurrence time is 3-4 min. A latitudinal gap is observed between the type 3 aurora and the type 1 cusp auroral activity on its poleward side. The type 1 aurora is characterized by a sharp equatorward boundary, located near 30° south of zenith, and a sequence of equatorward boundary intensifications (EBIs), each of which is fol lowed by poleward moving auroral forms (PMAFs). The fine-structure of these EBIs is seen in the 557.7 nm emission. EBIs recur at 1–2 min intervals, i.e., corresponding to the range of PC 4 pulsations (see also (Engebretson, Beck, Detrick, Rosenberg, Rairden, Mende, Arnoldy and Cahill Jr. 1994, Milan, Yeoman, Lester, Moen and Sandholt 1999)). It is anticipated that the EBIs are located in the close vicinity of the open-closed field line boundary. Thus, the type 3 aurora in the south is located on closed magnetospheric field lines. The emission gap between the type 3 and type 1 forms is thought to be a characteristic signature of the transition region near the open-closed boundary. The type 2 aurora is located at ~30–50° north of zenith. A type 2 brightening occurred at ~0820 UT, before this northernmost form faded out near 0825 UT.

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Near-Earth Space and Dayside Aurora

Illustrations of the responses of the dayside aurora to rapid transitions of the IMF orienta tion between south and north will be given below. First, we give a brief discussion of dayside convection patterns and their dependence on IMF orientation.

2.4

Convection Patterns

Figure 2.5 shows convection patterns in the polar upper atmosphere above 200 km altitude, where electrons and ions move at equal speeds, i.e., the so-called E×B drift. The six different patterns represent different IMF orientations as parameterized by the components in the geocentric sun-magnetosphere (GSM) Y-Z plane, i.e. normal to the earth-sun line. The upper panel shows lobe convection cells in a contracted polar cap, whereas the lower panel shows lobe (L) and merging (M) cells, after (Reiff and Burch 1985). There is a strong IMF related convection asymmetry in the cusp region, with northeastward (northwestward) convection corresponding to negative (positive) IMF polarity. A symmetric two-cell pattern is indicated for a due south IMF orientation. According to the more recent empirical results the convection throat (region of inflow to the polar cap on dayside) is generally displaced towards the prenoon (postnoon) side for positive (negative) polarity (Weimer 1995). For due north IMF, a two-cell convection pattern with reversed flow direction, characterized by equatorward flow at the cusp poleward boundary, is observed (see also (Crooker 1992)). In the next section we will describe the detailed response in the convection pattern and the associated cusp region aurora to a rapid transition of the IMF from south to north, and the activation of the merging (M) cell/type 1 cusp aurora following a southward turning.

2.5 Responses to IMF transitions … aurora and convection

2.5

[ 17 ]

Responses to IMF Transitions from South to North and vice versa: Aurora and Convection

Ground observations of the different phases of the evolution of the cusp aurora and the local plasma convection, in response to a rapid transition of the IMF from south to north, were recorded on Dec. 16, 1998 (Sandholt, Farrugia, Cowley, Lester, Denig, Cerisier, Milan, Moen, Trondsen and Lybekk 2000). Figure 2.6 shows meridian scanning photometer (MSP) observations at 630.0 nm from Ny Ålesund (76°MLAT) during the interval 0720–0830 UT on Dec. 16, 1998. The aurora was initially displaced well south of Ny Ålesund (76°MLAT), consistent with the strongly south ward IMF orientation before 0730 UT. This is the aurora referred to as type 1 in Figure 2.3. We note the auroral intensifications at 0731 and 0735 UT, the subsequent stepwise poleward expansion and weakening of the cusp aurora, reaching zenith of the station at ~0810 UT. These changes are all effects (type 2 aurora) in response to a rapid northward turning of the IMF, first activated at~0730 UT. The ionospheric ion flow (plasma convection) pattern which was observed by the CUTLASS Finland HF radar, is schematically illustrated in Figure 2.7. Initially, before the northward turn, a twin-cell ion flow pattern existed of the form expected for negative IMF and components (normal E×B drift). This initial state is indicated in Figure 2.7a. After the IMF northward turning, affecting the ionosphere from ~0730 UT, the plasma flow configuration went through different stages. The aurora responded initially with two impulsive auroral

[ 18 ]

Near-Earth Space and Dayside Aurora

brightening events during 0731–34 and 0735–40 UT (see Figure 2.6). These events generated intense clockwise ion flow vortices in the prenoon cusp, each lasting for ~3 min. The flow configuration during these events is indicated in Figure 2.7b. In the 0736 UT event, the strong auroral intensification was accompanied by a polar arc emanating from the cusp and the appearance of a lobe convection cell in the prenoon sector, characterized by clockwise vorticity (see Case 8 in chapter 4 for details). A postnoon convection cell, which is expected, but not directly observed in this case, is marked by dashed lines. After each of these events, the ion flow reverted to that appropriate to the previous southward IMF configuration (Figure 2.7a), which took ~15 min to decay away completely. A series of four poleward “steps” then occurred in the poleward border of the aurora, starting at ~0742 UT. The ion flows in the cusp region are consistent with a large weak clockwise convection in the prenoon sector, which pulses in concert with the auroral steps on few-minute timescales. The postnoon cell is assumed on the basis of logic and supporting data from observations from satellites in polar orbit. The resulting two-cell convection pattern, with equatorward (reverse) flow across the cusp poleward boundary, is illustrated in Figure 2.7d. The CUTLASS ion drift data (line of sight velocities) from the prenoon sector during

2.5 Responses to IMF transitions . . . aurora and convection

[ 19 ]

[ 20 ]

Near-Earth Space and Dayside Aurora

2.5 Responses to IMF transitions . . . aurora and convection

[ 21 ]

this interval are shown in Figure 2.8. The coordinate system (MLT–MLAT) is the same as in Figure 2.7. MLT meridians and the 70° and 80°latitude (MLAT) circles are indicated by dotted lines and circles, respectively. The approximate locations of the poleward and equatorward boundaries of the aurora are marked by solid curved lines. A beam swinging technique has been applied to derive ion drift vectors. We note the regions of blue and green colours, representing ion drift toward the radar (equatorward flow) in the 1000–1100 MLT sector at 0730, 0736, 0745, and 0757 UT, corresponding to auroral events shown in Figure 2.6. From 0745 UT onwards the poleward boundary of the cusp aurora expanded poleward, resulting in a latitudinal widening of the cusp emission band. In our view this is a result of the capture of magnetosheath flux tubes by the magnetosphere via magnetic reconnection taking place in both hemispheres (two-lobe events), as illustrated in Figure 2.9. This example illustrates the strong regulation of the aurora/precipitation and ionospheric plasma convection in the cusp region that is exerted by the IMF orientation. In the present case the transition from the standard two-cell convection pattern, representative of southward IMF, to the “reverse” two-cell pattern, representative of strongly northward IMF conditions, went through a 15 min long intermediate phase, characterized by strong transient auroral events and associated activations of small (lobe) convection cells intruding into the larger-scale convection pattern. By the technique of continuous monitoring from the ground, the evolution of the response of the magnetosphere-ionosphere system to a rapid change of external (solar wind/IMF) conditions can be followed from minute to minute. The evolution of the IMF-magnetosphere interconnection geometry for this case may be illustrated by Figure 2.9, after (Lockwood 1998). Figure 2.9a shows the evolution of re connected (interconnected) field lines for magnetopause reconnection during southward IMF, representing the conditions before 0730 UT in the case reported above. Figure 2.9b shows the case for northward IMF and reconnection in the northern hemisphere lobe (one lobe events), representing the condition in the transition phase during 0730–0740 UT on Dec. 16, 1998. Figure 2.9c shows the case for northward IMF and reconnection in both the northern and southern lobes (two-lobe events), giving rise to capture of magnetosheath (solar wind) plasma on closed magnetospheric field lines. The result is a poleward expansion and latitudinal broad ening of the cusp, which is exactly what was observed during the interval 0742–0810 UT in the reported case example. From the ground observations we may infer that the process occurs in a sequence of events rather than as a continuous process. Below we show an example of the responses in the midday and polar cap auroras to a rapid southward turning of the IMF. Figure 2.10 shows solar wind plasma and IMF observa tions obtained by spacecraft WIND during the interval 0900–1100 UT on January 21, 1999. The solar wind density is around and the speed is within . A rapid transition from strongly north to strongly south IMF orientation occurred during 0958–1000 UT. Ionospheric effects of the sharp southward turning around 0940 UT (see below), which implies that WIND sees the responsible solar wind ~20 min later. This is consistent with the spacecraft’s position at 1000 UT, i.e., at (-13, -43, 14) A first example of the cusp auroral response to a sharp southward turning of the IMF is shown in Figure 2.11. The field of view of the auroral meridian scanning photometer (MSP) is given in Figure 2.13 below. Before 0940 UT, corresponding to northward IMF orientation, the type 2 cusp aurora is located well north of zenith (76°MLAT). Then a brief intensification occurred at 0939 UT, which was followed by a significant equatorward shift at 0942 UT, a major intensification/equatorward shift at 0945 UT, and a sequence of three PMAFs during the interval 0945 – 1000 UT (onsets at 0945, 0950, and 0955 UT). Between 0935 and 0945 UT the cusp equatorward boundary shifted equatorward by ~200 km (2°MLAT). The activation of the type 1 cusp aurora was accompanied by a sharp onset of a magnetic deflection event recorded at the Svalbard magnetometer stations of magnetometers, as shown in Figure 2.12. The X

[ 22 ]

Near-Earth Space and Dayside Aurora

component went positive during the interval 0936 – 0943 UT. Inspection of the corresponding Z-component deflections (not shown) indicate that the source current in the ionosphere was centered at the latitude of station HOP – HOR (73° – 74°MLAT), which corresponds to the latitude of the auroral equatorward boundary (at 0945 UT). The positive X deflection refers to an eastward-directed ionospheric Hall current and a corresponding westward component of ionospheric convection. Westward convection is expected in the cusp region during the actual

2.5 Responses to IMF transitions . . . aurora and convection

[ 23 ]

[ 24 ]

Near-Earth Space and Dayside Aurora

positive IMF conditions (see lower right panel of Figure 2.5). Figure 2.13 shows Polar UVI images of the aurora in the northern hemisphere, taken at two times during the interval of the auroral plot in Figure 2.11 (0925 and 0938 UT). The coordinate system is magnetic local time (MLT)-magnetic latitude (MLAT). The fields of view of the auroral meridian scanning photometer (MSP) in Ny Ålesund, Svalbard and Danmarkshavn, Greenland are indicated by white stripes along the ~1300 and 1100 MLT meridians, respectively. The white crosses in the middle of the white stripes mark the positions of the stations. The upper panel, representing strongly northward IMF conditions at 0925 UT (Figure 2.10), shows a strong sun-aligned arc (called type 6 in Figure 2.3), extending across the polar cap from the dayside cusp region (near 1100 MLT) to the nightside auroral zone, near 0130 MLT. The cusp aurora along the Ny Ålesund meridian (~1300 MLT), as inferred from Figure 2.11, is located within ~77° –80°MLAT. The UVI image in the lower panel of Figure 2.13, taken at 0938 UT, just after the southward turning of the IMF, shows that the polar cap arc has partly disappeared. During the next 7 min (0938–0945 UT) the cusp aurora shifts equatorward to ~73°–75°MLAT, as illustrated in Figure 2.11. The auroral transition that took place during the interval 0935–0945 UT is also illustrated in Figure 2.14, representing the MSP auroral observations from Danmarkshavn, Greenland, at

2.5 Responses to IMF transitions . . . aurora and convection

[ 25 ]

[ 26 ]

Near-Earth Space and Dayside Aurora

2.5 Responses to IMF transitions . . . aurora and convection

[ 27 ]

[ 28 ]

Near-Earth Space and Dayside Aurora

77°MLAT. The approximate field of view of the MSP in Danmarkshavn is marked along the 1100 MLT meridian. Before ~0935 UT the cusp aurora, called type 2, is located to the north of zenith. On the poleward side of this is an auroral form with strong green line intensity, called type 6. This is the dayside extension of the polar cap arc seen in the upper panel of Figure 2.13. At 0945 UT, after the southward turning of the IMF, the type 2 form had been replaced by a type 1 aurora, located south of zenith (compare Figure 2.11), and the polar cap arc (type 6) had almost disappeared. The first appearance of the type 1 aurora occurred at 0938 UT, a few min after the estimated arrival at the magnetopause of the transition to southward IMF. This is a typical response in the cusp and polar cap auroras to southward turning of the IMF.

2.6

Remarks on Major Results

Strongly diverging opinions on the influence of the interplanetary magnetic field on the cusp position and dynamics have been offered over the last 20 years of dayside auroral observations. The different views are largely based on the different sensitivity of different observation tech niques (e.g. ground based vs. satellite observations). The different sensitivities of ground and satellite observations to the cusp emissions are illustrated by the comparison of simultaneous observations from the UVI imager on spacecraft Polar and the ground shown in Figures 2.11 and 2.13 above. One of the controversial issues related to the cusp dynamics is whether the equatorward shifts of the low-altitude cusp (the ionospheric signature of the magnetopause current layer) for southward oriented IMF is directly (“instantaneously”) caused by a negative or by a component (Carbary and Meng 1986), or by the current systems that develop time-delayed inside the magnetosphere during active times (substorms) (Eather 1985) (Stasiewics 1991). According to (Stasiewics 1991) “the cusp exists for arbitrary orientation of the IMF and hence it is not directly related to any process associated with a particular direction of the IMF”. (Lundin, Sandahl, Woch, Yamauchi, Elphinstone and Murphree 1992) reported “the persistency of the dayside auroral activity, an activity that depends little on the IMF component”. This view is based on satellite observations by for example the Viking spacecraft. Ground based observations lead to a strong modification of the above conclusions. The ground observations, which represent the proper sensitivity for dayside emissions in the cusp region, document the “instantaneous” effect of the IMF component. Thus, it turns out that ground-based observations are of critical importance for revealing the temporalspatial structure of the solar wind-magnetosphere interaction corresponding to the various IMF orientations. Detailed presentations of eight cases illustrating the association between IMF orientation and the dayside auroral forms and activities, are reported in Chapter 4. It is demonstrated in this study that the cusp region aurora (precipitation of solar wind (magnetosheath) origin plasma) is strongly regulated by the IMF orientation on an “instant aneous” basis. The regulation by the field components transverse to the earth–sun line, the Y and Z components, is particularly strong. The cusp aurora and the associated precipitation appear in different modes/configurations corresponding to different regimes of IMF orientation in the Y-Z plane. The “instantaneous” effects of IMF (the clock angle in the GSM Y-Z plane), in the form of auroral reconfigurations and latitudinal displacements, are observed both during southward and northward turnings of the IMF, as illustrated above. Furthermore, the ground magnetic signature of convection changes is observed near-simultaneously within a wide MLT sector (~0900–1500 MLT). Figure 2.15 shows six regimes of IMF clock angle, which is defined as the polar angle from the GSM Z axis to the IMF vector projected into the GSM Y-Z plane. Observations presented in this book indicate that the configuration of the cusp region aurora can be sorted by these

2.6 Remarks on Major Results

[ 29 ]

six clock angle regimes. These auroral configurations correspond roughly to the six convection configurations indicated in Figure 2.5. Thus, there are three main IMF clock angle regimes, component. The each of which is divided in two subregimes by the polarity of the IMF influence of the component is clearly seen in the cusp aurora, for example in the motion pattern of poleward moving auroral forms (PMAFs), as illustrated in Figures 2.3 and 2.7. In clock angle regime I (0°–45°; north) the cusp aurora in the northern winter hemisphere typically appears in the form of a rayed auroral band located at latitudes 75°–80°MLAT. This is the form we call type 2 (Figure 2.3). It is characterized by a sharp poleward boundary, while the intensity is decreasing more gradually at the equatorward boundary. The polar cap is characterized by sun-aligned arcs as shown in Figure 2.13. In IMF clock angle regime III (south) the aurora appears in the form of the type 1 emis sion band with equatorward boundary intensifications (EBIs), typically located within 70°– 75° MLAT. The polar cap is void of auroral arcs. When the IMF is in clock angle regime II (45° –135°; IMF orientation) we often observe the presence of both type 1 and 2 cusp auroral forms. This configuration is referred to as the bifurcated cusp aurora. The southernmost form is characterized by a series of EBIs, followed by PMAFs. The zonal motion of the latter forms is regulated by the IMF polarity.

[ 30 ]

Near-Earth Space and Dayside Aurora

The influence of the IMF component on the cusp region aurora is somewhat uncertain at present and further study is needed. Here we only note the following observation. The characteristic sequence of poleward boundary intensifications in the cusp aurora during north ward IMF conditions is observed in the winter hemisphere for both polarities. Assuming that this phenomenon is related to lobe reconnection (poleward of the cusp), this observation is contrary to expectations from the simple antiparallel merging model, according to which reconnection occurs in the favoured hemisphere (where fields arc most antiparallel) only (Crooker 1979). Our observations indicate that effects of field-line draping around the magnetospheric cavity are important and should be included in future models of solar windmagnetosphere coupling. The field draping may partly mask out the effect. When is large and the Y and Z components are small the cusp aurora is approaching a minimum intensity. When the IMF has large X and Y components the tilted phase front in the solar wind may give rise to the simultaneous presence of nearby located auroral forms which are related to IMF – magnetosphere interactions in different hemispheres, as illustrated in Figure 4.7 in case 1 of Chapter 4 (see (Maynard, Burke, Sandholt, Moen, Ober, Weimer, Egeland and Lester 2001)). An important result of our studies is the identification of the sequential brightenings of type 1 and 2 auroral forms during IMF orientation (regime II), that is, sequen tial intensifications at the equatorward and poleward cusp boundaries. This phenomenon is illustrated in Figure 2.16. The aurora in Figure 2.16 was observed when the IMF clock angle was ~115° Intensifications in the south (type 1) are associated with poleward moving auroral forms (PMAFs). Major events are initiated at 0700, 0707, 0711, and 0718 UT. Some of these events are followed by type 2 activations in the north, for example at 0705 and 0723 UT. These sequential activations of type 1 and 2 forms may be explained as follows. Figure 2.17a shows a schematic illustration of the IMF-magnetosphere interconnection geometry for an eastward oriented IMF. The dashed line shows the reconnection line on the magnetopause in the northern hemisphere. It is shown potentially continuous from low latitudes, where open flux production drives the “merging cell”, to high latitudes, where lobe reconnection drives the “lobe cell”. The northern hemisphere ion flow to which such reconnection will give rise is sketched in Figure 2.17b. In this diagram the heavy solid line indicates the open-closed field line boundary, while the dashed segment indicates the field lines which map to the magnetopause reconnection line where open flux production takes place. The “merging cell” streamlines cross this dashed line in moving from the closed flux region at lower latitudes to the open flux region at higher latitudes. The dashed line continues into the higher latitude region of open flux, indicating the field lines mapping to the reconnection line where lobe reconnection takes place. The “lobe cell” streamlines cross this line in circulating within the region of open flux. The numbered dotted lines schematically indicate lines of equal time (isochrones) since reconnection took place on the field lines downstream from the reconnection sites. These therefore indicate the nature of the cusp ion dispersion in the steady state, with ion energy decreasing with increasing time. Complex latitudinal dispersion patterns are possible in the noon sector, characterized by a relative minimum in the center of the precipitation band. The minimum lies at the interface between the “merging cell” at lower latitudes, and the “lobe cell” at higher latitudes. Those two regions of precipitation are inferred to correspond to the type 1 and type 2 cusp auroras discussed here. Cusp ion precipitation for IMF dominated IMF orientation of the type indicated in Figure 2.17b has been observed by (Woch and Lundin 1992). Recent observations by the UVI imager on the Polar spacecraft and the CUTLASS HF radar (Milan, Lester, Cowley and Brittnacher 2000a), as well as ground auroral observations in combination with radar data, as presented in chapter 4 below, suggest that reconnection can

2.6 Remarks on Major Results

[ 31 ]

often proceed in a wave-like manner propagating antisunward over the magnetopause, giving rise locally to “flux transfer events”. In the present circumstances, such a “reconnection wave” would lead first to an enhancement of the “merging cell” flow and type 1 auroras at lower latitudes, involving an equatorward motion of the auroras (and open-closed field line boundary) followed by poleward expansion. Shortly afterwards, this would be followed by enhancement of the lobe cell flow and type 2 auroras, involving a poleward expansion of these auroras followed by equatorward contraction. This picture thus provides a natural explanation for the auroral observations presented here, including the sequential activations of type 1 and 2 forms. Detailed information on the plasma processes operating at the solar windmagnetosphere interface in these cases, relating for example to the question of antiparallel vs. component reconnection, can best be resolved by in situ measurements from multi-spacecraft, preferentially in combination with remote sensing techniques on the ground. Such observations will be undertaken during the upcoming era of the multi-spacecraft project CLUSTER.

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Near-Earth Space and Dayside Aurora

Chapter 3

Optical Aurora

The auroral spectrum consists of a great number of spectral lines and bands from constituents in the upper atmosphere, excited by precipitating auroral particles. In this book, however, we will limit our discussion to those emissions most relevant to the subject topic, and refer the interested reader to other comprehensive treatments of all auroral phenomena. We will concentrate on the visible wavelength range of the optical spectrum. We will also concentrate on aurora in the dayside and polar cap, which have significantly different properties than nightside aurora.

3.1

Introduction

Some of the main differences between day- and nightside aurora are shown in Table 3.1. These differences are primarily driven by the very different energies of auroral particles, tracing to different auroral locations. By dayside we mean on that side of the auroral oval towards the sun, but remain aware that, depending on season and longitude, this side need not be sunlit. Observing the noon aurora while it is in total darkness at mid-winter has taught us a great deal of dayside auroral physics. The precipitated charged particles are guided by magnetic fields in their downward mo tion into the atmosphere. They suffer elastic and inelastic collisions, and through the latter transfer their energy to neutral particles by breaking molecules apart (dissociation), break ing neutral particles into electrons and ions (ionization), heating the atmosphere, producing bremsstrahlung (X-rays) under very high particle-energy conditions, and raising the bound electrons within atoms and molecules to higher energy states in their orbits around the atomic nuclei (excitation). The energy of these latter excited-state electron orbits is of course quant ized, with very specific orbital energy states. Thus, subsequent relaxation of the bound elec tron from its raised or excited energy state to a lower energy state releases a photon of a very specific energy or wavelength, unique to and characteristic of that atomic species. The gas particles in the atmosphere are treated as thermal, characterized by a temperature. However, the upper atmosphere must be considered in terms of very different temperatures: electron gas ion gas and three neutral gas temperatures. The latter define the degrees of freedom for a molecule: translational or kinetic energy, vibrational energy (e.g., along the axis of a diatomic molecule), and rotational energy (e.g., rotation of the axis of a diatomic molecule). The latter two are readily seen in infrared (IR) aurora seen only from space, and are key to auroral atmospheric thermal balance. The wide range of electron orbit states (i.e., quantized energy) for a given atom, and the range of atomic species in the atmosphere, yield a rich range of emission lines. Those [ 33 ]

[ 34 ]

Optical Aurora

wavelengths that reach the ground (visible wavelength range), and are sufficiently strong and separated from other emission lines, have long been studied with ground-based sensors for information about aurora.

3.1.1

Altitude Profiles

Particles penetrate in the atmosphere to a depth dependent on the particle energy. Even for a stream of single energy (mono-energetic) particles, this penetration is not a single height. The collision process involves random-walk energy loss, so each identical precipitating particle can stop at a different altitude within a range of altitudes. The altitude of greatest particle absorption is called the altitude of unity optical depth. Most particles are stopped within a and and ~60 km for atomic oxygen) neutral scale height (at F-region heights ~30 km for of the unity optical depth altitude. In a realistic case, the precipitating particles will have an energy spread, broadening the altitude range for particle absorption. Figure 3.1a illustrates the altitude profile of production rate of ionization, and steady state electron densities resulting from electron precipitation fluxes with a Maxwellian energy dis tribution, characteristic of continuous (diffuse) aurora, with isotropic incidence and energy (Strickland et al. 1983). The profiles are for four values of Maxwellian flux of characteristic energy. Figure 3.1b shows the same altitude profile, but for a Gaussian energy distribution, characteristic of discrete aurora (Strickland et al. 1983), and six values of Gaus sian maximum energy. The peak production rates are comparable, as expected for the same input energy flux, but about a neutral scale height lower in altitude. This discrepancy is because the Maxwellian energy distribution extends over a broader energy range than the Gaussian, and thus has more energetic particles, which in turn penetrate more deeply into the atmosphere. Figure 3.2 shows the same altitude profile again, but for proton precipitation with a Maxwellian energy distribution, characteristic of proton aurora (Jasperse and Basu 1982) and five characteristic energies. Figure 3.3 shows the altitude of maximum ion production

3.1 Introduction

[ 35 ]

rate vs. Maxwellian-characteristic energy, for ionization produced by electron (left) and pro ton (right) precipitation of particles with Maxwellian energy distributions and at isotropic incidence (Jursa 1985). For a comprehensive treatment see Strickland et al. (1983).

[ 36 ]

Optical Aurora

3.1 Introduction

[ 37 ]

Particle energies: The particle energies we refer to are much above the thermal energies normally found in the upper atmosphere, which are typically in the range tenths of an eV. Note that an electron with a representative temperature of 1000 K has a kinetic energy of 0.08617 eV. With reference to boundaries, as discussed in Chapter 2, particles precipitating from various magnetospheric regions are drawn from electron populations of typical energies of 0.1 keV in the magnetosheath, ~1 – l0 keV in the plasma sheet, and exceeding many tens of keV in the trapped radiation “Van Allen-” belt. These electrons coexist in these regions “large scale electrical neutrality” with protons, of energy roughly five times that of the electrons. Neutral pressure level: Although it is common to present work published in units of readily measured geo-potential altitude, i.e., number of km above the earth’s surface, it is most physically significant to think in terms of neutral pressure level. The stopping altitude for a particle is determined by its cross-section for collisions with a particle it passes, times the cumulative total number of particles it has passed, integrated over its trajectory path, which is dependent on its pitch angle. For a realistic multi-constituent atmosphere this is also summed over all constituents. The neutral pressure at its altitude of one optical depth directly defines this integral. The neutral pressure is that pressure which supports the net weight of all the gas above it. The difference between pressure level and geo-potential altitude is small at E-region heights, but by an altitude of 300 km in the F-region can be 15 km for a 1000 K difference in exospheric temperature.

3.1.2

Useful Relationships

Qualitatively, it is already apparent that F-region emissions are characteristic of soft (~0.1 keV) electrons, such as from the magnetosheath. F-region emissions are typical of dayside cusp au rora and polar cap aurora, particularly sun-aligned arcs. E-region emissions are, in contrast, characteristic of hard, that is, more energetic (~1– 10 keV) electrons. Partitioning of precipitating particle energies: Experimental data show that fast elec trons and protons produce about one electron-ion pair per 36 eV of their initial energy. Because the ionization potential of the atmospheric atoms and molecules is on average about 15 eV, over half of the energy then goes into the kinetic energy of the electron, which subsequently thermalizes. Interestingly, this is in sharp contrast to photoionization (vs. particle impact ionization here), where the constraint of conservation of momentum where m is mass and v is velocity) requires that the photoelectron carry away the overwhelming majority of the energy (mass times square of velocity). Thus, one can relate particle/energy fluxes to ionization production rates, and with some chemical recombination rate information, into asymptotic values — were steady-state electron density profiles to ever be achieved. Chemical recombination rates for molecular ions are long or comparable to discrete auroral time scales, but short relative to sun-aligned arc and dayside auroral time scales, a fact we shall make use of in later chapters. Rules of thumb for production rates: There is a particularly interesting and valuable property of absorption of electron energy by molecular nitrogen — leading to its Efirst region emission lines (see section 3.3). The probability of the ionization of the negative bands (Rees and Roble 1986) leading to excitation of the 391.4 and 427.8 nm emission bands, compared to the probability of all possible ionization processes, is nearly independent of energy between 0.5 – 20keV. Note that this represents E-region absorp tion and emission. Below 0.5 keV electron energy, different physics applies to absorp tion in the F-region, as will be discussed in chapters 5 and 6. For electron energies

Optical Aurora

[ 38 ]

above 0.5 keV, useful semiquantitative estimates can be made for properties of aurora in general, and the more intense sun-aligned arcs, but not weak sun-aligned arcs or cusp dayside aurora. A given electron energy flux above 0.5 keV will produce a given intensity of 391.4 and 427.8 nm emission, a given maximum (E-region) electron density (steady state, meaning for a flux steady or slowly varying over ~a minute), and a given E-layer critical frequency (for an HF of ordinary polarization mode). Note that

with in Hz and in electrons Observation of one serves as surrog ate measurement of the others. Note that the ratio of the auroral intensities of 427.8 to 391.4 nm emission is 0.3 (Vallance Jones 1974). Historically 391.4 nm emission was commonly observed (it is ~3 times brighter than 427.8 nm), but atmospheric scatter ing decreases rapidly with increasing optical wavelength, and 427.8 nm has become the standard now, especially for cusp data where achieving minimum scattered sunlight is critically important. Interrelationships of particle, optical, and ionospheric parameters for the continuous (diffuse) aurora and auroral E-layer are quantitatively as follows: energy flux of is equivalent to an auroral intensity of of 391.4 nm optical emission, or of 427.8 nm optical emission, of 3.25MHz, There is direct linear proportionality between , I (391.4), I (427.8), (see section 6.4.1) and thus where = I [391.4 nm] = = I [427.8 nm] = = =

Energy flux of precipitating auroral electrons

Column-integrated intensity of the

emission at 391.4 nm 0.3 intensity of 391.4 nm Fourth power of auroral E-layer critical frequency for the ordinary ray Electron density of the auroral E-layer at its altitude of maximum density

3.1.3 Auroral Emissions from Dayside Aurora and Sun-Aligned Arcs There are many excellent textbooks dealing comprehensively with the full range of auroral spectroscopy and its nature. The interested reader is referred for example to, Rees (1989), Vallance Jones (1974), Omholt (1971), and Chamberlain (1961). Here we confine ourselves to a discussion of the spectroscopic features most relevant to the topic of this book. Most important is atomic oxygen and its emissions, as the dominant F-region constituent, and dominant-tosole absorber of the particle fluxes that excite dayside aurora and weak sun-aligned arcs, the latter being present in the polar cap about half the time. For more intense sun-aligned arcs (> 1 kR), present in the polar cap a couple of percent of the time, harder electron fluxes are also present. These produce molecular E-region auroral emissions underlying the F-region emissions, although in the case of double (or multiple) arcs, do so only in the primary arc(s). Sections 3.1.1, and 3.1.2, lay the groundwork for our purposes in this book, and again the interested reader is referred to the bibliography for further discussion of molecular emission features in aurora. The atomic oxygen emissions most commonly observed from the ground are called “for bidden lines”. This simply means that they do not immediately (within relax back to their lower energy states as do “permitted lines”, but remain in their excited state for

3.2 Forbidden Atomic Lines in the Auroral Emissions

[ 39 ]

far longer times (~seconds). The two most commonly observed F-region lines are at 630.0 nm and 557.7 nm. The 630.0 nm state has a natural lifetime of about 110 s (see section 3.2), the statistically most probable time between excitation and natural relaxation by spontaneous emission of a photon. However, in a realistic atmosphere it is likely to be quenched, or de activated, by a collision with the nitrogen molecule well before that length of time (557.7 nm emission is so fast that this is not an issue for it). Thus, many excited atoms never emit a photon, but instead pass on their energy to a molecular nitrogen largely through vibrational excitation. The observed 630.0 nm intensity underestimates the excitation rate if quenching is not allowed for, and application to modelling of thermospheric thermal balance is also dis torted without allowance for quenching. In practice, for strong sun-aligned arcs, a reasonable estimate of lifetimes would be ~3/4 minute for 630.0 nm and 3/4 s for 557.7 nm; for dayside cusp aurora the 630.0 nm lifetime is a little longer.

3.1.4 Atomic vs. Molecular Emission The auroral spectrum consists of a great number of spectral lines and bands from constituents in the upper atmosphere, excited by precipitating auroral particles, with energies well above thermic energy. In this book we will concentrate on the visible region of the optical spectrum. Some of the main differences between day-and night-time aurora are shown in Table 3.1. As pointed out in Chapter 2, the energy of the particles bombarding the cusp and polar cap near magnitude noon are on average more than one order of magnitude less than those producing the night-time auroras. Due to the low energy, the dayside cusp and polar cap auroral emissions during quiet conditions emanate above 150 km, and with peak emissions, well above 200 km. Since the dominating emissions in dayside cusp and polar cap auroras are from above 200 km where the main constituents are atoms, furthermore, the cusp and cap aurora are F-region phenomena. They are produced in an altitude where collision frequency is relatively low. Consequently, atomic emissions, even from long-lived metastable atmospheric species, dominate the daytime cusp and polar cap auroral spectrum. Our discussion, then, will focus on auroral emissions from atomic species, i.e., the mech anisms accounting for the primary observations presented in this book. As the dominating emissions discussed are the (OI) 630.0 nm and 557.7 nm lines, we will first review the forbidden atomic lines. (A roman numeral following an atomic symbol refers to the ionization state, with I designating unionized, II designating singly ionized, etc.)

3.2

Forbidden Atomic Lines in the Auroral Emissions

Precipitating electrons and protons absorbed in the F-region are the main source of the dayside and weak polar cap auroras. As the dominant emissions observed and discussed here for these cases are the (OI) 630.0 and 557.7 nm lines, we will only review the forbidden atomic lines. Production of atomic auroral emission can be considered as the following three-step process. 1. As a result of the collision, the atmospheric constituents absorb energy; i.e., kinetic energy is deposited in the constituent. 2. Due to physical and chemical reactions within the species, bound electrons in the atom experience an orbital change, i.e., there is an impact excitation. 3. After a statistically calculable delay, the particle relaxes to a lower energy state, emitting a photon which carries off a discrete quanta of energy.

[ 40 ]

Optical Aurora

This process can be symbolized by the following two equations.

followed by radiation

Here, X is the atmospheric atom being excited, and (electron and ion) are the energetic auroral particles bombarding the atmosphere, while and is the energy of these particles after collisions. The shows that the particle has been excited. The dominant ions in our case will be protons The X in our case is most likely atomic oxygen, but could also be nitrogen. Equation 3.2 shows that X, by sending out photons, relaxes to a lower energy or ground state. The primary charged auroral particles gradually, through each collision, loose energy on their way into the dense atmosphere. In addition to excitation of atmospheric constituents, the probability of ionization is also large. In a standard atmosphere, the maximum penetra tion depth of a 1 keV electron is Via collisions with the atmospheric constituents, secondary electrons, energetic enough to both excite and ionize, are also produced. The most intense emission in dayside and polar cap aurora, during quiet conditions, is in the red line at 630.0 nm. This emission is produced when the state in atomic oxygen relaxes to the ground state. Fine structures of the electron shells in the ground state allow 636.4 nm emissions as well (see Figure 3.4). The forbidden oxygen line at 557.7 nm and the red doublet at 630 and 636.4 nm can be excited by the following process (where has less energy than ):

followed by

or

For the red doublet, we have

followed by

3.2 Forbidden Atomic Lines in the Auroral Emissions

[ 41 ]

where and have total electron spin of while has spin of (Vallance Jones 1974). Thus, transitions from (OI) yield prominent emissions in the midday aurora. The 630 nm (OI) emission can also be excited by other processes. The brightest visible feature of the aurora, the green line at 557.7nm, is thus due to the transition of an electron from the excited state to the state of atomic oxygen. Another commonly observed line, particularly in the polar cusp and cap, is the red line at 630 nm as the state relaxes to the ground state. If the electron gives up its full 4 eV in a single step, instead of two nominal 2 eV steps and then it emits a photon at 297.2 nm, that is, about half the wavelength of the emissions from the smaller energy steps (Figure 3.4). This line is not readily observed and is not further discussed here. Under special circumstances that lead to unusually high electron temperatures in the upper atmosphere, aurora can be thermally excited. Temperatures of the plasma can be so high that electrons in the high-energy tail of the thermal energy distribution have sufficient energy to excite optical aurora. Only the lowest energy states can be excited (e.g., the 630.0 nm redline emission) from the lowest excited state of atomic oxygen, the state. For electron temperatures in the upper atmosphere above ~3 000 K, these emissions can be seen. The physical process leading to hot electron gas and 630.0 nm emission leads to stable conditions for long periods of time just equatorward of the auroral zone. These stable red auroras are in fact stable auroral red (SAR) arcs. The auroral zone is the zone within which most intense (> keV particle excited) auroral arcs are observed. We suggest that this thermal excitation can occur for cusp and polar cap aurora, as well as under special thermal balance conditions for the ambient electron gas, although this suggestion has not yet been confirmed. Thermal excitation, important to thermal balance calculations, can be extremely important to (non-triangulation) mapping of processes. Thermal excitation comes from altitudes in the 300 – 500 km range, in contrast to particle excitation, which comes mainly from altitudes in the 200 – 300 km range. The probability of the excitation in Equations 3.3 and 3.4 is very small for energies but it is larger for electrons with energies <500 eV. Thus, the auroral green line, and particu larly the auroral red line, are created by auroral soft particles. Due to the relatively long lifetime of the auroral red line, this red aurora often forms diffuse patterns instead of discrete arcs (cf. chapter 4). During disturbed conditions, the whole sky may be covered with this “blood-red” aurora. It may look like a large fire for people who

[ 42 ]

Optical Aurora

are not used to aurora. History clearly shows that blood-red auroras have been mistaken for big-city fires several times (cf. Chapter 1). Using Bohr’s atom-model (cf. Figure 3.5), we can calculate the wavelength of the auroral emission from the following equation,

where is Planck’s constant, in J . s (Joule second), and is the velocity of light in If and is the energy in eV for two different electron orbits, then Equation 3.5 gives the wavelength in nm for the photon emitted when the electron falls from the higher-to the and Equation 3.5 can be lower-energy orbit. If we insert the quantitative values for written,

Both excitation and ionization often occur simultaneously, which can be written as

where is a secondary electron. Notice that the ion is created in an excited state. The energy difference between and and and in average, is 36 eV. If secondary electrons are released with, say, 20 eV, they have the ability to perform appreciable additional excitations.

3.3 Permitted Atomic Lines

[ 43 ]

Process 3.7 is followed by radiation, as was the case in Equation 3.2

The emissions generated in Equations 3.3 and 3.4 arc called forbidden transitions because the photons are not emitted spontaneously, i.e. within to The explanation is as follows. Transitions in excited atomic oxygen are forbidden by electric-dipole radiation. How ever, the probability for transitions in metastable levels of (OI) is large in an electric-quadruple and magnetic dipole, if the collision frequency is small (more discussion in Section 3.6).

3.3

Permitted Atomic Lines

There are also great numbers of permitted oxygen and nitrogen lines from higher excited states. Most of the permitted transitions observed in O and N have excitation potentials of and 20 – 30 eV above the ground state of the neutral particles. about 10 – 13 eV, and in The sodium doublet at 589.0 and 589.6 nm is occasionally observed in auroras. Also, has been observed on rare occasions. the helium line at 587.6 nm

3.4

Thermal Excitation in General

In addition to excitation by precipitating electrons, auroral excitation can also result from a thermal distribution of electrons. 630.0 nm: If the red arc is sufficiently high in altitude that it is insulated from the lower ionosphere/atmosphere to which it must ultimately cool by downward heat conduction, it may heat to high enough temperatures (>3000 K) that it produces thermal red-line emission. Let us discuss, then, the physical mechanism whereby 630-nm (OI) emission is excited by the process called thermal excitation. The ambient electron gas will have a thermal or Maxwellian distribution of energies, with a population decreasing exponentially with increasing energy. The fraction of the electron population above a fixed energy is obviously strongly dependent on the electron temperature. For electron temperatures much above 3 000 K, there may be enough electrons in the high-energy tail of the thermal distribution to excite detectable emission from O atoms. This excitation follows because the level is only 1.96 eV above the ground state, as shown in Figure 3.4. Thus, thermal electrons can produce excitation of 630 nm (OI) auroras. Stable auroral red arcs are formed on the equatorward edge of the auroral oval by this excitation process, provided that increased electron-gas heating from above, and/or decreased electron-gas cooling by the ionosphere below, allow the electron temperatures in the F-region to rise to well over 3000 – 4000 K (Kozyra, Valladares, Carlson, Buonsanto and Slater 1990). In short, thermal excitation of 630.0 nm results from electron impact, as does precipitating electron excitation. However, the electrons exceeding the 1.96 eV excitation threshold for this 630.0 nm line originate from a different source (tail of a hot thermal-velocity distribution, vs. secondary electrons from auroral acceleration to a very non-thermal energy and velocity distribution). This thermal excitation mechanism can only operate on emission lines for which the excitation energy threshold is low (e.g., for atomic oxygen the 630.0 nm threshold is very near 2 eV, and the 557.7 nm threshold is very near 4 eV).

[ 44 ]

3.5

Optical Aurora

Atmospheric Temperatures and Auroral Emissions

Other auroral emissions also depend on atmospheric gas temperatures. Atmospheric temperatures: For an unionized atomic gas, the temperature is a single num ber used to characterize the energy and velocity distribution of the particle population, as long as it is a close approximation to a Maxwellian distribution (cf. Figure 3.3) over the energy range for which the approximation needs to hold. The energy refers to trans lational energy; that is, it is based on the velocity of the particle. Note that for a partially ionized gas there are at least three important gas temperatures in the presence of external heating: the electron gas, ion gas, and neutral gas temperature, all of which are equal only after a sufficiently large relaxation time for thermal equillibrium to resume after turn-off of the external heat input. For a diatomic gas, two other temperatures come into play, because the pairs of atoms have two more degrees of freedom. They can vibrate along their axis, as a pair of weights on a sturdy spring. And, they can rotate about an axis. Thus, they can have trans lational, vibrational, and rotational energy. If a population of particle energies within each of these types of energy has a Maxwellian distribution, each can be characterized also by a single number, the vibrational and the rotational temperature. If all three temperatures are the same, the diatomic gas is in thermal equilibrium. For molecular nitrogen and oxygen in active aurora, the gases are not in thermal equilibrium, and take time to re-thermalize after auroral excitation. Laboratory experiments have shown that, contrary to initial theory, energy is exchanged among each of these three forms of diatomic energy. For ground-based observation of emissions in the visible spectrum, these collisions are of direct relevance only because they reduce the number of 630.0 nm photons seen from the ground. However, they are of considerable significance to thermal balance issues in the upper atmosphere. Vibrational heating of molecular nitrogen takes away most of the energy that would otherwise be stored in the 2 – 4eV energy range by atomic oxygen (note comment on strong deactivation of the first excited state of atomic oxygen), for altitudes much below ~225 km. As altitude decreases, the thermosphere becomes increasingly rich in molecular constituents. On average, when an atomic oxygen atom is excited to its first excited state, it takes about 2 minutes for the energy to be released again, by the escape of a 630.0 nm photon. When the time between collisions of an atomic oxygen atom with a molecular nitrogen molecule becomes much smaller than 2 minutes, a deactivating collision steals the energy instead. This means that the energygoes into the local molecular gas (“thermospheric heating”) vs. escaping the atmosphere as visible red light. The thermosphere cools by release of IR emissions, and by downward heat conduction. IR and UV Auroral Emissions: Molecules also can store energy in the form of vibrational energy (along the molecular axis) and/or rotational energy (along a transverse axis). Be cause of the close spacing of vibrational-energy levels, auroral emissions from molecules have bandwidths of nanometers, whereas atomic-line bandwidths are on the order of 0.1 nm or less. These emissions are of importance for many physical and practical mat ters, but will not be treated in detail here, as we limit discussion to more readily observ able emissions, and UV and IR emissions must be made from space. The current state of understanding and modeling of UV emissions in summarized very well in Strickland et al. (1983).

3.5 Atmospheric Temperatures and Auroral Emissions

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[ 46 ]

3.6

Optical Aurora

The Hydrogen Lines — Proton Auroras

Some weak, but important, hydrogen lines, first discovered by Lars Vegard (1880 – 1963) in 1939, exist in the auroral spectrum (Vegard 1955). The emissions at 656.3 nm and at 486.1 nm (often called the Balmer lines) result from excited hydrogen atoms that are produced when energetic protons bombard the atmosphere. The Balmer lines are the only emissions from hydrogen in the visible range. The excitation mechanism, illustrated in Figure 3.7, can be written

followed by the auroral emission

When a photon is emitted, it has a Doppler displacement that depends on the velocity of the emitting hydrogen atom and the angle between the velocity vector and the direction of the photon. The first realistic estimates of the auroral-particle energies (made before the space age) were based on such Doppler profiles. The hydrogen atom again collides:

and process 3.9 can start again. emission is obtained when an electron jumps from orbital 3 to orbital 2, which is is equivalent to an energy difference of 1.89 eV. Using Equation 3.6 we get emitted by an electron jump from orbit 2 to 4, equivalent to an energy difference of (3.4– 0.85) eV = 2,55 eV or The hydrogen atom formed in process 3.9 has almost the same velocity and direction as the original proton. The fast atom, after process 3.10, collides with an atmospheric particle (X) and may be reionized (cf. Equation 3.11) or excited. The latter is more likely to occur at low particle energies. An average particle goes through a great number of processes of electron capture and loss before it has lost its energy and is brought to rest in the upper atmosphere. As a fast particle (proton/atom) penetrates the atmosphere and slows down, it spends a large fraction of its time as a neutral atom. Therefore, its probability for emission of light increases until its energy drops below the excitation level. Figure 3.7 shows the photon and per proton as a function of residual range. When a photon is emitted, emission in is has a Doppler displacement that depends on the velocity of the emitting hydrogen atom and the angle between the velocity vector and the direction of the photon (Omholt 1971). The protons arrive in helical paths (Figure 3.6), spiralling around the magnetic lines of force with a given pitch angle. With a known distribution of an ensemble of protons of initial energies and pitch angles, we can estimate the total emission from hydrogen as a function of height, as well as the Doppler profile of the hydrogen light for any direction of observation. These computations are not difficult in principle, but somewhat cumbersome in practice (Omholt 1971). As a result of charge exchange, the proton auroras are more defocussed (i.e., diffuse) than the incident proton precipitation. These auroras occur in an oval displaced duskward of the electron oval (Figure 3.8) and have a response time different from that of electron auroras. Hydrogen optical emissions can be particularly valuable in the study of polar cap arcs as well as cusp aurora (and magnetopsheric configurations more generally), because their presence is of significance in relating upper atmospheric emissions to source regions of incoming

3.7 Characteristics of Auroral Emissions

[ 47 ]

excitation particles. For example, hydrogen ions flowing in alongside of electrons are common in theta aurora (see Section 5.2). We suggest that hydrogen ions are present in all strong sun-aligned arcs, and absent from weak sun-aligned arcs (see Chapter 5). Confirmation or correction of this hypothesis would solidify our understanding of the fundamental relationship between these ionospheric features and their solar wind/magnetospheric origins.

3.7

Characteristics of Auroral Emissions

Not all excited states lead to optical emissions. Some are quenched before they have time to release their excitation energy in the form of photons. Those excited states with residence times greater than times for collisional deactivation (quenching) will lose their energy to collision vs. losing energy by emission of a photon. This quenching is important for calculations of particle average-energy, net-energy input, and vertical and horizontal locations of auroras. The statistical residence time of an excited state before emission is determined by the Einstein transition probability. Governed by quantum-mechanical rules, allowed or permitted transitions occur very rapidly (in times on the order of Transitions that violate these selection rules are called “forbidden transitions”. They occur, but only after a much longer shown in (Equations 3.3 and 3.4) occurs in about time; for example, the transition for requires about 110s. The latter is so slow that much below 200km, 0.8s, and that for an atom is likely to suffer a collision that will knock it out of the state before it

[ 48 ]

Optical Aurora

has a chance to emit. Thus, the 630-nm (OI, or neutral oxygen) emission is expected to peak much above 200 km, even if its excitation peaks near an altitude of 100 km. This quenching by collision with other atmospheric constituents substantially reduces the states excited. The number of 630-nm photons emitted to less than the number of 1.96 eV used by the oxygen atom in exciting the state goes into vibrationally exciting the local atmospheric gases, instead of dissipating into the atmosphere. This affects the apparent location both vertically (low altitude excitation does not lead to low altitude emission) and horizontally (a horizontal motion of, say, 1 km/s common at polar altitudes leads the emission of 630 nm to be observed about 30 to 90km downstream from where the excitation occurs, for an excited state residence time of 30 to 90s after excitation and before emission). Sun-aligned arcs typically drift dawn-dusk at ~200 m/s, and thus may optically appear dawnward (or duskward) by ~ 10km of where the incoming exciting electrons excite the state. The emission time is also a statistical distribution of many different times, averaging the times noted here, so the 630 nm arc width observed for this red-line emission will be spread wider than the thickness of the exciting electron sheet. For the 557.7 emission the lifetime is ~0.8 s, so these considerations are minimal for green-line emission. Because infrared (IR)

emissions are in effect “heat emissions”, they are seen effectively only with very special and

expensive cooled sensors, from space. Ultraviolet (UV) images can also only be seen from

space, but more easily and less expensively.

3.8

Units of Auroral Intensities

An aurora appears as a luminous cloud, having an apparent brightness. Absorption within the visible spectrum is negligible. Hence, the apparent surface brightness is proportional to the integrated emission per unit volume along the line of sight. The surface brightness is used to define the intensity of an aurora. If the surface brightness I is measured in photons per square cm per second per steradian, represents the total emission in photons per square cm per second integrated along then the line of sight. This result is defined as the intensity of an aurora. The unit adopted for is the Rayleigh (R). One rayleigh is equal to an integrated emission rate of photons per square cm per column per second. Inclusion of “column” in the units refers to the unknown height of the column above the apparent source; it is included to show that this is a volume emission, not a true surface emission. The observed intensity of a particular auroral form depends on the direction of observation. A thin auroral layer covering a large part of the sky is most intense when viewed at low elevation angles. The auroral intensity in rayleighs at a particular wavelength from an incoming electron/proton beam, with isotropic pitch-angle distributions assumed at all energies, can be evaluated as the integral over the incoming particle energy spectrum of the differential electron/proton energy spectrum, times the total number of photons P(E) for a gas X at wavelengths (e.g., for O at 630.0 nm). For protons, P(E) depends strongly on the energy spectrum, and above 20keV contributes significantly to the output light. When classifying auroral intensities, the line used for reference is the green oxygen line at 557.7 nm, which is dominant in the wavelength region near the maximum sensitivity of the human eye. Typical intensities of nightside auroral arcs and bands vary from one to a few tens of kilorayleighs During an active period with a bright display, auroral intensity in or near the zenith may be several hundred Thus, as one near 550 nm is the visibility threshold for the dark-adapted naked eye, the optical aurora is a relatively weak but very dynamic optical phenomenon. The large variations in intensity are closely correlated with the net downward particle energy. In order for the aurora to be visible to the eye, the particle energy input to the

3.9 Summary

[ 49 ]

atmosphere must be about one or approximately For a mediumstrong aurora, about 10 km wide and 1 000km long, approximately are needed, which is comparable to the power capacity of a large power plant. Because only about 1 percent of the particle input to the atmosphere is used to produce visible light, it is clear that there is an enormous quantity of energy deposited in the upper strata of the atmosphere during each auroral night.

3.9

Summary

The 630-nm (OI) emission, which is created by auroral soft primaries, is seen as the red au rora, and forms the diffuse background radiation in which the discrete arcs are embedded (Figure 3.9). “Blood-red” auroras are produced by low-energy (< 1 keV) electrons. An ideal ized view looking down on the polar regions at the footprint of auroral particle precipitation is illustrated in Figure 3.10, which distinguishes between two patterns of precipitation. One ap proximates a circle, whose center is displaced away from the sun by 5–10 degrees of latitude, and traces where discrete aurora are usually found. The other approximates a circle centered on the magnetic pole, and traces where diffuse or continuous aurora are usually found, as from the trapped radiation loss cone. The majority of the aurora are yellow-green, but they sometimes appear grey. They appear grey because they are so weak in intensity that they are below the color threshold of the human eye, so therefore appear to the observer to be grey. Such observations by the eye refer to low particle fluxes. Figure 3.11 shows how the ratios of the main visual emissions change relative to one another as a function of altitude. Specifically, blood-red auroras dominate in the altitude region above 200 km, whereas a magenta color predominates below approximately 100 km.

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Optical Aurora

Blood red is the (OI) emission at 630 – 636.4nm, yellow green is the (OI) emission at 557.7 nm, and magenta is a combination of and emissions near 600 nm and first-negative-band emissions in the blue end of the spectrum. The statistical residence time of an excited state before emission leads to allowed or permitted transitions to occur very rapidly (in times on the order of Transitions that violate these selection rules are called forbidden transitions. They do occur, but only after a much longer time; for example, the transitions leading to 557.7 nm emission occur in about 0.8s, and for 630.0 nm, transitions require about 110 s. The latter is so slow that much below

3.9 Summary

[ 51 ]

200 km, an atom is likely to suffer a collision that will propel it out of the state before it has a chance to emit. Thus, the 630-nm (OI, or neutral oxygen) emission is expected to peak above 200km, even if its excitation peaks near 100 km. This quenching by collision with other atmospheric constituents substantially reduces the number of 630-nm photons emitted below the number of states excited. The 1.96 eV used by the oxygen atom in exciting the state goes into vibrationally exciting the local atmospheric gases, instead of dissipating into the atmosphere (Figure 3.4). A few definitions and categorizations should help to put these ideas of auroral emission processes and rates in perspective. The photon-emission intensity

where is the density of the excited-state emitting molecules and is the Einstein coefficient. The density of excited molecules is the ration P/L: the excitation rate divided by the loss rate L must include all collisional per unit volume deactivation (or quenching) processes of the excited state by each quenching species, weighted by its rate coefficient Auroral Photography: On a clear, dark winter night, the aurora may look bright to the dark-adapted naked eye. Still, it is not easy to take good pictures of the aurora. Even for clearly visible displays, the exposure required may be measured in seconds, therefore, a tripod is essential. Cusp and most polar cap sun-aligned arcs are ten or more times weaker.

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Chapter 4

Dayside Auroral Forms and Activities The auroral luminosity in the day sector sunward of the 0600-1800 MLT terminator consists of a variety of different auroral forms and activities, reflecting the rich structure/dynamics of the dayside magnetosphere with its outer boundary regimes towards the shocked solar wind, the magnetosheath.

4.1

Introduction

The detailed dynamics of these auroral types can only be monitored by continuous observations from the ground with high-resolution instruments. A simple schematic illustration of dayside auroral forms and their association with merging (M) and lobe (L) convection cells, is shown in Figure 4.1. A major distinction is drawn between the forms/activities in the so-called cusp region and those in the mid-morning and postnoon sectors. By cusp region auroras we mean those forms representing the ionospheric footprint of “the most direct entry of magnetosheath plasma”, generally occurring in the sector ~0900 – 1500 MLT / 70–80°MLAT. The auroras in this sector are those which are most sensitive to the dynamical solar wind-magnetosphere coupling processes. This is reflected in the high sensitivity to the solar wind plasma parameters (density, bulk speed, dynamic pressure) and the intensity and orientation of the interplanetary magnetic field (IMF). Thus, we may distinguish between different configurations of the cusp region aurora, corresponding to different regimes of IMF orientation. The two basic cusp region forms observed to dominate during intervals of southward and northward IMF orientation, respectively, we call types 1 and 2. The type 1 aurora is typically located within 70°–75°MLAT, whereas the type 2 form is generally located at higher latitudes, within 75° –80°MLAT. Both these forms typically appear as a se quence of brightening events with a mean recurrence time of 5 min (sec below). For southward IMF the auroral activity is characterized by equatorward boundary intensifications (EBIs), each of which is followed by poleward moving auroral forms (PMAFs), called forms 1a in Figure 4.1. During positive (negative) IMF polarity PMAFs move westward (eastward). For northward IMF the most marked feature is the intensification occurring at the poleward boundary of the pre-existing type 2 luminosity, so-called poleward boundary intensifications (PBIs). Both EBIs and PBIs are marked by heavy curved lines in Figure 4.1. For the purpose of illustrating the auroral dynamics in the day sector, we have decided to use observations at the two oxygen lines at 630.0 and 557.7 nm. This selection is based [ 53 ]

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Dayside Auroral Forms and Activities

4.1 Introduction

[ 55 ]

on tradition and the fact that the red and the green lines are most sensitive to the precipita tion of magnetosheath-origin electrons and low-altitude electron acceleration events, respect ively (Rees and Luckey 1974, Rees and Roble 1986). Although the emission altitudes vary from case to case, typical altitudes of these emissions in the cusp region aurora are 250 — 300 and 150km, respectively. Statistically there is a minimum in the green line intensity in the cusp region. This has been called the “midday gap” aurora (Dandekar and Pike 1978), charac terized by a green line intensity below 1 kR. The red line intensity in the cusp region typically lies in the range of 2 – 10 kR. This region is characterized by high fluxes of magnetosheath origin plasma, i.e. electron precipitation fluxes at energies below ~200 eV and protons below ~3 keV (Newell and Meng 1988). The auroral forms in the pre- and postnoon sectors, outside the cusp region, called types 4 and 5 in Figure 4.1, are generally more intense in the green line, with intensities reaching 10s of kRs. produced by electrons at around ~ 1 keV. They often appear as multiple fan arcs or rayed bands. These forms have been documented by space-borne auroral imagery (Meng and Lundin 1986, Elphinstone, Hearn, Murphree, Cogger, Johnson and Vo 1993). They often cover a wide range of latitudes. The type 3 form in Figure 4.1 is a diffuse aurora, typically appearing in the form of a pulsating green line (557.7 nm) emission, but very weak in the red line at 630.0 nm. It is related to the precipitation of 1–10 keV electrons from dayside extension of the central plasma sheet. These electrons are drifting through the morning hours from the nightside before precipitating. As indicated in Figure 4.1, the type 3 aurora is basically a prenoon phenomenon. In the following sections, we illustrate the various auroral forms and activities occurring on the dayside with selected case examples, placing special emphasis on the cusp region. The technique of continuous ground-based observations is particularly useful to distinguish between temporal and spatial features of the dayside auroral precipitation. Transitions between the different auroral types and activities are observed as the optical instruments rotate with the earth, as well as in response to temporal changes in the solar wind plasma and the IMF orientation. The examples presented below are selected with the purpose of illustrating how the cusp region aurora is regulated by the IMF orientation. During relatively quiet solar conditions, the IMF affecting the earth’s magnetosphere is often oriented according to the Parker spiral, characterized by dominating components in the radial (X) and east-west (Y) directions (Hakamada and Akasofu 1982). Associated with interactions between solar wind streams of different origin in the solar atmosphere, the streamstream interactions, or solar events such as flares and coronal mass ejections, significant IMF fluctuations occur that may lead to a large IMF component in the plane transverse to the sun-earth line (Hakamada and Akasofu 1982, Tsurutani and Ho 1999). In such cases the earth’s magnetosphere is strongly forced by the IMF/solar wind via magnetic interconnec tion (reconnection). The interconnection geometry and the associated energy and momentum transfers from the solar wind to the magnetosphere-ionosphere system change strongly with the variable IMF orientation (Perrault and Akasofu 1978). The magnetosphere responds par ticularly strongly to rotations of the IMF in the plane transverse to the sun-earth direction, the GSM Y-Z plane (Farrugia, Scudder, Freeman, Janoo, Lu, Quinn, Arnoldy, Torbert, Burlaga, Ogilvie, Lepping, Lazarus, Steinberg, Gratton and Rostoker 1998c). The IMF orientation in the Y-Z plane can be represented by the polar angle with respect to and components the Z-axis, the so-called IMF clock angle The polarities of the IMF give rise to interesting pre-postnoon and inter-hemispheric asymmetries of the solar windmagnetosphere coupling, respectively. The dayside aurora and the plasma convection pattern in the cusp region are very sensitive to this IMF-regulated variability of the coupling geometry. Thus, the dayside aurora/convection configurations may be sorted according to the following eight main categories of IMF orientation:

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Dayside Auroral Forms and Activities I)

II) III)

IV) V) VI) VII) VIII) We will discuss data examples showing the cusp region auroral signatures corresponding to the various IMF orientations as specified by these eight categories. Particular emphasis will be placed on responses to southward and northward rotations of the IMF, both smooth rotations as well as sharp discontinuities. In the 8 days of observations discussed below, all eight IMF categories are represented. Ancillary data are used to place the auroral observations in the context of particle precipitation and plasma convection. The association with convection is schematically indicated in Figure 4.1. Satellite observations of particle precipitation above the aurora are used to identify the plasma sources in the outer magnetospheric boundary layers and source processes corresponding to the different forms/activities. This identification is based on the present classification of particle precipitation regimes in the dayside magnetosphere (Newell and Meng 1994). Information on the ionospheric convection ion drift) is obtained by ground-based radars and magnetometers, and satellite-borne drift meters. In several of the selected case studies reported below, the auroral observations are combined with ionospheric ion drift data obtained from ground-based radars and/or satellites in polar orbit. Information on the particle precipitation source of the aurora is available for some of the case studies, representing both IMF south and north conditions. We will mostly focus on the energy spectra of electron and proton fluxes for the purpose of identifying the source plasma. Furthermore, the energy versus latitude dispersion signatures in the proton data from the DMSP spacecraft can be used to infer important features of the mechanisms of plasma entry through the magnetopause.

4.2

Case 1: December 3, 1997

4.2.1

Auroral Observations

Figure 4.2 shows an overview of the Ny Ålesund auroral meridian scanning photomter (MSP) observations at 630.0 and 557.7nm for the interval 0600–1100 UT (~0900–1400MLT). The plot shows line of sight intensities along the magnetic meridian as a function of zenith angle and time. South is to the right and north to the left in each panel. Zenith in Ny Ålesund corresponds to 76.1°MLAT. The prcnoon interval from 0600 to ~0730 UT (I) is characterized by multiple arcs appearing in both emission lines and spanning a rather large latitude range (~75°–80°MLAT). This is the type 4 aurora in Figure 4.1. In interval II (0730–0800 UT) a strong emission band is observed in the vicinity of zenith. Interval III (0750–0845 UT; ~1100 – 1200 MLT) shows a red-dominated “midday gap” aurora, where the green line emission is almost absent. A regime of enhanced auroral emission intensity/activity extending to lower latitudes is observed during 0840 – 0900 UT (interval IV). Then the aurora became generally weaker and was displaced towards north in interval V (0900 – 0925 UT). Interval VI (0925 – 1100 UT) is characterized by the appearance of forms located alternatively to the north and south of zenith. Thus, intervals of equatorward migration of the aurora were interrupted by shorter periods of activations of higher-latitude forms. The latter are tentatively interpreted

4.2 Case 1: December 3, 1997

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Dayside Auroral Forms and Activities

as type 2 forms. The IMF orientation (not shown) was characterized by negative positive and negative or zero The activation of the high-latitude aurora in the cusp region at 0917 UT can be related to a rapid northward turning of the IMF. In this case the clock angle went from 120 to 90°. Panel c) of Figure 4.3 shows a color-coded contour plot of the red line emission observed in the interval 0830 – 0930 UT. Panels a) and b) show estimates of the interplanetary electric field applied to the dayside magnetopause, i.e., the geo-effective electric field, in the same interval, using Wind to ground signal propagation delays of 44 and 58.5 min, respectively. The estimates are based on the formula of the geo-effective electric field given by (Kan and Lee 1979). A rather weak “midday gap” aurora is observed during 0830 – 0840 UT. It was followed at 0840 UT by auroral intensification, equatorward motion of the equatorward boundary and more intense poleward moving forms (PMAFs) during the interval 0840 – 0900 UT. The average PMAF recurrence time is ~4 min. The PMAF sequence went on during 0900–0920 UT, at reduced intensity and at slightly higher latitudes. In this interval (V) the luminosity consists of two latitudinally separated branches of auroral emission. Brightenings of the northernmost branch are seen at 0907 and 0917 UT, marked type 2 in the figure. A new regime of bright lower-latitude forms was initiated at ~0927 UT. This represents the start of the interval marked VI in Figure 4.2. We now consider the possible relationship between auroral features and (or IMF orientation), applying the two propagation delays. First we note that the strong type 2 brightening events at the poleward boundary of the pre-existing cusp aurora at 0907 and 0917 UT correspond to two abrupt decreases of (58.5 min lag). Next we note that the strong type 1 auroral brightening at 0847 UT may be related to the abrupt increase in (44 min lag) at 0803 UT (Wind time). It has been suggested that the two mentioned auroral types and corresponding Wind-ground delays refer to two different coupling sites at the magnetopause, taking into account the possible effect of the strong IMF X component in the present case (Maynard et al. 2001) (see Figure 4.7 below). Labels M and N in panels a) and b) of Figure 4.3 refer to IMF-magnetosphere interactions sites in the southern (M) and northern (N) hemispheres, respectively. Figure 4.4a shows 557.7nm images of the prenoon sector discrete aurora taken at 0703, 0705, and 0710 UT. The aurora appears in the form of east-west oriented multiple arcs (type 4) straddling zenith in the ~1000MLT sector. The bright arcs at 0703 UT occurred at the time of a magnetic impulse event (see below). Figure 4.4b shows three 630.0 nm images of the midday sector aurora taken at 0821, 0841, and 0851 UT. The upper panel (0821 UT) illustrates the phenomenon of “midday gap” aurora, characterized by an emission minimum within a 2 – 3 hr long sector near noon, with stronger emissions on either side. At 0841 UT (middle panel) a more intense aurora is expanding in from the eastern (postnoon) side. At 0851 UT (bottom panel) a more intense aurora (brightening event) has appeared in the midday sector (compare Figure 4.3). Figure 4.4c shows the two-dimensional auroral expansion during 0915 – 0925 UT of the bright event (type 2) also seen in the MSP plot in Figure 4.3. The field of view of the MSP instrument in Ny Ålesund is indicated by a white arrowed line. The initial brightening in the postnoon sector was followed by a westward expansion across the 1200 MLT meridian into the prenoon sector. This motion pattern is typical for type 2 forms observed during positive IMF conditions. Figure 4.4d shows 557.7nm images of the postnoon sector aurora taken during the interval 1014 – 1018 UT, i.e., representing the postnoon multiple arcs (type 5). The arcs/arc-fragments are fanning out towards eastern (dusk) side. The interval 1014 – 1018 UT represents the intervals of equatorward expansion of the region of multiple arcs in interval VI in Figure 4.2.

4.2 Case 1: December 3, 1997

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Dayside Auroral Forms and Activities

4.2 Case 1: December 3, 1997

4.2.2

[ 61 ]

Magnetic Observations

Figure 4.5 shows X component magnetograms from the Svalbard part of the IMAGE mag netometer chain, covering the latitude range 71° – 76°MLAT. Interval I (0600 – 0740 UT) is characterized by PC 5 pulsations at all stations. A clear magnetic impulse event is seen near 0710 UT, characterized by a phase shift with latitude in the X-component. At this time multiple arcs were observed around zenith in Ny Ålesund (Figure 4.4 a). Intervals II and III (0740–0830 UT) are rather quiet, particularly the period 0755 – 0830 UT. Then a positive convection bay is observed in interval IV, during 0830 – 0905 UT, with the largest deflections at the three southernmost stations (HOR, HOP, BJN). Two deflection maxima occurred at ~0850 and ~0902 UT, centered at the latitude of Hornsund (74°MLAT). These events correspond to two clear equatorward excursions of the cusp aurora (Figure 4.3). The convection bay in interval IV corresponds to the auroral intensification and equatorward expansion documented in Figures 4.3 and 4.4b.

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Dayside Auroral Forms and Activities

4.2 Case 1: December 3, 1997

4.2.3

[ 63 ]

Case Review

The IMF condition during this case is characterized by a positive IMF component and negative or zero IMF The component was strongly negative during most of the period. Figure 4.6 shows a schematic summary plot of the auroral observations. Several transitions between auroral types/activities were observed as the ground station rotated with the Earth from prenoon to postnoon hours. The first transition (at ~0720 UT/1020 MLT) is between the mid-morning multiple arcs (interval I; type 4) and the “near-cusp” aurora (interval II). Recent studies indicate that the mid-morning arcs are related to boundary plasma sheet (BPS) precipitation which is connected with a filamented mixing region (magnetosheath and magnetospheric plasmas) in a flank boundary layer, and which furthermore represents current filaments within the region 1 current system (Farrugia, Sandholt, Maynard, Burke, Scudder, Ober, Moen and Russell 2000) (see also Case 2). The entry into the “midday gap” aurora (interval III) occurred at ~0750 UT (1050 MLT). The “midday gap” aurora (1050–1140 MLT), presumably corresponding to the particle pre cipitation category “cusp proper” (Lundin, Woch and Yamauchi 1991), is in the literature characterized by the absence of green line emission (Dandekar and Pike 1978). In the present case some temporal/spatial structure of the red line emission (similar to PMAFs) was ob served even in this region. The location of the “midday gap” aurora on the prenoon side (1050-1140 MLT) during positive IMF conditions is consistent with the statistical location

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Dayside Auroral Forms and Activities

of the convection throat (merging gap) (see Weimer 1995) (Figure 4.75). From 0830 UT/1130MLT onwards (interval IV) the “midday gap” was gradually replaced by stronger auroral forms moving westward and poleward (PMAFs). The auroral equatorward boundary moved to lower latitudes, accompanied by an enhanced DPY convection current (see Figure 4.5. The auroral activities observed during the intervals 0847 – 0902 UT and 0902 – 0917 UT may be explained by IMF-magnetospherc interactions with the same solar wind segment occurring at different times in the southern and northern hemispheres, respectively, as indicated in Figure 4.7. According to this interpretation, the southern hemisphere interaction (marked M) gives rise to enhanced merging (reconnection) and a sequence of PMAFs during 0847– 0902 UT, whereas the interaction in the vicinity of the northern cusp (marked N) excites the characteristic auroral bifurcation events during 0902 – 0917 UT, consisting of type 1 and 2 activations (see Figure 4.3). The higher-latitude (type 2) aurora is interpreted in terms of lobe reconnection, taking place during the more northerly IMF orientation IMF category VI) recorded by Wind at ~0808 UT and during 0818 – 0825 UT. Thus, the two brightening events observed in the poleward branch of auroral emission during 0907 – 0910 UT and 0917 – 0925 UT belong to our class type 2 aurora (Figure 4.1). They are associated with dips in (58 min). A possible IMF-magnetosphere interaction geometry for bifurcation events associated with northern hemisphere coupling during positive IMF conditions is indicated in Figure 2.17. Some of the equatorward boundary intensifications in the type 1 aurora seen in Figure 4.3, occurring at frequencies in the PC 4 range, seem to be directly driven by IMF fluctuations, which give rise to corresponding fluctuations in Intervals with such auroral “pulsations” are: 0847 – 0855, 0905 – 0915, and 0920 – 0930 UT. The strong auroral intensification near zenith at 0927 UT may correspond to the southward turning recorded by Wind at 0844 UT, assuming southern hemisphere merging at a time delay of 44 min.

4.2 Case 1: December 3, 1997

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Schematic illustrations of the auroral conditions in intervals III, IV, and V are shown in Figure 4.8. The upper and middle panels indicate the “midday gap” aurora and cusp intensification/expansion with associated PMAFs, respectively. Interval VI (0930 – 1130 UT) is characterized by postnoon fan arcs (type 5) and the occasional appearance of westward (sunward) expanding forms (type 2) at higher latitudes. Finally, we would like to point out the following major observational features: 1) the location of the “midday gap” aurora in the prenoon sector during the prevailing positive /negative conditions, 2) the close association between intensification and equatorward/westward expansions of type 1 auroral forms and enhanced magnetic deflection of the type DPY (Friis-Christensen and Wilhjelm 1975) during 0847–0905 UT (The positive X-component magnetic deflection corresponds to an eastward-directed ionospheric Hall current/westward convection.), which is attributed to magnetic merging in the southern hemi sphere, 3) the activation of higher – latitude type 2 forms, accompanied by southwestward con vection (rocket data not shown here), is initiated in the postnoon sector and expands westward across the 1200 MLT meridian, and may be accompanied by lobe cell convection (Sandholt, Farrugia, Moen, Cowley and Lybekk 1998b, Sandholt, Farrugia, Cowley, Lester, Moen, Ly bekk and Trondsen 1999b) (see also case 7), and 4) adjacent auroral forms may be related to plasma sources in two different hemispheres, as indicated in Figure 4.7.

4.3 Case 2: November 30, 1997 4.3.1

Solar Wind and IMF Observations

Figure 4.9a shows solar wind proton and magnetic field observations for the interval 0300 – 0900 UT on Nov. 30, 1997. Wind was at an average position of (196, -3, 27) (GSE coordinates) during this time, i.e. very close to the sun-earth line and thus ideally placed to monitor the solar wind which affects the magnetosphere. Shown from top to bottom are the proton density, temperature, bulk speed, dynamic pressure, the total field intensity and its GSM components and the IMF clock angle. Six abrupt changes in the plasma and/or magnetic field conditions are marked by vertical lines. Line A marks the abrupt northward turning of the IMF at 0530 UT, leading to a 20 min period of very small IMF clock angles. Line B marks the following southward turning. Line C marks a strong enhancement in plasma density, dynamic pressure, and the field intensity recorded at 0713 UT. Line D marks the IMF northward turning at 0750 UT. Lines E and F mark density/pressure drops at ~0810 and 0820 UT. Figure 4.9b shows proton and magnetic field observations for the interval 0900 – 1100 UT. The magnetic field is characterized by essentially two orientations, separated by a clear and sharp discontinuity at 1002 UT. Before this, the IMF is characterized by strong northward and sunward fields. The clock angle is less than 40 degrees for 2 hours. After the discontinuity, the field points south (clock angle fluctuating around 140°) for 30 min, and then rotates slowly to a more westward orientation. At the discontinuity, the north-south component of the field changes from 2.4 nT to -4.2 nT in 9 s; decreases but remains positive; while hardly changes at all. A slight (less than 10%) decrease in dynamic pressure also occurs, which seems change and which in this resolution takes about 3 min to complete, i.e., to follow the is small and not impulsive. Minor increases of the clock angle before the major southward turning were observed at 0942 and 0955 UT. The delay time for the IMF seen at Wind to affect the ionosphere may be calculated by assuming that the propagation is radial and at the average convection speed of the wind (420 km/s). For an average dynamic pressure of 2.8 nPa, the magnetopause is at 9.6 We then use gas dynamics to obtain the time the IMF front takes to cross the subsolar

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magnetosheath (5 min). The total delay from Wind to the subsolar magnetopause is then 52 min, to which must be added a couple of min for information to be communicated from here to the ionosphere. We shall work below with a delay of 54 min for the interval after 0900 UT. The effect of the sharp southward turning at 1002 UT is thus expected to affect the ionosphere at around 1056 UT.

4.3.2

Auroral Observations

Figure 4.10 shows a color-coded contour plot of the MSP observations from Ny Ålesund during the interval 0600 – 0650 UT. The red and green lines are shown in the upper and lower panels, respectively. We note the three latitudinally separated forms labelled A, B, and C. The green line intensity is weak in form A, intermediate in form B, and strong in form C. Auroral form C is characterized by strong intensity modulations. Nine brightenings occurred during the 50 min interval shown, implying a mean recurrence period of 5.5 min. A similar, but weaker modulation is observed in forms A and B. This aurora belongs to the category of the midmorning multiple arcs which were labeled type 4 in Figure 4.1. The magnetospheric plasma source of these forms will be discussed later, when the ground observations are combined with plasma observations from spacecraft Polar. Figure 4.11 shows all-sky images of the latitudinally separated mid-morning fan arcs ob served during the interval 0630–0640 UT (~1000 MLT). We note the brightening of the south ernmost arc (called C in Figure 4.10) at 0634 UT. This is one in a sequence of brightenings (Figure 4.10). The magnetic meridian is indicated in the images by the dashed line marked MN (magnetic north). Four latitudinally separated auroral forms can be seen within these images. Figure 4.12, representing the noon sector (1100 – 1300 MLT), shows the presence of two latitudinally separated auroral forms, the red line dominated cusp aurora north of zenith, and the pulsating green line emission (type 3) south of zenith. In the latter region the red line aurora is very weak. The abrupt intensification of both the red line emission in the cusp aurora north of zenith (see upper panel) and a strong green line enhancement mainly south of zenith (see lower panel) occurs at~0810 UT. This corresponds to the rapid enhancement of the solar wind density (35– /dynamic pressure recorded by WIND at 0713 UT (Figure 4.9a). This density/pressure change was followed by an intense type 2 cusp emission, particularly during the interval 0830 to 0850 UT, corresponding to strongly northward IMF orientation. During the interval 0900 – 1000 UT, when the solar wind density drops back to a more normal value (10 the cusp aurora is much weaker and may be referred to as a “midday gap” aurora (Dandekar and Pike 1978). We also notice the type 3 green line auroral brightenings at ~0900 – 0905 and ~0912 – 15 UT, corresponding to the two step drop in solar wind dynamic pressure recorded by Wind at 0808 and 0820 UT (events E and F in Figure 4.9a). The ground magnetic signature is reported below. Color-coded contour plots of the MSP data in Figure 4.13 show the details of the postnoon auroral activity observed from Ny Ålesund in response to the southward turning of the IMF recorded by WIND at 1002 UT. The auroral intensity is shown as a function of zenith angle (vertical axis) and time for the interval 1050–1150 UT. The first major brightening occurred north of zenith at 1100 UT, and was followed by more intense brightenings at 1102 and 1108 UT, both at ~ 15° south of zenith. These forms expanded poleward during the next 5 min before the activation of a higher-latitude form took place in the late phase of the events. The auroral activity was reduced between 1115 and 1122 UT. Then a new brightening sequence is observed during the period 1122 – 1150 UT. Major brightenings were often followed by poleward moving forms, which faded well north of zenith. Seven such events are observed within 1122–1150 UT, implying a mean recurrence time of 4 min.

4.3 Case 2: November 30, 1997

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Figures 4.14a, b, and c show all-sky image sequences of the 630.0 nm aurora, representing the intervals before and after the main southward turning of the IMF, i.e. 1039–1054 UT, 1056 – 1106 UT and 1100 – 1117 UT, respectively. The aurora in Figures (a) and (b) is projec ted to an earth-centered sphere at 250 km with the grid representing geographic coordinates. The latitude circles at 70° and 80° as well as meridian lines separated by 10° are shown. Svalbard is in the center of the image, with the northeastern part of Greenland to the left. The circle represents the boundary of the camera field of view. In the first sequence (1039–1054 UT) the strong red-line aurora is limited to the region

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between the Greenland east coast and Svalbard, i.e., within 1130 – 1330MLT. A minor auroral brightening with associated expansion is seen in the last two images, representing 1051 and 1054 UT. The second sequence (1056–1106 UT) includes the first major expansion event, charac terized by brightening, equatorward shift, and eastward expansion. The eastern boundary of the major cusp region form reached the Ny Ålesund meridian (~1430 MLT) at 1102 UT and expanded further eastward during the next few minutes. Thus, the auroral form entered the field of view of the Ny Ålesund MSP, as evidenced by the observations in Figure 4.13a. The auroral images reveal that the successive auroral brightenings seen in the Ny Ålesund MSP records at 1102 and 1108 UT (Figure 4.13) are due to repetitive eastward expansions of a midday auroral form (type 1 cusp aurora) into the field of view of the MSP, which at this time scans approximately along the ~1430 MLT meridian. The disappearance of the most equatorward aurora in the MSP record during ~1115–1122 UT (Figure 4.13) corresponds to the westward contraction of the type 1 auroral form (see Figure 4.14c).

4.3.3

Magnetic Observations

Figure 4.15 shows Svalbard magnetograms for the interval 0800–1000 UT. We note the mag netic impulse events at 0810–0820 UT, 0904 and at 0914 UT, marked by vertical guidelines in the figure. All three events are associated with abrupt intensifications of green line auroral emission (type 3) on the equatorward side of the cusp (Figure 4.12). The first of these events is triggered by a rapid dynamic pressure enhancement (WIND record at 0713 UT), while the later two are triggered by rapid decreases of dynamic pressure (recorded by WIND at 0810 and 0820 UT). Figures 4.16 show H- and Z-component magnetograms from six cusp latitude stations on

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Greeland and Svalbard. The station locations are indicated in Figure 4.20. We note the en hancements of magnetic disturbance (DPY convection current) at 1057 and 1120 UT (marked by vertical lines) as well as the subsequent modulations in the magnetic deflection. The positive H-deflection at HOP corresponds to an eastward-directed Hall current (represent ing westward (sunward)-directed convection) located to the south of Ny Ålesund (negative Z-comp.). The negative H-component in the Greenland sector corresponds to a westward Hall current (eastward convection), which is consistent with the prevailing negative IMF polarity (Figure 4.9). The onsets of the convection bays at 1057 and 1120 UT are closely associated with major cusp auroral expansions reported above. The sequence of auroral events (poleward moving forms) observed in the interval 1122–1150 UT (see Figure 4.13) corresponds to the series of H-component modulations/pulsations observed both in the pre- and postnoon sectors. The same aurora-magnetic correspondence holds during the interval 1057–1115 UT. The recovery of the magnetic deflection to the background level at ~1115 UT corresponds to the westward contraction of the cusp aurora at this time. The locations of the magnetometer stations are indicated in Figure 4.20.

4.3.4

Combined ground and satellite observations of mid-morning multiple arcs

Figure 4.17 shows that the footprint of the Polar spacecraft was well within the field of view of the ground optical instruments in Ny Ålesund during the interval 0530 – 0710 UT. The Polar footprint was close to the ground site in Ny Ålesund at 0630 UT. This conjunction allows us to study the magnetospheric plasma sources, which are coupled via the magnetic field lines to the auroral forms detected from the ground in the mid-morning sector (see Figure 4.10). Figure 4.18 shows that during the interval 0530 – 0642 UT, the satellite (at an altitude of 6 ) crossed a boundary layer on closed field lines where magnetospheric and magnetosheath plasmas are mixed. This region contains filaments where magnetospheric electron and ion fluxes are enhanced and are associated with field-aligned current structures embedded within the large-scale Region 1 (Rl) current. High fluxes of magnetosheath electrons (<200 eV) are observed during the interval 0550 – 0642 UT. Polar entered into the magnetosphere proper (central plasma sheet) at 0642 UT. After 0642 UT the low-energy magnetosheath plasma component (electrons below 200 eV) was absent. The filaments of enhanced fluxes of mag netospheric particles on the background of the soft magnetosheath plasma correspond to the auroral fan arcs (type 4) observed from the ground (Farrugia et al. 2000). The observation geometry for this case is indicated in Figure 4.19 for the time 0630 UT, when the Polar footprint was close to the ground site in Ny Ålesund, and for 0710 UT, when the POLAR footpoint was closer to the southern boundary of the field of view (lower panel). The figure shows four latitudinally separated type 4 arcs. Three of these have been marked A, B, and C in Figures 4.10 and 4.11 The Polar observations show that these forms map to distinct plasma structures in the mixing boundary layer and corresponding filamentary struc tures in Region 1 field-aligned current (marked R1 in the figure). The particle precipitation in this regime belongs to the category boundary plasma sheet (BPS) (see (Newell, Burke, Sanc hez, Meng, Greenspan and Clauer 1991)). Temporal variations present in the auroral arcs correlate with PC 5 pulsations and are probably related to modulations in the interplanetary electric field. The auroral observations indicate that the filamented mixing region persisted for many tens of minutes, suggestive of a spatial structuring. The data suggest further that the filamented, mixing region is an important source of the R1 current and the associated mid-morning arcs.

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4.3.5

Dayside Auroral Forms and Activities

Case Review

This example illustrates several major features of the structure and dynamics of the dayside aurora, and its regulation by the IMF and solar wind conditions. The three features we focus on are: 1) combined ground and satellite observations relating to the mid-morning arcs, 2) auroral and magnetic responses at cusp and sub-cusp latitudes to rapid variations of solar wind density/dynamic pressure and, 3) cusp expansion events following the southward turning of the IMF recorded by Wind at 1002 UT. The plasma source of the multiple type 4 arcs is documented by the observations from Polar. It is a flank boundary layer consisting of a mixing of magnetosheath-origin and magnetospheric particles. The auroral arcs correspond to a filamentation of the mixing layer, the filaments being characterized by enhanced intensity of the magnetospheric plasma component, on a background of magnetosheath type plasma. Magnetic field observations from the satellite show that the auroral forms correspond to field-aligned current structures within the largescale region 1 current system (Farrugia et al. 2000). The auroral forms and the ground magnetic field are pulsating at frequencies in the PC 5 range. When Polar entered the field lines of the diffuse type 3 aurora further south, at 0642 UT, magnetosheath-type plasma was no longer observed. The plasma regime of the type 3 aurora consists of quasi-trapped plasmasheet plasma. The field-aligned current here is the region 2 system. Resonance oscillations of the

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magnetic field are observed from both the satellite and the ground in this regime. A clear example of the midday gap aurora (cusp emission with very weak green line intens ity) was observed within 0800 – 1000 UT (1100 – 1300 MLT) during northward IMF orientation (clock angle regime CAR 1). A strong intensification of the red-line emission in the cusp aurora occurred at 0810 UT, in response to a solar wind density enhancement from 10 to recorded by WIND at 0710 UT. The subsequent decrease of solar wind density, recorded by WIND at 0808 UT, gave rise to a corresponding decrease of the cusp red-line emission at ~0900 UT. Thus, the interval of enhanced cusp emission during 0810–0900 UT corresponds to the interval of enhanced solar wind density observed by WIND during 0713 – 0808 UT. The magnetic impulse events at 0810, 0904, 0914 UT, which were accompanied by enhanced green line emission at sub-cusp latitudes, show the characteristics of travelling convection vor tices (TCVs) (Friis-Christensen, McHenry, Clauer and Vennerstøm 1988). The association between optical and magnetic effects of TCVs has previously been documented (Luhr, Lockwood, Sandholt, Hansen and Moretto 1996). The present example illustrates the connection with solar wind dynamic pressure pulses. We focus in particular on the detailed response of the cusp aurora to the rapid southward turning of the IMF recorded by Wind at 1002 UT, during negative IMF conditions. Ini tially, before the IMF southward turning, the cusp aurora consisted of two weak, latitudinally separated forms as observed within the 1100–1300 MLT sector. Minor brightenings occurred at 1045/1051 UT. The auroral response to the major southward turning, involving a change of clock angle from 45 to 135°, is schematically indicated in the upper panel of Figure 4.20. The major auroral form before the southward turning is marked A in the figure. The auroral activations after the southward turning may be separated into three phases: 1) strong intensification and 100 km equatorward shift of the cusp aurora at 1200MLT (1058 UT), as observed by the MSP in Danmarkshavn, Greenland and in the all-sky images in Figure 4.14, 2) eastward and poleward expansions of the type 1 cusp aurora, reaching the Ny Ålesund meridian (~1400 MLT) by 1102 UT (Figures 4.13 and 4.14), marked by form B in Figure 4.20, 3) a third phase characterized by the activation of a higher-latitude type 2 form, marked by form D in Figure 4.20, and westward retreat of the lower-latitude form after 1115 UT. Form C in the figure represents a discrete aurora in the post-noon sector (type 5), which is also activated by the southward turning of the IMF. A new cusp eastward expansion event reached the Ny Ålesund meridian at 1122 UT. It was followed by a sequence of auroral brightenings and PMAFs within 1122–1200 UT, corres ponding to more steady southward IMF conditions. In this interval the auroral equatorward boundary migrated equatorward. Near – simultaneous (within 3 min) enhancements of the DPY convection current are ob served in the ground magnetograms, both locally in the 1500 MLT region as well as in the prenoon sector (~0900 MLT), associated with the major cusp auroral expansions at ~1100 and ~1120 UT. These observations are interpreted as follows. The initial auroral activity, which occurred during the 10–15 min interval after 1057 UT, may be considered to be the response to initial pulses of dayside magnetopause reconnection in the presence of negative IMF as described by Cowley (1998). The Greenland magnetometer data indicate that a very wide sector of the dayside iono sphere is activated near-simultaneously by the IMF southward turning. This observation is in agreement with recent observations of the convection response detected by ground-based radars (Shepherd, Greenwald and Ruohoniemi 1999). Thus, the auroral and magnetic obser vations may reflect the dynamics of a large-scale composite convection pattern such as that indicated in the lower panel of Figure 4.20, consisting of merging and lobe convection cells. The approximate locations of critical stations of magnetic and auroral observations are indic

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4.4 Case 3: January 11, 1993

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ated in the figure, as are the isochrones of cusp ion precipitation. We refer to this condition as the bifurcated cusp (Sandholt, Farrugia and Cowley 1998a). In this case, the cusp ion precipitation will show falling ion energies with increasing latitude at lower latitudes (type 1 precipitation), giving way to increasing ion energies with increasing latitude (type 2 precipita tion) poleward of this. The ion energy dispersion profile is thus “V-shaped” in this case. The minimum ion energy at the bottom of the “V” will be observed on the streamline for which the time since reconnection is a maximum at a particular local time, which will occur in the vicinity of the boundary between merging cell and lobe cell streamlines. Examples of these different types of cusp ion dispersion can be found, for example, in the study of Viking space craft plasma data by (Woch and Lundin 1992). The intermediate or “hybrid” type discussed here is shown in their Figure 3, and their results indicate that these are generally observed for values. small (positive or negative) IMF The present observations are in agreement with the idea of the opening of the cusp in asso ciation with the southward turning of the IMF (Crooker, Toffoletto and Gussenhoven 1991), and the recent observations of rapid equatorward and longitudinal expansions of the re lated convection enhancement (Greenwald, Ruohoniemi, Baker, Bristow, Sofko, Villain, Lester and Slavin 1999) and (Shepherd et al. 1999). Furthermore, the present continuous groundbased observations demonstrate the pulsed nature of the cusp region aurora and the plasma convection (see the H-component modulations at GDH-ATU-HOP), in line with the ideas of (Southwood 1987) and (Cowley, Freeman, Lockwood and Smith 1991a). The activation of the type 2 aurora in the late phase of the events corresponds to the activation of lobe cell convection, as will also be demonstrated in cases below (see also chapter 2). The H-deflection at the highest-latitude station in Greenland (NRD; 81°MLAT) represents in our view lobe cell convection. As indicated in Figure 4.20, the deflection at station NRD is recorded in the close vicinity of the type 2 aurora, which is the high-latitude component of the auroral events. After the three successive auroral/magnetic events observed during the interval ~1058-1115 UT, the auroral/magnetic activity subsided until 1120 UT, when a long activity sequence was initiated. The reduced activity during ~1115-1120 UT may be due to a magnetospheric reconfiguration associated with the increase of the IMF clock angle beyond 140°, which can be seen in Figure 4.9c.

4.4

Case 3: January 11, 1993

We now present a case of cusp region auroral activity in the postnoon sector, during conditions similar to those in the previous case for which radar data are present, which allows us to in vestigate the local convection pattern. The data are EISCAT radar observations of ionospheric convection in the vicinity of an equatorward boundary intensification, which was followed by a poleward moving auroral form. An overview of the observations corresponding to 1055 UT on Jan. 11, 1993 is given in Figure 4.21. It shows the pattern of equivalent convection derived from magnetograms obtained at stations on Svalbard and Greenland. We note the similarities with the pattern observed on Nov. 30, 1997 (Figure 4.20). Figure 4.21 also shows the fields of view of the MSPs in Ny Ålesund (NYÅ) and Danmarkshavn (DMH) and the UHF and VHF antennas of the EISCAT radar in Troms are marked by meridian lines. A heavy curved line marks the main feature (rotational reversal) of the local ionospheric convection pattern, as derived from the radar observations (see below). The poleward moving/expanding auroral form (PMAF) located between Greenland and Svalbard is marked by the double-hatched area in the figure as inferred from all-sky camera observations. A dashed line marks the estimated position of the projected reconnection line in the sector of the observations. Figure 4.22 shows H-component magnetograms from Greenland and Svalbard for the in terval 0600–1200 UT. We note the marked intensifications of DPY convection currents in the

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intervals 0930-1000 and 1030-1100 UT and will here focus on the latter. The DPY deflection maximizing at 1055 UT is marked by a vertical dashed line in Figure 4.22. Ionospheric ion drifts (line of sight velocities), observed with the UHF and VHF antennas of the EISCAT radar, are shown in Figure 4.23 (compare Figure 4.21). Red color indicates high velocities away from the radar. The intervals marked I, II, and III in Figure 4.23 correspond to periods of successively increasing magnetic disturbance (see Figure 4.22). We note that the equator ward boundary of the high-speed flow area is moving equatorward in the interval 1030-1100 UT. The 1055 UT event (see Figure 4.21) is marked by a vertical dashed line. The regions of high-speed (>400 m/s) poleward flows (red color), as measured by the UHF and VHF antennas, are marked by thick bars in Figure 4.21. Figure 4.24 shows MSP observations from Ny Ålesund for the interval 1000-1110 UT on January 11, 1993. We note the significant equatorward shift of the auroral equatorward boundary between 1020 and 1100 UT and the sequence of 8 poleward moving auroral forms during 1000-1100 UT. The interval 1005-1020 UT is characterized by two auroral bifurcation events, i.e., sequential brightenings of type 1 and 2 forms which are separated in latitude. Here we focus in particular on the equatorward boundary intensification (EBI) at 1056-1059

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UT, and the poleward moving form emanating from this EBI during 1059-1103UT. This event has been marked in Figure 4.21, just before it entered into the field of view of the photometer in Ny Ålesund. From these observations we conclude that the 1056-1103 UT auroral event on Jan. 11, 1993 was observed in a region characterized by a rotational convection reversal, with the poleward ion flow component being >400 m/s. Since the disturbance level on Jan. 11, 1993 was higher than that on Nov. 30, 1997, the postnoon convection reversal was displaced further equatorward on the former day. The similarities of the auroral activity and equivalent convection pattern on Jan. 11, 1993 and Nov. 30, 1997 are evident. A more detailed description of the Jan. 11, 1993 data is given by (Sandholt et al. 1998d).

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4.4.1

Dayside Auroral Forms and Activities

Case Review

This example is used to document the pattern of local ionospheric convection in the region of PMAFs (type 1a) in the postnoon cusp aurora, corresponding to negative IMF and conditions (see lower panel of Figure 4.1). In the present case the IMF orientation is inferred by analogy with other cases, like that of Nov. 30, 1997. Ion drift observations by the EIS CAT radar show the association between events of enhanced convection and PMAFs/erosion events during the interval 1020-1100 UT. Furthermore, the local convection associated with the auroral event at 1055 UT appeared as a rotational convection reversal with a large (~ 400 m/s) antisunward flow across the auroral cquatorward boundary. Assuming that the latter auroral boundary marks the close vicinity of the open-closed field line boundary (mag netic separatrix) (see discussion below) the EISCAT data document the presence of ion flow (EXB-drift) across the magnetic separatrix. This is one of the definitions of magnetic recon nection (Vasyliunas 1994). A case of rotational convection reversal in the cusp region during similar conditions has been reported by (Baker, Greenwald, Ruohoniemi, Dudeney, Pinnock, Newell, Greenspan and Meng 1990, Baker, Rodger and Lu 1997). The suggestion that east-west aligned PMAFs could be associated with poleward plasma convection across an east-west aligned open-closed field line boundary was first made by (Horwitz

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and Akasofu 1977). The data presented here, in combination with the observed particle precip itation characteristics of PMAFs (see cases 4 and 7), confirm the presence of this phenomenon.

4.5 4.5.1

Case 4: January 3, 1995 IMF Observations

Figure 4.25 shows in the first four panels the total field and its GSM components on January 3, 1995, from 0600 to 0800 UT. The fifth panel gives the IMF clock angle, and the last panel shows the “epsilon parameter”, a measure of the electromagnetic power (Poynting flux) into the magnetosphere, here expressed in mW (Perrault and Akasofu 1978). The data, which are of ~90 s resolution, were acquired by the MFI and SWE instruments on the Wind Major changes in the spacecraft. The GSE position coordinates of Wind were IMF orientation, which are reflected in the cusp aurora, have been marked by vertical lines: (1) a northward turning at 0640 UT, (2) a southward turning at ~0705 UT, (3) a polarity dominated) to a more radial field change at ~0725 UT, and a change from a southward dominated) at ~0735 UT. The field is (generally) northward (category IV) in interval I, rotates steadily southward (category II) in interval II, and points strongly southward (category I) in interval III. Whereas IMF was mostly strongly positive in intervals I and II, it was negative in interval III. We note the significant amount of estimated Poynting flux (epsilon parameter) entering the magnetosphere when exceeds 90°.

in interval I to 590 km in intervals II The solar wind speed increased from 560 km

and III. The corresponding values of the solar wind dynamic pressure were 5 and 6 nPa. Figure 4.26 shows magnetic field data from IMP 8 located in the magnetosheath (inside the bow-shock). The same main features as observed from Wind are identified hero. We note in particular the following transitions: 1) northward turning at 0705 UT, 2) southward turnings transition from positive to negative polarity at 0755 UT. at 0723/0732 UT, and 3) IMF In the next section we report the auroral responses to these changes in IMF orientation.

4.5.2

Auroral Observations

Figure 4.27 shows MSP observations of the auroral intensities at 630.0 nm and 557.7 nm for the interval 0700-0800 UT (~ 1000-1100 MLT) on January 3, 1995. The emission intensities are shown color-coded according to the bottom scale, and plotted as a function of zenith angle and UT. After the equatorward boundary intensification at 0705 UT, the aurora during interval 0710-0730 UT consists of two latitudinally separated forms/branches, one centered at 30°-40°N (type 2) and another centered at 40°-50°S (type 1). A major intensification of the southernmost form, accompanied by a significant equatorward shift of its equatorward boundary, occurred at 0730 UT. This intensification and shift were followed in the interval 0730-0750 UT by a classical sequence of PMAFs, corresponding to the strongly southward component. During 0730-0740 UT, PMAFs are followed IMF orientation and the positive by the activation of the northern auroral branch (type 2 form). This is the phenomenon of auroral bifurcation events. From their initial brightening to their fading phase, these events span a latitude range along the meridian of ~500 km. The intensity maxima in the events are near 10 kR, while minima in between are around 5 kR or less, giving a relative modulation of about a factor of two. Similar and even higher intensity contrasts are observed in the green line aurora at 557.7 nm. While inside the events the green line is several kRs strong, its intensity is less than 1 kR in between events. The next transition in the auroral morphology, marked by the disappearance of PMAFs, occurred at 0750 UT, around the time of transition component. Between 0752 and 0800 UT, the aurora was from positive to negative IMF

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limited to a much more narrow latitude range than in the previous interval. There are some intensity modulations, but major PMAFs are not observed.

4.5.3

Magnetic Observations

Figures 4.28a and b show X- and Z-component magnetograms from the Svalbard stations. We note the following features of the observations. Three distinct transient increases of the X-component deflection were observed in association with the three major auroral events (PMAFs) in the interval 0725-0750 UT. Immediately after the fading of the last of these events (at 0750 UT) the X deflection went negative. This is the time of the auroral transition from PMAF sequence to a much more latitudinally narrow band of emission in the south. The IMF clock angle increased to near 180°, before a transition to negative polarity took place. The X deflection at 0745 UT maximizes at the latitude of Hornsund, HOR (74.0°MLAT), where the Z deflection is a minimum. This indicates that the ionospheric current in the maximum phase was centered at the latitude of HOR (74.0°MLAT), which is near the latitude of the corresponding equatorward boundary intensification in the aurora.

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4.5.4

Dayside Auroral Forms and Activities

Observations of Particle Precipitation

Figure 4.29 shows particle precipitation data from spacecraft DMSP F11, relating to one of the poleward moving auroral events/forms in Figure 4.27. We shall focus on the observations made during the interval when the satellite was in the close vicinity of the Ny Ålesund me ridian, that is, during 0735:00-0735:50 UT (see Figure 4.30). In this region, near the major equatorward boundary intensification shown in Figure 4.27, the electron and ion precipitation fluxes were characterized by average energies of 100-200 eV and 1 keV, respectively. These are typical characteristics of magnetosheath-origin particles. The ion flux shows a sharp lowenergy cutoff at 500 eV. At ~0735:50 UT F11 crossed into a zone of much more energetic electron precipitation (plasma sheet) with average energy of ~10 keV. The ion convection sense in the region of the PMAFs was antisunward. The geometry of the satellite pass in rela tion to the local convection pattern, estimated from the ground disturbance field, is indicated in the next section.

4.6 Case Review

4.6

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Case Review

This case shows auroral responses to changes in the IMF clock angle, i.e., northward and southward rotations as well as change of IMF polarity. The auroral observations are combined with particle precipitation data from satellite DMSP F11. The observation geometry is schematically illustrated in Figure 4.30. The northward turning of the IMF recorded by IMP 8 at ~0705 UT was followed by the activation of a type 2 form in the north at 0710 UT, giving rise to a 2-branch auroral config uration, which persisted until slightly after the southward turning at 0725 UT (Figure 4.27b). The rotation from northward to southward orientation was accompanied by the disappearance of the type 2 aurora (Figure 4.30) in the north, and the activation of a strongly pulsed type 1 aurora south of zenith (at 0730 UT). The latter appeared as a sequence of equatorward bound ary intensifications, each of which was followed by PMAFs, and in some cases, activations of type 2 aurora, i.e., giving rise to transient auroral bifurcations. This observation refers to the prenoon sector (~1030-1100 MLT) during the prevailing positive IMF conditions. The change to negative polarity was followed by the local disappearance of PMAFs along the 1100 MLT meridian. The local ground magnetic data (Figure 4.28) strongly indicate that this change of auroral configuration was related to a corresponding change of the IMF convection pattern in the cusp region (see (Hep pner and Maynard 1987) and (Weimer 1995)). The local magnetograms further indicate that the auroral brightening events were accompanied by a corresponding intensification of the ionospheric Hall current (DPY convection current). Observations of particle precipitation from a satellite in polar orbit are used to illustrate a characteristic feature of the particle precipitation in the vicinity of the PMAFs. As indicated in Figure 4.30 the satellite DMSP F11 crossed over type 5 forms in the 1400-1500 MLT sector, a form we call type 1b in the 1100-1300 MLT sector, then in the vicinity of a PMAF (type 1a) near 1030 MLT, before entering the more diffuse plasma sheet aurora/precipitation (type 3)

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near 1000 MLT (see Figure 4.29). The satellite intersected the Ny Ålesund meridian in the close vicinity of the equatorward boundary of the cusp aurora, at the latitude of magnetometer station HOR (74°MLAT), at 0735 UT (Figure 4.30). We focus attention on the sharp lowenergy ion cutoff observed at this time. This type of precipitation feature has been shown to be consistent with an interpretation in terms of pulsed magnetopause reconnection (Lockwood and Smith 1992). The present observations were first reported by (Øieroset, Saudholt, Luhr, Denig and Moretto 1997b). Another similar example of observations of particle precipitation associated with PMAFs is given in Case 7. In Figure 4.31 the PMAF recorded during the interval 0730-0739 UT is placed in the context of the plasma convection pattern derived from magnetometer records in Greenland and Svalbard, as well as cusp precipitation recorded by the spacecraft DMSP F11. The temporalspatial evolution of the associated ground magnetic deflection event (maximum disturbance) during 0730-0739 UT is marked by crosses in the figure. We note that the indicated convection pattern is very similar to that derived from a statistical study of ground and satellite data for the actual IMF condition and season (see (Weimer 1995)).

4.7 Case 5: January 12, 1997

4.7 4.7.1

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Case 5: January 12, 1997 Solar Wind and IMF Observations

Figure 4.32 shows interplanetary magnetic field (IMF) observations from spacecraft WIND for the interval 0600-0900 UT on Jan. 12, 1997. At 0600 UT. Wind was located at (102.3, -54.6, Panels from top to bottom show the total field, the IMF components -5.8)

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4.7 Case 5: January 12, 1997

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the IMF clock angle in the GSM Y-Z plane, and the “epsilon-parameter”. Figure 4.33 shows a plot of the plasma and field observations during 0900-1000 UT. In Figures 4.32 and 4.33 we note in particular the clock angle variations during the period 0630-0715 UT (three sub intervals are marked by horizontal bar), the transition from positive to negative at 0740 UT, return to positive at 0808 UT, the rapid northward turning at 0822 UT, the southward orientation (clock angle regime CAR 2) during turning at 0917 UT, and the 0925-1000 UT. The signal propagation time from the spacecraft to the ground is ~25-30 min in this case.

4.7.2

Auroral Observations

Figures 4.34a, b, and c show color plots of photometer observations of the red line aurora for the period 0700-1100 UT (1000-1400 MLT). These plots illustrate the regulation of the latitudinal location and configuration of the cusp aurora by IMF orientation. Figure 4.34a shows the transition from a regime dominated by a sequence of two-phase auroral brightenings (initial brightening in the south, typically followed 2-3 min later by a second brightening to the north during 0700-0750 UT), to a regime dominated by brightenings of the southern branch only (0750-0830 UT). In the latter interval the northern branch was absent. The three sub-intervals of strong “bifurcation events” during 0700-0710 UT, 0717-0730 UT, and 0740-0747 UT, characterized by both type 1 and 2 brightenings, correspond to the intervals marked by horizontal bars in the clock angle panel in Figure 4.32e, when a Wind to ground signal propagation delay of 28 min is taken into account. The two-dimensional evolution of the bifurcation event that occurred during 0740-0747 UT is shown in Figure 4.35. The brightening of the equatorward boundary during 0740-0742 is followed during 0742-0745 UT by a westward expansion of a form located further north. The latter form (type 2 aurora) faded out after 0747 UT. Thus, the two branches of the aurora were simultaneously present during the interval 0742-0747 UT. Figure 4.34b shows a characteristic bifurcation and poleward motion of the cusp aurora during the interval 0848-0910 UT, associated with the northward turning of the IMF recor ded by Wind at 0822 UT (delay of 26 min). During the interval 0848-0910 UT the auroral poleward boundary moved from ~73° to ~ 78°MLAT. The auroral bifurcation, consisting of two latitudinally separated forms, persisted at reduced auroral intensity, during the interval 0910-0930 UT. A strongly northward IMF orientation (clock angle < 10-15°) during 0902-0917 UT (Wind time) gave rise to the disappearance of the southern auroral branch and a further reduced auroral intensity. A re-activation of the aurora, in the form of two latitudinally separated forms, occurred at 0938 UT, in response to the rapid southward turning recorded by Wind at 0917 UT (delay of 21 min). The auroral equatorward boundary expanded equatorward during 0945-0955 UT. An all-sky image illustrating the two-dimensional evolution of auroral configuration from 0949 UT, via 0951 UT, to 0956 UT, is given in Figure 4.36. The type 1 and 2 cusp forms have been marked by labels. It is seen that the type 1 form in the center of the image expanded eastward and equatorward in this interval. The bright form in the eastern part of the field of view is the postnoon aurora called type 5 in Figure 4.1. Figure 4.34c shows that the interval 1005-1030 UT is characterized by a latitudinally wide band of auroral emissions/activities, corresponding to an IMF orientation dominated by a component (category III IMF orientation). We note a sequence of type strongly negative 1 brightenings in the vicinity of zenith and a weaker type 2 aurora in the north.

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4.7 Case 5: January 12, 1997

4.7.3

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Case Review

In this example, we observed transitions among many different auroral configurations cor responding to different IMF orientations, involving both positive and negative and components. A slow southward rotation of the IMF gave rise to a corresponding equatorward motion of the auroral equatorward boundary in the prenoon cusp region. During 0630-0750 UT (0930-1050 MLT) this equatorward motion was accompanied by the occurrence of equat orward boundary (type 1) intensifications, PMAFs, and auroral bifurcation events, in the prenoon sector. Figure 4.37 illustrates local magnetic deflections associated with the isolated auroral event observed in the interval 0740-0747 UT. The initial brightening in the south at 0740-0742 UT is accompanied by a magnetic deflection maximizing at station HOR, located at 74°MLAT. The label M in the figure marks the association with merging cell convection (see Figure 4.1). The subsequent brightening event, taking place further north during 0743-0747 UT, is accompan ied by a separate magnetic disturbance at the optical site in Ny Ålesund (NAL; 76°MLAT). The transition in the magnetic activity at 0743 UT is illustrated by the rapid change of the Z-component deflection from negative to positive at station LYR (75°MLAT). The label L in Figure 4.37 marks the association with lobe cell convection, as is also illustrated schematically in Figure 4.1. The interval 0742-47 UT is therefore a good illustration of the excitation of a transient event of type 2 aurora/lobe cell convection, triggered by variations of the IMF ori entation (see Figure 4.32). A similar distinction between two magnetic deflection components

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corresponding to merging and lobe convection cells has been made by (Friis-Christensen 1989). Figure 4.38 places the local observations of the PMAF sequence in the context of the largescale auroral oval dynamics. It shows a selection of UVI images obtained from the POLAR satellite, illustrating the oval dynamics during the interval 0630-0830 UT (from (Brittnacher et al. 1999)). From this figure we can see that the PMAF sequence observed from Svalbard during the interval 0700-0750 UT (within ~1000-1100 MLT/72-77°MLAT) is located inside the dayside gap in the UV emission displayed in Figure 4.38, during the phases of growth and early expansion of a substorm. This observation is consistent with previous studies of occur rence of dayside poleward moving auroral forms in relation to substorm phase (see (Vorobjev, Gustafsson, Starkov, Feldstein and Shevnina 1975) and (Sandholt, Deehr, Egeland, Lybekk, Viereck and Romick 1986)). Schematic illustrations of the auroral morphologies during positive and negative IMF conditions are given in Figure 4.1. Auroral bifurcations, characterized by latitudinally separated type 1 and type 2 forms, were observed when the IMF clock angle was in the range 45°-140° but disappeared when the IMF clock angle increased beyond ~140°. The south ernmost auroral form (type 1) in a bifurcation configuration disappeared only when the IMF orientation was strongly northward as during the interval 0930-0938 UT. We note the close associations between IMF orientation, auroral activity, and ground magnetic deflection with positive (negative) IMF corresponding to the presence (absence) of PMAFs/bifurcations in the prenoon sector (1100 MLT). As in case 4, PMAFs/bifurcations conditions is associated with positive X deflections in the local ground during positive magnetograms. prenoon-postnoon The present data confirm earlier observations of an IMF

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asymmetry in PMAF occurrence (Sandholt, Moen, Rudland, Opsvik, Denig and Hansen 1993, Karlson, Øieroset. Moen and Sandholt 1996) and recent studies of the dayside plasma convec tion (Weimer 1995, Weimer 1996). In our case study, the occurrence of PMAFs has for the first time been placed in the context of UV observations of the auroral oval from space. Our data show that a PMAF sequence occurred in the sector of dayside gap in the UV auroral emission. This dayside gap in UV auroral observations from space is a typical signature of the cusp region convection throat. The type 1 auroral activity observed in the postnoon sector during southward and was accompanied by negative X component magnetic deflections (see (Sandholt negative and Farrugia 1999)). This is consistent with the IMF convection pattern in the

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4.7 Case 5: January 12, 1997

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4.8 Case 6: November 22, 1995

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cusp region (Heppner and Maynard 1987, Weimer 1995). The observations of cases 4 and 5 strongly support the presence of the IMF cusp auroral forms as indicated in Figure 4.1. The inferred convection configurations during the interval 0945-1030 UT may be repres ented by the summary illustration of Figure 4.39. The various panels represent different IMF negative conditions, represented by clock angle reorientations in the Y-Z plane during gimes CAR 1 (strongly northward; panel (a)) and CAR 2 (intermediate) in different phases of the evolution of the IMF in this case. The simultaneous presence of lobe and merging cells, and the corresponding type 1 and 2 auroras for a wide range of clock angles, are typical features during this period. The intensification of the type 1 aurora and associated negative X component magnetic deflections south of Ny Ålesund near 0950 UT are represented by panel (b) of Figure 4.39. The reactivation of the type 2 form in the northwest at 1007 UT, its sub sequent evolution, and the persistence of the type 1 activity (see Figures 4.34b, c and 4.36b), are represented by panels (c) and (d). Data from this case study have been previously pub lished by (Sandholt, Farrugia, Moen, Noraberg, Lybekk, Sten and Hansen 1998c, Sandholt et al. 1998c, Sandholt and Farrugia 1999).

4.8 4.8.1

Case 6: November 22, 1995 Solar Wind and IMF Observations

Figure 4.40 presents solar wind proton and magnetic field observations made on November 22, 1995, for the period 0500-0900 UT. These data are high-resolution data obtained from WIND. The WIND spacecraft was earthbound with position vector (GSE) of (81, 48, 6) Panels 1–4 show proton density, temperature, bulk speed and solar wind dynamic pressure. and The magnetic field panels of Figure 4.40 show the total field strength, the components of the field in nT (GSM coordinates), and the IMF clock angle. Below we shall focus on the interval 0600-0800 UT, when various combinations of IMF components are real within 2-5 nT most of ized. We note in particular the following characteristics: (1) IMF the time, (2) changing from (a) negative to positive at 0604 UT, followed by near zero values (very small clock angle) until 0655 UT, (b) to positive at 0655 UT, and (c) back to negative values at 0722 UT. These changes will be related to the observations of the dayside aurora. The proton data show a dense and slow solar wind. The bulk speed gradually increases over the 6 hours from 320 to 350km/s. Using the bulk speed as the relevant convection speed, we estimate a time delay for signals seen at WIND to arrive at the magnetopause of ~30 min. and during the period 0600-0800 UT it has The solar wind density is throughout > an average value of The three IMF directional discontinuities in the period of interest, namely at 0604, 0655, and at 0722 UT, have been marked by vertical lines in Figure 4.40. There are no interplanetary shocks present. The last change in IMF occurs at the same time as a reduction in solar wind density from 30 to The total field is relatively constant at 5–6nT. To order the auroral observations, we find it convenient to subdivide the interval into four regions marked I–IV in the figure.

4.8.2

Auroral and Magnetic Observations

Figure 4.41 shows a color plot of red and green line emissions for the interval 0710-0740 UT. It illustrates the multiplicity of latitudinally separated forms during a strongly northward IMF orientation. The different forms have been labelled A, B, C, and D. The southernmost

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4.8 Case 6: November 22, 1995

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emission (D) is a pulsating diffuse aurora covering a relatively large range of zenith angles from zenith and southward. This is the aurora labelled type 3 in Figure 4.1. In this aurora, the green line emission is significantly stronger than the red line. This is in marked contrast to the aurora further north, which consists of multiple rayed bands (A – B – C) that are mostly aligned in the east-west direction. Form B in Figure 4.41 is characterized by a very high red-to-green intensity ratio. The green line is almost absent (less than 500 R) in this aurora, which appeared as a prominent feature around 0700 UT (1000 MLT) and persisted until the end of the observation period at 1000 UT (1300 MLT). This is the form called “midday gap” aurora, presumably corresponding to particle intrusion from the solar wind in the magnetic cusp, i.e. the particle channel marked 0 in Figure 2.1. A form labelled A, containing a significant green line emission in addition to the red, is located at the poleward boundary of form B during the interval 0700 – 0800 UT (1000– 1100 MLT). Auroral forms/events located at the equatorward boundary of form B, called form C, were observed in the periods 0700–0725 UT and 0820–0840 UT (see also Figure 4.42). Form B moved poleward in the interval 0700-0800 UT. The diffuse aurora (form D) disap peared from the field of view of the MSP (Ny-Ålesund meridian) at 0800 UT (1100 MLT). The recurrence time of brightenings in form D (type 3 aurora) is °2 min, placing it in the PC 4 frequency range. After 0800 UT the red dominated form B is the dominating aurora, as seen in Figure 4.42. We note the bright cusp aurora during the interval 0820-0838 UT, with discrete forms at its equatorward boundary. This auroral observation will be discussed in relation to the actual IMF orientation and the local ionospheric convection and ground magnetic disturbance. Figure 4.43 shows X-component magnetograms from the IMAGE stations Ny Ålesund, Hornsund, Hopen, and Bjørnøya. Major changes in the deflection characteristics are marked by vertical lines at ~0635 and 0800 UT. We also note the negative X deflections in station NAL centered at 0610 and 0830 UT, corresponding to strong auroral forms labelled C in Figure 4.42. These magnetic and auroral observations occurred during strongly negative IMF conditions.

4.8.3 Ionospheric Ion Drift Observations In this section ion drift observations in the vicinity of the auroral forms reported above will be presented. These data were obtained from the CUTLASS radar, which consists of two HF radars, one located at Hankasalmi, Finland, and one at Pykvibbaer, Iceland (Lester, Jones, Robinson, Thomas, Yeoman, Pellinen, Huuskonen, Opgenoorth, Persson, Pellinen-Wannberg and Häggström 1997). The two radars form part of the larger Super Dual Auroral Radar Network (Greenwald, Bristow, Sofko, Senior, Cerisier and Szabo 1995). These radars measure a number of parameters, including the intensity of the received backscatter, the line-of-sight velocity of the backscatter, and the width of the received spectrum, the so-called spectral width. The discussion is limited to the line-of-sight velocity. During the interval of interest, each radar operated a scan with 16 different beam directions, with a dwell time in each beam direction of 7 seconds and a full scan time of 2 minutes. The range resolution along each beam was 45km. Between 06 and 09 UT on November 22, 1995, ionospheric scatter was received by the Finland radar only; consequently only line-of-sight measurements of the ionospheric flows can be presented. Figure 4.44 shows a map of northern Scandinavia, Svalbard, and the northeastern part of Greenland. The fields of view of the Finland CUTLASS radar and the MSP in Ny Ålesund are indicated. Also indicated is the pass of spacecraft DMSP F13 over Svalbard for the period 0855–0858 UT on November 22, 1995.

4.8 Case 6: November 22, 1995

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4.8 Case 6: November 22, 1995

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Figure 4.45 presents radar measurements of line of sight velocity for the interval 0732-0738 UT. These data represent the subinterval (III) of positive IMF when the auroral observations show the cusp form called B in Figure 4.41. Strong backscatter is limited to the sector to the east of the photometer scanning meridian (indicated in the lower right panel). This region of scatter is located between 1100 and 1300 MLT and poleward of 77° MLAT. Velocities of less than 400 m/s are observed in the latitudinal zone of cusp-type auroras (forms B and C). The flow is somewhat disordered, but the toward (equatorward; green) flow is dominating to the northeast of Svalbard. Figure 4.46 shows similar radar data for the period 0816-0832 UT, representing the negative IMF conditions after the transition recorded by Wind at 0722 UT (interval labelled IV in Figure 4.40). The flow pattern is characterized by away drift (red) in the northeast and toward drift (green) in the north-west. A flow pattern with this counterclockwise vorticity is consistent with the theoretical flow predicted for the observed negative IMF polarity (Reiff and Burch 1985) (see Figure 2.5). This flow pattern was observed during the interval of strongly enhanced cusp auroral emission (see Figure 4.42) and negative X component magnetic deflection.

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4.8.4

Dayside Auroral Forms and Activities

Observations of Particle Precipitation

As mentioned previously, observations of particle precipitation from satellites in polar orbit are very useful in providing information on magnetospheric plasma sources of the different dayside auroral forms (Newell and Meng 1994). In this section, data from a pass of satellite DMSP F13 in the vicinity of Svalbard is used as a supplement to the ground observations reported above. The DMSP spacecraft overflew Svalbard at 0857 UT, as indicated in the maps shown in Figures 4.44. The satellite track was aligned almost normal to the photometer scanning meridian through Ny-Ålesund, i.e., it was a post-noon to pre-noon pass. Thus, the 0857 UT pass intersected the southern part of a cusp-like aurora (form B) located north of Ny Ålesund. The cusp precipitation zone observed by the satellite, projected to auroral altitude, is shown by the bar along the sub-satellite track in Figure 4.44. Particle spectrograms are given in

4.8 Case 6: November 22, 1995

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Figure 4.47, showing the following characteristics at the time of intersection with auroral form B: 1) patch of electrons in the energy range below 300 eV (average energy: 100 eV), 2) patch of ions with energy below 3 keV (average energy: 2-3 keV).

4.8.5

Case Review

This case example documents a rich latitudinal structure of auroral emission in the vicin ity of the cusp for positive IMF conditions, and its relationship with IMF plasma flow pattern and current configuration. The data comprise a few-hour long interval consisting of subintervals with different component, i.e., positive, near zero, and negative. The auroral observations were combined with simultaneous ionospheric ion flow data obtained by the CUTLASS Finland radar. The essential points of our findings may be summarized as follows: 1) A correspondence exists between variations in the latitudinal structure of the aurora/particle precipitation, and ionospheric plasma flow pattern in the cusp region (precip itation of magnetosheath-type particles; electrons at energies < 300 eV, ions < 3 keV). 2) The aurora is characterized by three latitudinally separated forms in the cusp region: the so-called “cusp proper” / “midday gap” aurora (form B) with discrete forms on its poleward (form A) and equatorward (form C) sides, and a fourth form (D; diffuse aurora due to plasma sheet electrons at energies of ~ 1-10 keV; not shown) equatorward of forms B/C. 3) Forms A and C

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contain enhanced green line intensities (a few kRs), in addition to red line intensities, which are comparable to those in form B, indicating accelerated magnetosheath electrons. 4) Form A appears as a pulsating green line emission at the cusp poleward boundary in the ~1000 1100 MLT sector during positive or near-zero IMF (strongly northward IMF). 5) The ion flow configuration in the cusp region is determined by IMF according to theoretical expectation. A possible correspondence between auroral forms A, B, and C and the classification of particle precipitations in the cusp/cleft region reported by (Kremser and Lundin 1990), which was based on observations from the VIKING spacecraft, is indicated in Figure 4.48. Most evidently, the so-called “cusp proper” (or “midday gap” aurora), characterized by soft magnetosheath-origin particles with no acceleration events, corresponds to our auroral form B, which is strongly dominated by the red line emission (with very weak green line). The sheath plasma may enter the magnetosphere in the exterior cusp in this case (see Figure 2.1). According to (Kremser and Lundin 1990), precipitation in the cusp proper is surrounded by precipitation zones on its poleward and equatorward boundaries, the latter containing signa

4.8 Case 6: November 22, 1995

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tures of field-aligned acceleration. We find that these precipitation zones may correspond to auroral forms A and C in our case example. A schematic illustration of the association between auroral forms and ionospheric ion drift/field-aligned current systems is indicated in Figure 4.49. Intervals of positive and negat were observed to be associated with convection patterns characteristic of the respective ive lobe cells, i.e., dusk-to-dawn and dawn-to-dusk flows, respectively (Reiff and Burch 1985). These auroral and ionospheric ion flow observations may be discussed in terms of existing models of field-aligned currents in the cusp region, which are responsible for the electric field coupling from the solar wind to the cusp ionosphere (Clauer and Banks 1986, Clauer and Friis-Christensen 1988). The present observations are consistent with the pattern shown in Figure 4.49 (see also (Sandholt 1991)). Previous statistical ion drift observations showed small and disordered flows when the

and

components were of comparable magnitude (Greenwald et al. 1995, Ruohoniemi and Greenwald 1996). Lobe cell flow tended to occur only in the hemisphere where high-latitude reconnection is favored. In our case example a rather well-organized lobe cell is observed for negative IMF conditions, when the ratio was ~ 0.8 (Figure 4.46). Furthermore, transitions from one state (flow vorticity) of lobe cell convection to the other, in associ ation with IMF directional discontinuities involving polarity changes, are indicated in the represents ground magnetic deflections (Figure 4.43). Interval III (positive the favored condition for high-latitude (lobe) reconnection in the north (most antiparallel magnetic fields), whereas intervals I and IV represents an unfavored condition, according to the antiparallel merging hypothesis.

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The present observations, in combination with previous reports (Sandholt, Farrugia, Øieroset, Stauning and Cowley 1996, Øieroset, Sandholt, Denig and Cowley 1997a), indicate that au roral form B is a characteristic feature of the cusp under northward IMF. It is suggested that forms A and C, located at its poleward and equatorward boundaries, are related to field-aligned currents coupled to source plasmas in the high-latitude and low-latitude boundary layers, respectively. Electron acceleration signatures in forms A and C are preferentially observed in association with respectively positive and negative IMF polarities. The schematic illustration of Figure 4.50 shows a possible magnetic configuration at high latitudes resulting from reconnection tailward of the cusp (Cowley 1981) (and references therein) and (Gosling et al. 1991). The latitudinal location of the observed auroral forms (see Figure 4.41), and a possible correspondence with the respective source plasmas, are in dicated in the figure. The latter association is to a large extent based on the identification of

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auroral form B as a signature of unaccelerated magnetosheath plasma, which can enter the magnetosphere on the reconnected flux tube equatorward of the reconnection site, as indicated in the figure (Crooker 1992). Alternatively, magnetosheath plasma may enter in a cross-field diffusive process in the near-neutral point area, as suggested by (Woch and Lundin 1992). Electron acceleration at the poleward (form A) and equatorward (form C) boundaries of the cusp may be related to field-aligned currents connected to the high-latitude boundary layer (HLBL; the plasma mantle), and the LLBL/entry layer, respectively (see Figure 4.50 and discussion below). Signatures of electron acceleration in the VIKING data were seen at the cusp equatorward boundary, not at the poleward boundary, according to (Kremser and Lundin 1990). If narrow forms with green line intensities above 1 kR are taken as signatures of fieldaligned electron acceleration, these events are present both in auroral forms A and C in our case example, i.e. at the cusp poleward and equatorward boundaries. Furthermore, we find that the intensities of forms A and C may be related to the IMF Form A is most pronounced during positive or zero orientation, in particular its conditions (intervals II and III), whereas form C is most intense during negative

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(intervals I and IV). This is in agreement with previous observations reported by (Øieroset et al. 1997a). (III) and negative (IV) were observed to be associated with con Intervals of positive vection patterns in the cusp region, patterns characteristic of the lobe cells, i.e., dusk-to-dawn and dawn-to-dusk flows, respectively, consistent with the model of (Reiff and Burch 1985). These auroral and ionospheric ion flow observations may also be discussed in terms of models of field-aligned currents in the cusp region, currents that may be responsible for the electric field coupling from the solar wind to the cusp ionosphere (Clauer and Banks 1986, Clauer and Friis-Christensen 1988). The association of discrete auroral forms and upward-directed field-aligned currents is well known (Bythrow and Potemra 1987). Such a relationship has pre viously been documented at the cusp poleward boundary during northward IMF conditions (Sandholt 1991). The idea is that strong auroral forms at the cusp poleward boundary (type A) primarily occur in association with a northward electric field/westward convection in the cusp region, be cause this configuration is consistent with upward-directed current sheets at the cusp poleward boundary, as illustrated in Figure 4.49 (upper left panel). Another condition for the presence of upward-directed large-scale field-aligned current is the case of IMF convec in the pre-noon sector (Figure 4.49, lower panel). This case is represented by tion (small interval II in our example (see aurora in Figure 4.41).

4.9 4.9.1

Case 7: November 20, 1995 Solar Wind and IMF Observations

Figure 4.51 shows solar wind proton and magnetic field observations from spacecraft Wind for the interval 0600-0800 UT on Nov. 20, 1995. Wind was located at (98, 45, 5) Initially we focus on the interval 0645-0730 UT, when the IMF rotated slowly from south (clock angle category I) to east (clock angle ~90°; category VII) orientation. ~170°; The start of the IMF rotation is marked by vertical guideline at 0645 UT in Figure 4.51. was small throughout. A rapid southward turning was recorded by Wind at 0752 UT. The identification of the auroral response to this southward turning is used to derive the Wind to earth propagation delay. The delay obtained of 45 min indicates that the interval of slow northward IMF rotation from south to east during 0645-0730 UT corresponds to the ground auroral observations during the interval 0730-0815 UT. Dashed vertical lines mark the interval (0700-0752 UT) corresponding to the sequence of auroral events we focus on, when the signal propagation time delay from Wind to ground is taken into account. This interval is characterized by a slow IMF rotation within the clock angle range 140°-90°. The solar wind plasma parameters were quite stable with typical values of proton density, bulk speed, and dynamic pressure at and 1.7 nPa, respectively.

4.9.2

Auroral Observations

Figure 4.52 shows MSP data for the interval 0730-0840 UT on November 20, 1995. In the interval 0730-0750 UT, the equatorward boundary of the cusp aurora (type 1) was located at a relatively constant latitude, corresponding to zenith angles around 40° south (~73°MLAT). A classical PMAF is observed during 0735-0740 UT. A new activity regime started around 0745 UT. This new regime is characterized by poleward migration of the equatorward boundary during 0750-0835 UT and a sequence of two-phase auroral activations, marked A, B, C, and D in Figure 4.52. The sequence consists of the three strong events (B, C and D) during ~0805 0818 UT, 0820-0827 UT and 0827-0840 UT, in addition to the weaker event at 0750-0758

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UT (A). We notice that each event starts with an initial brightening in the south (marked Al, Bl, C1, and D1 in the figure). The corresponding higher-latitude events are marked A2, B2, C2, and D2. The poleward boundary expands poleward, characterizing these events. Each individual event lasted ~10 min, and resulted in a temporary widening of the zone of auroral emission. The strong enhancements in the red line intensity at the poleward boundary were accompanied by similar intensifications in the green line (557.7nm) emission (not shown). Event D2 was particularly strong in both the red and green emissions. The poleward boundary of the aurora in its most expanded phase was located at ~80°MLAT. Thus, the auroral activity we focus on was located in the latitude range ~75°-80° MLAT. At 0838 UT a new brightening event occurred at lower latitudes, well south of the zenith of the station, i.e., an equatorward excursion of the auroral equatorward boundary. This we take to be the response to the southward turning of the IMF measured by Wind at 0752 UT, as mentioned above. At this point the auroral bifurcation sequence comes to an end.

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4.9.3 Plasma Convection Figure 4.53 shows ionospheric ion drift observations in the area around Svalbard during the times (a) 0735 UT, (b) 0755 UT, (c) 0809 UT, and (d) 0826 UT. These observations illus trate the local convection pattern during the interval of the auroral observations shown in Figure 4.52. The location along the MSP meridian of the PMAF recorded during 0735-0740 UT is marked by the arrowed line in panel (a). Panel (a) shows a standard two-cell convection pattern characterized by strong antisunward flow centered in the 0900-1100 MLT sector, with sunward “return” flows near 1200 and 0800-0900 MLT. The lagged IMF observed at Wind is mainly directed southward, but with a positive component. The observed convection pattern is that expected for the merging cell (M) during strongly southward IMF orientation component (Weimer 1995). and a positive Panel (b) of Figure 4.53, representing 0755 UT, shows a more distorted two-cell pattern (marked M), and, in addition, a zone of strong westward flow at high latitudes (marked L), within ~76°-80°MLAT/1100-1300 MLT. The equatorward part of this zonal flow, which we identify as a lobe cell (L), is colocated with the auroral expansion event marked A2 in Figure 4.52. The lagged IMF at Wind has a clock angle of ~130°, with negative Z and positive Panels (c) and (d) of Figure 4.53, representing 0809 and 0826 UT, respectively, show a distorted two-cell pattern (M) and a lobe cell (L) flow in the ~76-80°/1100-1400 MLT region. The L-cell is characterized by strong westward flow originating in the postnoon sector (blue in the figure), and extending to beyond the MSP meridian (near 1100 MLT). The latitudinal locations of auroral expansion events marked B2 and C2 in Figure 4.52 are marked by bars along the MSP meridian in panels (c) and (d). The equatorward part of the auroral emission, also characterized by quasi-periodic intensifications, is marked by labels A1, B1, C1, and D1 in Figure 4.52. At this time the IMF is almost directly eastward.

4.9.4 Ground and Satellite Observations: Aurora and Particle Precipitation Figure 4.54 is an illustration of the observation geometry with the location of three auroral forms representing the different phases of the evolution of PMAFs. The track of satellite DMSP F13 during the interval 0738-0741 UT. Thus, the satellite traversed the three forms forms (marked c-b-a) at different stages of their evolution during 0738-0741 UT. The location of the auroral forms, the satellite track, and the fields of view of the optical instruments in Ny Ålesund are marked in the figure. The poleward motion of the auroral poleward boundary (see Figure 4.52) is marked by the arrow along the scanning meridian. Forms a, b, and c may be identified in Figure 4.52 as the most equatorward, intermediate and most poleward part, respectively, of the aurora in the interval 0738-41 UT. Form c is seen as a clear maximum in the 630.0 nm aurora centered at 15° north of zenith. This aurora appears to be the later phase of a form which had been activated around 0735 UT at ~30° south of zenith and faded away at 30°-40° north of zenith 7-10 min later. Thus, this form expanded poleward at an average speed of reaching a latitude of ~ 78° MLAT before fading. Forms b and a mark weaker emissions traversed by the satellite during 0739 0741 UT, and located within 0°-40° south of zenith at 630.0 nm. Figure 4.55 shows particle precipitation data and ionospheric ion drift for the time inter val 0735-0744 UT, obtained by the DMSP F13 spacecraft. The panels from top to bottom the represent the ion (dotted) and electron differential precipitation fluxes average energy of the ions (dotted) and electrons (eV), color-coded electron and ion spectra in the 30 eV - 30 keV energy range, and the horizontal (cross track, in blue) and vertical ion We shall focus on the observations made during the interval 0738-0741 flow speeds

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UT, when F13 crossed from 12.7 to 10.1 MLT, reaching highest magnetic latitudes corres ponding to an MLAT slightly higher than that of Ny Ålesund. During this time interval, F13 crossed over the three auroral forms described above and displayed in Figures 4.52 and 4.54. In the region corresponding to the auroral forms a, b, and c, the electron and ion precipitation fluxes were characterized by average energies within 100-200 eV and 1-2 keV, respectively. At

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~0741 UT, F13 crossed into a broad zone of much more energetic electron precipitation, with average energy of ~10 keV. The ion convection sense in the zone of the three auroral forms was antisunward, with a typical speed of in the direction normal to the satellite track. Low-energy cutoffs in the ion precipitation are clearly observed in this interval. The pat tern consists of three latitudinal zones of nearly constant cutoff energy versus latitude located at ~100, 317, and 679 eV, respectively (marked c, b, and a in the figure). There is a corres pondence between the three steps in the ion low-energy cutoff (staircase type) and the three auroral forms observed from the ground.

4.9.5

Case Review

In this case we document the association between aurora and lobe cell convection during a dominated IMF orientation, characterized by a sequence of auroral activations/expansions along the cusp poleward boundary. Each auroral event appears as an initial brightening in the south (type 1 aurora), followed by a type 2 activation/expansion further north. Twodimensional observations of the evolution of this type of auroral expansion events during

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positive IMF show that they are typically initiated in the postnoon sector, before expanding westward across the 1200 MLT meridian (see Figures 4.4c and 4.35). This case furthermore illustrates the auroral response in the cusp region to a slow, continuous rotation of the IMF from south to east orientation. Onset of the specific auroral activity and the lobe cell convection were observed at an IMF clock angle of ~140°. Thus, the major sequence of type 2 auroral expansion events, occurring at high latitudes (76°-80°MLAT), was observed when the IMF clock angle traversed the range ~140-90°. The simultaneous radar observations show clear indications of lobe cell convection north of Svalbard, during the prevailing positive IMF condition. This case shows the opposite flow vorticity compared to case 6 (see Figure 4.46), when had opposite polarity. Thus, the two cases illustrate the presence of bursts of magnetospheric lobe cell convection and associated auroras during positive and negative IMF conditions. The presence of lobe cell convection for a orientation (even with negative component) has been modelled by (Crooker, Lyon and Fedder 1998) and observed by (Huang, Sofko, Murr, Hughes and Moretto 1999). In this example we document the association between aurora and plasma convection during a sequence of lobe cell activations under a stable, positive IMF condition. The observed association between aurora and convection in 1030-1130 MLT sector is as illustrated in the upper panel of Figure 4.1. Figure 4.56 shows the MHD model ionospheric convection pattern in the northern hemi sphere, after (Crooker et al. 1998). The merging and lobe (shaded) convection cells of the model are perfectly consistent with the auroral and ion drift observations of the present case (see Figures 4.52 and 4.53). A further illustration of the association between merging and lobe convection cells and the corresponding auroras (applicable to the present case) is shown in Figure 2.17 in Chapter 2. This same figure illustrates the IMF-magnetosphere interconnection geometry. We refer to this condition as the bifurcated cusp (Sandholt et al. 1998a). In this case the

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cusp ion precipitation will show falling ion energies with increasing latitude at lower latitudes (type 1 precipitation), giving way to increasing ion energies with increasing latitude (type 2 precipitation) poleward of this. The ion energy dispersion profile is thus “V-shaped” in this case. The minimum ion energy at the bottom of the “V” will be observed on the streamline for which the time since reconnection is a maximum at a particular local time, which will occur in the vicinity of the boundary between merging cell and lobe cell streamlines. Examples of these different types of cusp ion dispersion can be found, for example, in the study of Viking spacecraft plasma data by (Woch and Lundin 1992). The intermediate or “hybrid” type discussed here is shown in their Figure 3, and their results indicate that these are generally observed for small (positive or negative) IMF values. A combination of ground and satellite observations of PMAFs during the state of southward IMF orientation, with IMF clock angle > 140° (before 0740 UT in the present case), shows the following features. Poleward expanding type 1 auroral forms in the noon sector correspond to a staircase type dispersion signature in the ion precipitation flux, with the low-energy cutoff increasing with decreasing latitude, as observed from satellites in polar orbit (Farrugia, Sandholt, Denig and Torbert 1998a). This is an expected signature of pulsed magnetopause reconnection (Lockwood and Smith 1994). The low-energy ion cutoff is a time of flight effect. The longer time of flight from the magnetopause combined with ExB drift means that the lowenergy ions precipitate at higher latitudes than the higher-energy ions. The same phenomenon was observed in Case 4 (Figure 4.29).

4.10 4.10.1

Case 8: December 16, 1998 Solar Wind and IMF Observations

Solar wind magnetic field and plasma data for the time interval from 0600 UT to 0900 UT from the Wind spacecraft are shown in Figure 4.57. During this time, the spacecraft was in the solar wind on the dawn side of the magnetosphere at (5, -28, 19) (GSE coordinates). Here we focus on the sharp southward and northward rotations of the IMF recorded by Wind at 0622 and ~0735 UT, respectively. The northward turning (0735 UT; marked by vertical line) was accompanied by an enhancement of the solar wind dynamic pressure from ~2 to ~5 nPa, the signature of which was recorded by ground magnetometers at ~0728 UT. During the southward IMF interval, the component is strongly negative. After the northward turning, the IMF is strongly northward-pointing for ~ 37 min (clock angle ~10°-15°), with nT and the minor component The temperature and the bulk speed remain relatively constant at and respectively. The period of strongly northward IMF orientation (small clock angle) lasts until ~0812 UT, after which the component (category VIII). Below we will concentrate on the IMF is dominated by the response of the cusp aurora to the rapid northward turning of the IMF and the subsequent 37 min interval of strongly northward IMF orientation. As a background for that study, we show briefly the response to the earlier southward turning.

4.10.2

Auroral Observations

MSP observations of the auroral emission at 630.0 nm and 557.7 nm obtained from Ny Ålesund during the interval 0630-0700 UT (0930-1000 MLT) are displayed in Figure 4.58. The inter val is characterized by the equatorward motion of the auroral equatorward boundary, and several strong auroral brightening events along this boundary. Each equatorward boundary intensification (EBI) is followed by poleward expansion, fading, and new activation at higher latitude. Prominent EBIs are seen at ~0642, 0646, 0649 and 0655 UT, indicating a recurrence

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period varying between 3-6 min. We note the presence of a demarcation line, representing a minimum intensity, between the EBIs and the later re-brightenings to the north. The equatorward expansion of the aurora is the response to the southward turning of the IMF recorded by Wind at ~0622 UT. All-sky data for this case (not shown) document the eastward and subsequent northward motion of the auroral events/forms (Thorolfsson, Cerisier, Lockwood, Sandholt, Senior and Lester 2000). After the arrival of the northward turning of the IMF at ~0730 UT, the aurora commences a long poleward expansion, which continues throughout the interval shown, bringing the auroral poleward boundary back to zenith (~76°MLAT) at 0810 UT (see Figure 2.6). As shown in Figure 4.59, the poleward motion is accompanied by a decreasing auroral intensity. The reduced intensity and poleward motion are responses to the northward turning of the IMF. The poleward motion up to 0800 UT consists of discrete events at 0735, 0745, 0750, and 0756 UT (see also Figure 2.6 in Ch. 2). Most of these events are characterized by a 1-2 min, initial poleward leap, followed by a longer interval (~5 min) of steady or slightly decreasing latitude, most evident in the poleward boundary of the red aurora. The poleward retreat of the poleward boundary of the red aurora results in a latitudinal widening of the band of red emission during 0742-0805 UT. Figure 4.60 shows a sequence of six all-sky images at 557.7 nm for the interval 0733-0736 UT, covering the initial phase of the second auroral brightening event shown in Figure 4.59.

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The initial brightening appears as a bright spot located slightly west of the MSP meridian in the second (0734:15 UT) image. A careful inspection reveals a faint polar arc emanating from the bright spot. The next four images, representing the interval 0734:45-0736:15 UT, show the eastward motion of both the bright spot and the polar arc. The bright spot crossed the MSP meridian at 0734:45 UT. The are manifests itself as a transient form extending poleward (towards zenith) from the intensified cusp emission, which can also be seen in the lower panel of Figure 4.59. The polar arc lasted for 2 min (0734-0736 UT). The arc intensity is observed to decrease with distance away from the bright spot at the cusp poleward boundary. The transient arc marks the initial phase of the event of enhanced cusp emission, which lasted until 0940 UT. A schematic summary of the observation geometry in magnetic latitude/magnetic local time (MLAT/MLT) coordinates, with the cusp poleward boundary (solid curved line), the bright spot (circle), and the polar arc at 0734 and 0736 UT, is shown in Figure 4.61. The fields of view of the optical instruments and beam 9 of the CUTLASS Finland radar are also marked in the figure.

4.10.3

Radar Observations: Ionospheric Ion Drift

Figure 4.62a shows line of sight ion drift velocities obtained by beam 9 of the CUTLASS Finland HF radar during the interval 0720-0750 UT. The radar was operating a non-standard mode during the interval, hence the variable time resolution of the data (for a full description of the mode see (Thorolfsson et al. 2000)). The field of view of beam 9 is indicated in Figure 4.61. Onsets of auroral brightening events are marked by vertical lines. The second

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vertical line in the figure, representing 0735 UT, marks the time when the polar auroral arc had just crossed the MSP scanning meridian during its eastward motion in the 1000-1100 MLT sector. At this time (0735-0736 UT), a transition from away (northward; red) flow to toward (equatorward; green) flow is observed within 70°-75°MLAT, in beam 9. This state lasted until 0738 UT, when a return to away flows occurred. A new event of strong equatorward flow occurred at 0742-43 UT. Here we focus on the 0735-0738 UT event. A spatial plot of the ion drift pattern in the area around Svalbard at 0736 UT is shown in Figure 4.62b. Ion drift vectors have been superimposed using a beam-swinging algorithm given by (Villain, Greenwald, Baker and Ruohoniemi 1987) and (Ruohoniemi, Greenwald, Baker, Villain, Hanuise and Kelly 1989). Relatively strong eastward flow is seen poleward of the cusp, in the northeastern part of the backscatter region. At lower latitudes the flow is westward, with a significant equatorward component in the 1000-1100 MLT sector. The spatial plot of line of sight velocities is reproduced at higher resolution in Figure 4.63. Velocities are color-coded according to the scale to the right. Blue is toward (equatorward), and red is away (antisunward) from the radar. A schematic convection pattern consistent with the observed line of sight velocities is indicated. It consists of the remnant merging (M) cell convection in the north and the transient lobe (L) cell intruding during the auroral brightening event. The latter is marked by the hatched area. A region of expected upward field aligned current associated with a flow reversal is indicated by small dotted circles. The radar observations are consistent with relatively strong eastward flow poleward of the cusp, in the northeastern part of the backscatter region. The flow in the vicinity of the cusp aurora

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is westward, with a significant equatorward component in the 1000-1100 MLT sector.

4.10.4

Satellite Observations: Particle Precipitation

Figure 4.64 shows the trajectories of the DMSP F13 and F14 spacecraft and the intervals during which they detected precipitation of magnetosheath ions and electrons on the dayside around 0800 UT. The latter intervals are marked by thick bars along the satellite tracks. The spacecraft passes are given in an MLAT-MLT format. The fields of view of the MSP and all-sky cameras at Ny Ålesund (large dot) for 0800 UT are shown by the arrowed line along the 1100 MLT meridian and the circle, respectively. Thus, the F 13 pass intersected the MSP field of view just south of the station at 0800:40 UT, at the poleward boundary of the cusp aurora, shown by a heavy bar along the Ny Ålesund meridian (Figure 4.64). The orbit of the F14 spacecraft intersected the same latitude range on the postnoon side as shown. This satellite detected a regime of similar magnetosheath precipitation as F 13, and significant equatorward convection during the interval 0756:30 to 0757:20 UT within the MLAT range 75° to 73° MLAT, centered at 1330 MLT. The traversal of this precipitation is also marked

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by a bar along the trajectory. The particle data from the F13 pass are shown in Figure 4.65. The number fluxes and average energy of electrons and ions (dotted) are shown in the first two panels. The third and fourth panels show color-coded electron and ion differential number fluxes as a function of energy and time. The bottom panel shows the horizontal (across track component) and vertical ion flows. We concentrate on observations made after 0800 UT, when the satellite is in the immediate vicinity of the field of view of the optical instruments. From 0800:40 UT to 0802:20 UT, intense fluxes of electrons and ions are encountered, both of which, as the spectrograms show, are of typical magnetosheath energies (i.e. ion energy < 2 keV and electron

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energy < 300 eV, with average energies of 1-2 keV and 200 eV, respectively). After 0802:20 UT, the magnetosheath component is strongly depleted, and the higher-energy electrons are enhanced (average electron energy increases to 1 keV). Thus, after 0802:20 UT the spacecraft encounters the boundary plasma sheet and the central plasma sheet, but these regimes are of minor concern in this study. We note the equatorward ion flow component observed during 0800:00-0800:40 UT. An even stronger equatorward flow was observed by the F14

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spacecraft during 0756:00-0757:20 UT (see below). When the magnetosheath populations were observed, the poleward boundary of the cusp aurora had reached 15° south of zenith at Ny Ålesund. This location agrees well with the poleward boundary of the magnetosheath electron and ion fluxes, which was observed by the F13 spacecraft at ~75.5°MLAT at 0800:40 UT (Figure 4.64). Thus the observed precipita tion at this time corresponds to one episode in the sequence of poleward auroral retractions. The sharp precipitation boundary recorded by F13 at 0800:40 UT corresponds to the sharp poleward boundary of the aurora seen in Figures 4.59.

4.10.5

Case Review

This case illustrates the auroral and convection responses in the cusp region associated with rapid southward and northward turnings of the IMF. The interval of southward IMF is charac terized by a sequence of PMAFs emanating from the auroral equatorward boundary. The latter boundary migrated equatorward from zenith (76°MLAT) to 60°south of zenith (~73°MLAT) in 30 min. The PMAF sequence is observed to be associated with a characteristic plasma convection pattern that is consistent with the model of (Southwood 1987). This pattern

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consists of antisunward flow across the PMAFs and equatorward return flow on its sides, as illustrated schematically in Figure 4.66, after (Thorolfsson et al. 2000). The second major feature of this case is the response of the cusp aurora following the large, rapid northward turning of the IMF recorded by WIND at 0735 UT. In the 37 minutelong interval of strongly northward IMF (clock angle less than 15°), the polar cusp aurora was characterized by a sequence of brightening/expansion events, which were accompanied by events of enhanced equatorward convection at the polar cap boundary. The gross features of the relationship between aurora and convection for this case are indicated in Figure 2.7. The ionospheric response to the northward turning may be separated in three phases. Phase 1 is the first ionospheric signature, which can be associated with the northward turning. This is a magnetic impulse event (MIE) observed within a wide range of latitudes equatorwards from the cusp at ~0727-30 UT (Sandholt et al. 2000). This impulse event is accompanied by a moderate auroral brightening. The event is possibly triggered by the rapid enhancement of solar wind density (dynamic pressure), which accompanied the actual IMF directional discon tinuity. Phase 2 (0730-0745 UT) consists of two discrete episodes of auroral intensification, each of which is accompanied by short-lived (~3 min), localized events of equatorward con vection. These events are both followed by the re-forming of the standard distorted two-cell pattern, which is reminiscent of the previous interval of strongly southward IMF. Thus, phase 2 consists of a mixture of ionospheric signatures attributed to reconnection processes taking place both poleward and equatorward of the cusp. Phase 3, which occupies the next 25 min (0745-0810 UT), is entirely regulated by the strongly northward IMF orientation. It is charac terized by a sequence of poleward expansions and latitudinal broadenings of the cusp emission band and events of reverse convection (Figures 4.59 and 4.62). In this case the cusp aurora, which we call type 2, is characterized by a sharp poleward boundary. The connection between the cusp dynamics and the excitation of the transient polar arc in phase 2 is indicated schematically in Figure 4.67. It adds an important element to the previ ously reported observations, having important implications for the magnetosphere-ionosphere coupling during the phase of transition from low-latitude to high-latitude reconnection. Polar cap arcs are generally associated with flow lines of polar plasma flow and gradients of plasma flow during northward IMF orientation (Burke et al. 1982, Gussenhoven 1982, Hardy

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et al. 1982). Polar arcs are furthermore associated with a pair of meso-scale field-aligned currents, the outflowing component being carried by sheets of incoming electrons, and an associated ionospheric flow channel (Chiu 1989, Weiss, Weber, Reiff, Sharber, Winningham, Primdahl, Mikkelsen, Seifring and Wescott 1993). In the present case, the transient arc and the associated channel of equatorward (sunward) convection are observed to intrude into a large-scale convection pattern established during the previous southward IMF interval (decays totally after 15 min of northward IMF by the end of phase 2). A region of strong flow shear develops during the activation and expansion of the lobe cell, between the eastward flow at higher latitudes driven by continuing field tension, and semi-stagnant plasma just joined to northward IMF and westward flowing magnetosheath. The sense of the shear is such as to require upward field-aligned current, as illustrated in Figure 4.67. One result is the polar arc emanating from the cusp poleward boundary. Furthermore, we suggest that the eastward motion of the polar arc is related to the evolution of the IMF-magnetosphere interconnection geometry during the event onset (ex (7 nT) pansion of the lobe reconnection site), taking into account the strongly northward and small negative condition. The band of new enhanced cusp precipitation (Figure 4.60), marked in Figure 4.67, represents, in our view, the ionospheric footprint of the lobe reconnection process. The particle source of the cusp aurora is documented by overflights by the satellites DMSP F12, F13, and F14. The particle precipitation at the cusp poleward boundary at ~0800 UT/1100 MLT and ~0757 UT/1400 MLT shows the following characteristic features. It consists of magnetosheath ions and electrons and shows a transition from “reverse” ion dis

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persion/equatorward plasma flow at the poleward boundary of the cusp aurora to closed low-latitude boundary layer (LLBL) flux flowing tailward at lower latitudes. The latter cat egory of the particle precipitation has been identified by Newell as corresponding to LLBL plasma located on closed field lines (Newell and Meng 1998). We note that the transition from equatorward (sunward) to antisunward (tailward) flow was observed both at prenoon (~1100 MLT/F13) and postnoon (~1400 MLT/F14) local times, indicating a twin cell pat tern. The aurora was east-west oriented with a sharp poleward boundary, which is typical of northward IMF conditions. The combined ground and satellite data indicate that the zone of precipitation of magnetosheath-origin plasma extended across the longitude range 0900-1400 MLT. Based on the documented association between particle source, aurora, and convection events, we use the continuous ground based observations of the aurora, in combination with the IMF observations, to infer essential features of the related coupling process between the solar wind and the magnetosphere. The main feature in the auroral data is the stepwise poleward expansion of the sharp poleward border of the cusp emission in the interval ~0742 0810 UT, which leads to a significant latitudinal widening in the band of cusp precipitation. This poleward expansion, taken together with “reversed” flow cells and cusp ion dispersion, is strongly suggestive of the pulsed “capture” of northward-directed magnetosheath flux tubes by the magnetosphere, due to sequential lobe reconnections involving both the southern and northern hemispheres (see Figure 2.9c). In our view, the reported data sets can be explained in terms of the specific scenario of solar wind/magnetosphere coupling described by (Song and Russell 1992). The process leads to a field-line bundle with both ends connected to the ionosphere (see (Reiff and Burch 1985) and (Le, Russell, Gosling and Thomsen 1996)). The duration of the reconnection bursts may be estimated from the duration of the auroral expansion phase (1-2 min). A schematic illustration of the association between the individual flux capture events and the poleward step events at the poleward boundary of the aurora is schematically indicated in Figure 4.68. In summary, the association between the dynamic cusp aurora and pulses of “reverse” convection in the northern winter hemisphere during strongly northward IMF has been doc umented. These observations are explained in terms of a scenario of sequential capture of magnetosheath plasma by the magnetosphere by high-latitude recounection in one hemisphere (phase 2: 0730-0740 UT) and two hemispheres (phase 3: 0740-0810 UT), with associated elec tromagnetic coupling to the ionosphere at cusp and subcusp latitudes. Thus, a central feature of our interpretation of the multi-instrument observations is that a phase of one-lobe recon nection (0730-0740 UT) was followed by a phase of two-lobe events (0745-0810 UT), which terminated when the IMF became radial.

4.11

Summary of Observations

The observations presented in the previous sections are included to illustrate the dynamics of the system of dayside auroral forms indicated schematically in Figures 2.3 and 4.1. The main results are reviewed below.

4.11.1

Classification of Dayside Forms

The different auroral types we postulated in Figure 4.1 have been exemplified in the cases described above, i.e. cusp region forms (types 0, 1 and 2), morning sector diffuse aurora (type 3), mid-morning multiple arcs (type 4), and postnoon multiple arcs (type 5). Here we give a review of characteristic properties of the different types of forms. The characteristics of the auroral morphology in the different sectors are described in the following sections.

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4.11 Summary of Observations

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Cusp region auroras: type 1 forms

The type 1 aurora is the dominating auroral form found in the midday sector and is related to the precipitation of magnetosheath plasma during southward IMF orientation (or, more precisely, when a strong northward IMF component is not present). As indicated in Figure 4.1, the type 1 aurora occurs in two variants, a dynamic sequence of brightening events/PMAFs, also called type 1a, and another, more latitudinally restricted form, called type 1b. Some of the characteristics of the type 1a aurora are, 1) a rayed band, appearing strongly in both the 630.0 nm and 557.7 nm emissions; 2) occurrence generally within the sector 0800-1500 MLT/70°-75°MLAT; 3) appearance often in the form of a sequence of equatorward boundary intensifications (EBIs), in the Pc 3-4 frequency range; 4) some EBIs are followed by poleward moving auroral forms (PMAFs); 5) a PMAF sequence is usually surrounded (in the east-west dimension) by the more latitudinally restricted type 1b form. Characteristics of the type 1a PMAFs are; a) strong enhancements of the red- and green line intensities with respect to the background emission; b) typically sharp equatorward boundary, with the intensity decreasing more gradually towards higher latitudes; c) a lifetime of each event in the PMAF sequence of typically 10 min, with a recurrence period of ~4-8 min (see Figure 4.80 below); d) PMAFs appearing as rayed bands that may rebrighten one or several times during the interval of poleward expansion (see (Fasel 1995)); e) regulation of the local time of occurrence and the east-west component of motion by the IMF polarity (see cases 4 and 5), as indicated in Figure 4.1. In case 5, PMAF occurrence is placed in the context of substorm phase and the global UV images of the auroral oval, observed from the POLAR satellite. PMAFs are typically observed during the growth and expansion phases of substorms. There are no causal links to substorm processes. The auroral form we called type 1b in Figure 4.1 is located in the region immediately outside the PMAF regime, where plasma convection turns from east-west dominated (sunward) to the more northward flow in the throat region (Neudegg, Cowley, McWilliams, Lester, Yeoman, Sigwarth, Haerendel, Baumjohann, Auster, Fornacon and Georgescu 2001). In the postnoon sector this is the region of strong upward-directed Region 1 field aligned current. The corresponding aurora shows strong green and red line emissions which are often subject to pulsations in the Pc 4-5 range. This aurora has been observed to intensify when the IMF turns southward (Neudegg et al. 2001). The characteristics of the associated particle precipitation is indicated in Figure 4.29 and in Figure 6 of (Neudegg et al. 2001). We suggest that the type 1a and 1b forms represent a substructure of the cusp morphology during strong IMF conditions which is referred to as double cusp by (Wing, Newell and Ruohoniemi 2001). Illustrative examples are shown in Figures 2.4, 4.52, 4.34c, and 4.58. Cusp region auroras: type 2 forms

The form we call type 2 aurora is the dominating auroral form in the midday sector during northward IMF orientation (or when a strong southward IMF component is not present), and is related to the precipitation of accelerated magnetosheath-origin plasma. An example of the “pulsating” green line emission in this auroral type is shown in Figure 4.69. Other prominent examples are found in Cases 1, 6, 7, and 8. Some of the general characteristics of type 2 aurora are, 1) strong emissions both at 630.0 and 557.7nm, 2) occurrence generally within the sector 0900-1500 MLT/75°-80°MLAT; 3) appearing predominantly in the postnoon or prenoon sector depending on IMF polarity (Milan, Lester, Cowley and Brittnacher 2000b), 4) dependent east-west motion of brightening events, 5) a poleward boundary typically more sharp than the equatorward boundary (see Case 8); 6) appearance as an east-west oriented emission band with long rays; 7) the longitudinal extent can be large, as demonstrated in Case 8, when (0800 UT) the auroral band covered the range from 0900 to 1400 MLT (during strongly

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northward IMF), or short (east-west propagating ray bundles during strong conditions); 8) occurrence often in the form of a sequence of poleward boundary intensifications, often accompanied by poleward “step” events, resulting in the poleward expansion of the auroral poleward boundary and associated latitudinal widening of the cusp emission band (see Case 8); 9) each poleward “step” is typically followed by equatorward retreat and fading, as shown in Figure 4.69; 10) auroral intensity typically decreasing towards a low background level (“ground state” cusp) when the IMF rotates from northward to a radial orientation (see Case 8). Cusp region auroras: bifurcation events

When the IMF has a significant component the cusp dynamics is often characterized by a series of auroral bifurcations, i.e., sequential brightenings in two latitudinally separated auroral forms/branches, called types 1 and 2. In the MSP records the equatorward boundary intensification is followed typically within a few min by a second intensification to the north. An example of this type of activity during positive IMF conditions is given in Figure 4.70. Near-simultaneous brightenings in two latitudinally separated branches of the aurora occurred at 0918, 0921, 0923, 0930, 0935, and 0944 UT. All-sky data (see Cases 1, 2, and 5) show that such MSP signatures observed near noon result from longitudinal expansions into the MSP polarity, field of view of two latitudinally separated auroral forms. Depending on IMF they expand from the eastern (postnoon) or western (prenoon) side. Such observations are strongly indicative of pulsations in a system of strongly coupled field-aligned currents (FACs). It is suggested that optical observations reported here reflect pulsations in the coupled system of region 1/0 FACs in the cusp region discussed by (Ohtani and Higuchi 2000). A possible association between auroral forms and patterns of convection/FACs is indicated in Figure 4.71 (see (Sandholt et al. 1998a)). A last example of the dynamic cusp aurora during intervals of strong IMF component as revealed by meridian scanning photometer observations, is shown in Figure 4.72. We note the sharp equatorward boundary which is subject to intensifications (EBIs) typically recurring at ~2 min intervals. The green line emission reveals the finer details of the temporal/spatial structure. The evolution of these auroral events in relation to plasma convection is indicated schematically in section (B) of Figure 4.77 below. Mid-morning diffuse aurora (type 3)

Examples of mid-morning diffuse auroras are given in Chapter 2 and in Cases 1, 2, 5, and 6. This aurora, called type 3, is totally dominated by the green line emission (red line is weak), and is located equatorward of discrete forms in the prenoon sector. It typically appears as a pulsating diffuse aurora in the 0800-1100 MLT sector. From the spectral information and morphology, we deduce that it is related to the precipitation of relatively high-energy magnetospheric electrons (plasma sheet) drifting around from the nightside. Figure 4.41 of Case 6 illustrates that during northward IMF orientation, the type 3 emission extends up to the equatorward border of the type 2 cusp aurora. In most cases the type 3 emission disappears around 0800 UT (1100 MLT). Typically, the type 3 auroral pulsations occur in the Pc 5 frequency range. Furthermore, during strongly northward IMF the poleward boundary of the type 3 aurora is subject to a clear wave motion (Figure 4.73). This may be a signature of waves (T~10 min) on the plasma sheet boundary. Figures 2.4 and 4.73 illustrate that during southward IMF orientation, there is typically a small latitudinal gap between the type 3 and type 1 auroras with the latter located on its poleward side. On January 8, 1999, the pulsating type 3 form faded out at 0825 UT (~1130 MLT). In the Case 2 example, it was shown that the type 3 aurora was located within the regime of the Region 2 field-aligned current (Farrugia et al. 2000).

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Mid-morning multiple arcs (type 4)

Type 4 auroras are illustrated in Cases 1 and 2. Figures 4.10, 4.11 and 4.19 illustrate the case 2 example of multiple fan arcs in the mid-morning sector, as well as the transition to the type 2 cusp region aurora near the 1000 MLT meridian. The actual conjunction with spacecraft Polar shows that these forms map to a filamented layer of mixed plasmas of mag netosheath and magnetospheric origin, located along the dawn flank, on field lines where the mid-morning Region 1 field-aligned current is flowing. The particle precipitation in this re gion, corresponding to the multiple arcs, is mainly that classified as BPS plasma (Sandholt, Jacobsen, Lybekk, Egeland, Meng, Newell, Rich and Weber 1989b, Newell et al. 1991, Farru gia et al. 2000). It is mainly located on sunward convecting field lines. The arc brightness is strongly modulated by variations in the IMF north-south component, with the strongest arcs occurring during negative The multiple discrete arcs are pulsating at frequencies in the Pc 4-5 range. The associated magnetic pulsations above the ionosphere and at the ground level are documented in the case 2 example. The northernmost aurora in this sector (0800-1000 MLT), which is dominated by the red line emission, is often ascribed to LLBL precipitation (Sandholt et al. 1989b). The geometry of the magnetosphere-ionosphere coupling and the field-aligned current is indicated schematically in Figure 2.2 (ref. Chapter 2), after (Siscoe et al. 1991) (see also Figure 11 of (Woch, Lundin, Potemra and Shapshak 1994)). Our observations in Case 2 show that the Region 1 FAC consists of filaments of enhanced magnetospheric-origin particle fluxes, which are coupled to the multiple fan-shaped auroral arcs of type 4 (see Figure 4.19). According to the model of (Siscoe et al. 1991), a large fraction of the R1 FAC in this region, generated by the solar wind-LLBL (flank boundary layer) interaction, is located on sunward convecting field lines. The latter is consistent with the observations reported in Case 2. The

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Case 2 observations furthermore indicate that the intensity of auroral arcs are strongly mod ulated by IMF variations/fluctuations and the associated variations in the interplanetary electric field applied to the magnetosphere. In the model of (Siscoe et al. 1991), the R1 cur via a voltage generator in the high-latitude rent amplitude is strongly regulated by IMF boundary layer. We note that the strong magnetospheric plasma component observed in the current filaments (which is mixed with a background of magnetosheath plasma) indicates that the magnetospheric plasma (BPS) plays an active role in the current generation process. The mid-morning arcs/bands, often appearing as “bright spots” when observed from space, have been observed to brighten in association with the arrival of interplanetary shocks (IPs) (Zhou and Tsurutani 1999). Discrete arcs in the postnoon sector (type 5)

An example of the type 5 arcs in the postnoon sector occurs in our Case 1 (after 1000 UT, see Figures 4.4 and 4.2) (see also (Moen, Sandholt, Lockwood, Egeland and Fukui 1994)). This auroral type is typically seen in the MSP records from Ny Ålesund during the interval 1000-1300 UT (1300-1600 MLT). Thus, it comprises the region of the so-called 1500 MLT “bright spot” identified in satellite observations (Lui, Ventkatesan and Murphree 1989). Most likely the “bright spot” corresponds to forms like those seen in Figure 4.4d. The auroral phenomenon is characterized by intermittent bright arcs that often wind up in vortical struc tures (spiral forms), located within the regime of sunward convection, in the vicinity of the postnoon convection reversal ((Moen et al. 1994) and (Farrugia, Sandholt and Burlaga 1994)). The actual auroral morphology and its mapping to the dusk flank boundary layer is discussed by (Elphinstone et al. 1993) (see also (Meng and Lundin 1986)). Particle precipitation char acteristics in this local time sector have been studied by (Evans 1985) and more recently by (Liou, Newell, Meng, Sotirelis, Brittnacher and Parks 1999). The latter study indicates that the plasma source of this precipitation is the dayside extension of the plasma sheet. Thus, it is likely that both the mid-morning arcs (type 4; see Case 2) and the postnoon arcs (type 5) correspond to the so-called boundary plasma sheet (BPS) precipitation, representing a spatial structuring (filamentation) of the region 1 field-aligned current.

4.11.2

Cusp Aurora and Plasma Convection in Relation to the IMF

The cusp region observations reported here are organized into eight categories of IMF ori entation. In cases when the component can be neglected, the IMF orientation may be classified by three regimes of clock angle in the GSM Y-Z plane, each of which is divided in two sub-groups, according to IMF polarity (see Figure 2.15 in Chapter 2). Clock angle regimes

The present observations illustrate how strongly the aurora in the cusp region is regulated by the IMF orientation. An auroral classification in three different configurations, corresponding has been proposed by (Sandholt et al. to three specific regimes of the IMF clock angle 1998c). Here we consider the following three clock angle regimes (CARs): (CAR 1), (CAR 2), and (CAR 3). The three regimes represent IMF orientations that are referred to as north, intermediate and south. The cusp region aurora corresponding to CAR 1 (north) is what we call type 2 (see above). The aurora corresponding to the intermediate clock angle regime (CAR 2). i.e. dominated typically covers a wider range of zenith angles/latitudes, extending from below by IMF 75°MLAT to ~78-79°MLAT. Bright emissions/forms are observed both at the equatorward and poleward boundaries of the latitudinally wide emission band (see Case 7). These forms

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we identify as the type 1 and 2 forms, respectively. The typical dynamical evolution of this aurora consists of a brightening sequence with the individual events characterized by an initial intensification in the south (the type 1 form), which is followed by a brightening further north (the type 2 form). This activity, which we call auroral bifurcation events, is, for example, seen to be activated during the slow IMF rotation from CAR 3 to CAR 2 in Case 7, i.e., when the clock angle became <140°, and was positive. Negative examples of the same phenomenon (bifurcation) are given in Figures 4.13, 4.58, and 4.72. The aurora corresponding to strongly southward IMF orientation (CAR 3), is what we call type 1. In this case the type 2 aurora is absent. The initial brightening of the type 1a events (PMAFs) occurs in the vicinity of a convection channel within the merging cell, as documented in cases 3, 7, and 8. The ionospheric plasma convection within PMAFs shows a strong poleward component (in addition to the east-west component; see Cases 3 and 7). The ground magnetic disturbance associated with PMAFs consists of a sequence of polewardpropagating X-deflections, as demonstrated in Case 4. Response to Southward Turning of the IMF

The clearest examples of the response of the cusp region aurora to southward turnings of the IMF are the 0940 UT event on Jan. 21, 1999, reported in Ch. 2 (Figure 2.11); the ~1100 UT event in Case 2; the 0730 UT event in Case 4; and the 0630 UT event in Case 8. A schematic summary of the main features of the southward turning response in Case 2 is given in Figure 4.20. The initial response consists of a significant equatorward boundary intensification, both in the red line at 630.0 nm and the green line at 557.7 nm, and a ~100 km equatorward shift of the equatorward boundary in the midday sector. This brightening/latitudinal shift is followed by a sequence of longitudinal auroral expansions/contractions in the cusp region. In the expansion phase during the prevailing negative IMF conditions on Nov. 30, 1997 (Case 2), the cusp aurora was observed to reach beyond the 1430 MLT meridian. Each auroral event, lasting ~10-15 min, may be divided into three phases: 1) brightening/equatorward expansion, 2) longitudinal and poleward expansions, and 3) new activation at higher latitudes (type 2 aurora). Phase 3 occurs when the IMF clock angle is <140° after the southward turning. Longitudinal expansions of the cusp aurora into the prenoon sector in respose to rapid southward turning of the IMF during positive conditions have been reported by (Sandholt et al. 1998c, Farrugia, Sandholt, Moen and Arnoldy 1998b). On January 10, 1997, a cusp form expanded westward and reached beyond the 0900 MLT meridian during a strongly southward condition in an interplanetary magnetic cloud (Farrugia et al. 1998b). The auroral and magnetic observations reported above are in line with the response in ionospheric plasma convection reported by (Greenwald et al. 1999, Shepherd et al. 1999). The magnetometer data reported in Case 2 indicate that the ionospheric convection response to the southward turning occurred near-simultaneously within a wide local time sector (0900-1500 MLT) (see Figure 4.16). A last example of responses in dayside auroras, to a sharp southward turning of the IMF is shown in Figure 4.73. This time we consider both the types 1 and 2 cusp auroras as well as the type 3 form relating to the dayside extension of the plasma sheet. The two panels of Figure 4.73 shows red and green line observations of the aurora from our MSP in Ny Ålesund (line of sight intensity vesus zenith angle). Intensities are color-coded as shown below each panel for the period 0800-0900 UT (1100-1200 MLT). The red line shows first a form centered at ~55°NZ (type 2) and persisting until 0830 UT. At 0826 UT a second red-dominated form appear 40° south of zenith. This is type 1. Types 1 and 2 thus coexist for ~4 min., after which type 2 disappear and only type 1 persists. Thereafter the latitude of type 1 moved slowly poleward and at 0837 UT the aurora splits into two branches. This behaviour we

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have called an auroral bifurcation. The northern branch is weaker and the southern branch is subject to sporadic, intermittent brightenings (T~2 min). The fine structure of these forms is revealed in the green line panel. South of the cusp is an aurora documented by the green line attributing to its high-energy source in the plasma sheet. This is the type 3 aurora. It is generally located at zenith angles of 40° SZ or below. Corrugations of its poleward boundary are evident at ~0810 and 0822 UT. These may be waves on the plasma sheet boundary. This aurora disappears suddenly from the field of view at 0826-27 UT. This corresponds to the time when the type 1 first appears. The type 3 reappears in the FOV at a reduced intensity and pursues an intermittent existence during the time when the cusp is bifurcated. Its poleward edge is also at lower latitude and a visible gap has appeared between the type 1 cusp emission and the type 3 plasma sheet aurora. The appearance of the type 1 aurora and the strong depletion of the type 3 at 0826/27 UT is the response to a rapid southward turning of the IMF from strongly northward which was recorded by the Wind spacecraft at to strongly southward 0813-14 UT (Sandholt and Farrugia 2002). In our view the appearance of the type 1 cusp aurora is related to the entry of magnetosheath plasma along newly opened field lines, whereas the simultaneous depletion/equatorward shift of the type 3 aurora is related to the escape to the magnetosheath of higher-energy plasma sheet particles (Scholer, Ipavich, Gloeckler, Hovestadt and Klecker 1981, Gosling et al. 1990b). The latter phenomenon reduces the source for the type 3 aurora. This is seen very clearly in the form of the disappearance of the emission in southernmost part of green line panel of Figure 4.73. Such a rapid erosion of the dayside plasma sheet witnessed in the aurora, and the corresponding equatorward shift of the open/closed field line boundary, seem to be consistent with the the Cowley-Lockwood model of ionospheric response to a pulse of enhanced magnetopause reconnection (Cowley and Lockwood 1992). It is furthermore in line with the reasoning of (Lockwood 1997) concern ing the location of the open/closed field line boundary in relation to plasma sheet particle precipitation, appropriate to intervals of enhanced reconnection. The brief overlap of the types 1 and 2 aurorae during 0826/27-0830 UT indicates the time interval from the onset of reconnection at sub-cusp latitudes to the disappearance of high-latitude (lobe) reconnection, a time that commensurates with the duration required for a newly reconnected field line at low latitudes to reach the site of high-latitude reconnection. The co-existence of type 1 and 2 cusp forms after 0837 UT corresponds to the IMF ori entation going into an intermediate state (clock angle fluctuating around a mean value of 100°), after the 8 min interval of strongly southward IMF. An alternative source of the more stable component of the type 2 aurora, in the form of diffusive entry in the exterior cusp (Sandahl, Lundin, Yamauchi, Eklund, Safrankova, Nemecek, Kudela, Lepping, Lin, Lut senko and Sauvaud 1997), marked 0 in Figure 2.1, is not excluded. Finally, we note the isolated event observed during 0826-0829 UT (Figure 4.73), consisting of a fast propagation poleward of auroral emission, originating at the initial brightening of the type 1 aurora. We suggest that this corresponds to a poleward propagating Alfvenic shock formed in the reconnection layer emanating from the X-line in the subsolar region (Lockwood, Cowley and Onsager 1996). dependence

The occurrence pattern and longitudinal motions of auroral brightening events (PMAFs) are strongly regulated by IMF polarity. Similar patterns of east-west motion are observed for both type 1 and 2 forms. For positive (negative) the events expand westward (eastward) regulation and are more often observed on the prenoon (postnoon) side. The indicated IMF

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of PMAF occurrence can be illustrated by a histogram presentation of the MSP observations of PMAFs that have been published by the University of Oslo group during the last 15 years (1985-2000). These data consist of 17 positive and 12 negative cases. The events were sorted in 30 min bins of local time of occurrence (within the MLT sector 0500-1500), and plotted in Figure 4.74. The PMAF distribution shows a clear prenoon-postnoon asymmetry polarity. The positive cases (maximum occurrence) are shifted to the related to IMF prenoon side, while the negative cases preferentially occur on the postnoon side. The PMAFs observed during positive expand in the westward (dawnward) direction, while the negative cases move eastward (duskward). This is also the case when positive events are observed postnoon (see (Sandholt, Lockwood, Oguti, Cowley, Freeman, Lybekk, Egeland events are observed prenoon (see Case 8).

and Willis 1990)), and when negative An image sequence illustrating the motion pattern during positive

is given in Fig are shown in Figures 4.14 and 4.36. The auroral ure 4.35. Illustrative cases for negative events are accompanied by significant (~50-100 nT) deflections in the local ground magnetic and field. H-component ground magnetic deflections are generally positive for positive negative for negative cases.

Examples of positive

events are given in cases 1, 4, 5, and 7, while cases 2, 3, and 5 contain negative examples. The IMF prenoon-postnoon asymmetry of type 1 auroral events is particularly clearly illustrated by the case 4 and 5 examples. In both cases, intervals of PMAF occurrence, as observed by MSPs in the prenoon sector during positive polarity at ~0750 UT (~1100 MLT). conditions, were terminated by switches to negative This pattern is summarized in Figure 4.1. Figures 4.75a and b show the general convection patterns in the winter hemisphere for southward IMF orientation corresponding to positive and negative IMF conditions, re spectively, after (Weimer 1995). The local time (MLT) span of the throat region at cusp the latitude (70°-75°MLAT) are indicated for both cases. In the case of negative IMF it is more like 0800-1200 MLT span is approximately 1100-1500 MLT, whereas for positive is similar MLT. Thus, the longitudinal shift of the daysidc convection throat with IMF PMAF occurrence is usually limited to the 0800-1300 to that of PMAFs. For positive MLT sector, as confirmed by the observations in our cases 1, 4, and 5, and the statistics in Figure 4.74. In many of the positive cases, the PMAF sequence came to an end around 0900 UT (1200 MLT), when the station rotated with the earth into the postnoon sector. Two examples were documented in our Cases 2 and 8, the former showing events ex negative panding well into the postnoon sector (around 1100 UT/1400-1500 MLT), while the latter sequence was observed as early as 0630-0700 UT (~0930-1000 MLT). Overall, the observa tions indicate that there is a close association between the location of the convection throat and PMAF occurrence. Furthermore, the location of pulsed flow events in the dayside iono sphere, as observed by HF radar, shows an IMF regulation very similar to that of PMAFs (see (Provan, Yeoman and Cowley 1999).) The combined optical and EISCAT radar data of Case 3 document the presence of an tisunward plasma flow across the equatorward boundary of the poleward moving/expanding auroral forms (PMAFs). Similar ion flows across the type 1 cusp aurora (PMAFs) are ob served by the CUTLASS radar in Case 7 (around 0740 UT). Observations by DMSP satellites in cases 1, 4, and 7 show presence of latitudinally restricted channels of enhanced (1-2 km/s) convection colocated with the type 1 cusp aurora. The signature in ground magnetometer data is a convection bay of 50-100 nT deflection in the X-component, i.e., so-called DPY convection current (Friis-Christensen and Wilhjelm 1975), which is centered at the equatorward boundary of the cusp aurora (see Cases 1, 3, 4, and 5). The X deflections at cusp latitudes are gener ally positive (negative) during intervals of positive (negative) IMF component, consistent with the convection pattern. The auroral equatorward boundary intensifications

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(EBIs) are accompanied by a correspondingly pulsed magnetic deflection (see Cases 2, 4, and 5). The combined observations of optical aurora, ionospheric ion drift, and ground magnetic deflections allow us to conclude that the cusp auroral brightening events (often followed by PMAFs) are typically accompanied by bursts of northwestward or northeastward convection, polarity, respectively. A northward flow component corresponding to positive and negative is always observed in the region where EBIs/PMAFs are observed. Antisunward convection in

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EBIs with return flows in the surrounding regions (which are consistent with the flux transfer event (FTE) model of (Southwood 1987)) has been documented in Cases 7 (Figure 4.53a) and 8 (Figure 4.66). Response to northward turning of the IMF

Illustrating examples of the response of the cusp aurora to northward turning of the IMF are the 0840-0910 UT event in Case 5 and the ~0730-0810 UT event in Case 8. An additional example is given here in Figure 4.76. Figure 4.76 shows MSP observations from Ny Ålesund for the intervals 0700-0800 and 0900-1000 UT, respectively, on Jan. 15, 1999. Auroral intensities at 630.0 and 557.7 nm are plotted as a function of zenith angle and time. The IMF clock angle is given in the top panel. We note the rapid northward turning recorded by ACE at ~0820 UT (clock angle changing from 150° to <20°), the return to southward IMF around 0830 UT, and the slow rotation towards 90° at ~0855 UT. The auroral responses to these IMF variations are observed at a ~1 hr delay, due to signal propagation from ACE. We note the auroral activations/poleward expansions recorded at ~0925 UT and ~0955 UT. The latter appears as a clear auroral bifurcation. In all of the three mentioned cases, the northward turning was preceded by longer intervals of strongly southward IMF, with corresponding type 1 auroras located well to the south of zenith in Ny Ålesund (~76°MLAT). The northward turnings of the IMF activated a different auroral form (type 2), characterized by brightenings (also in the green line emission) and pole ward expansions of poleward boundary. The poleward expansion at times occurs in discrete steps (Case 8). In two of these cases (5 and 8), the poleward motion lasted ~30 min and was accompanied by large polar cap contractions (200-300 km in 30 min), whereas the event in Figure 4.76 was short (10 min), and no significant polar cap contraction occurred. These pole ward boundary intensifications are accompanied by negative Y-component ground magnetic deflections and sunward (“reverse”) plasma convection in the polar cap and at the poleward boundary of the cusp aurora. The convection signature is documented with observations from ground-based radar (CUTLASS) and satellites in polar orbit (DMSP). The observations in Case 8 are consistent with initial (after northward turning) activations of small lobe convec tion cells intruding into the standard two-cell pattern (0730-0740 UT) and the later (0740-0810 UT) formation of “reverse” twin-cell convection centered at the cusp poleward boundary (see Figure 2.7). From the continuous ground observations, we infer that the aurora and the plasma convection in this case both consist of a sequence of coherent activations, each lasting ~5 min. The different types of poleward boundary activations associated with northward turnings of the IMF may be explained by different modes of lobe reconnection, depending on the IMF clock angle. The stepwise latitudinal broadening of the auroral band of cusp emission observed during strongly northward IMF (small in Case 8 may be explained by a sequence of lobe reconnection events involving both hemispheres (so-called two-lobe events), with associated capture of magnetosheath plasma by the magnetosphere (see (Song and Russell 1992)) in discrete events. The examples characterized by a large IMF component (clock angle regime 2; see Case 7) are different, and may be described by single lobe events in the northern hemisphere. Azimuthal

IMF orientation

This IMF category is characterized by the simultaneous presence of type 1 and 2 auroral forms spanning a wide latitudinal zone in the cusp region, as illustrated in the previous section. Good examples, for which aurora and ion drift/ground magnetic data are available, are reported in cases 1, 2, 5, 6, and 7. The events are characterized by (negative cases are, on the other X/positive Y) deflections of moderate intensity. The positive

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hand, characterized by (positive X/negative Y) deflection bays. The radar observations of

show the following characteristic pattern within

ionospheric ion drift in case 7 (IMF the field of view of the radar: a composite pattern of lobe cell in the north (westward flow within 76°-80°MLAT, associated with sequence of westward expanding type 2 auroral events),

and a distorted merging cell convection in the south (northwestward flow within 74°-76°MLAT,

associated with auroral equatorward boundary intensifications (type l))(see Figure 4.53). This

clockwise vorticity of the lobe cell is consistent with the positive IMF

polarity (see (Reiff and Burch 1985)). A transition from positive to negative IMF polarity in case 6 gave rise to a corresponding change of ion flow vorticity, as is reflected in the ground magnetic deflection (Figure 4.43). The composite pattern of merging and lobe convection cells and associated auroras (see Figures 2.17 and 4.77) are consistent with the predictions of (Reiff and Burch 1985) (see also (Crooker et al. 1998, Huang et al. 1999)). The individual events in the quasi-periodic sequence of type 2 brightening/expansion events for positive are initiated on the postnoon side and subsequently expand westward across the 1200 MLT meridian (see Figures 4.4c and 4.35). The simultaneous presence of type 1 and 2 auroras within the field of view of the optical instruments has been referred to as the bifurcated cusp aurora (Sandholt et al. 1998c, Sandholt et al. 1998b). Schematic illustrations of the aurora-convection configurations for positive and negative IMF conditions are shown in Figure 4.77. The association between the propagation of types 1 and 2 auroral surges and plasma convection within merging and lobe convection cells are indicated. The lobe cell circulates plasma entirely within the open field line region in the polar cap, whereas in the merging cell flux is transported across the open closed field line boundary, from the closed to open regime. Thus, this schematic illustration accounts for the following essential features of the observations: 1) the auroral bifurcation in meridian scanning 2) the association of the two photometer (MSP) records during positive and negative IMF components of the auroral bifurcation with merging and lobe convection cells, 3) the IMF dependent local time asymmetry of i) cusp particle precipitation (Newell, Meng, Sibeck and Lepping 1989), ii) occurrence and motion of poleward moving auroral forms (PMAFs), iii) plasma convection, iv) ground magnetic deflection. An illustration of combined ground and satellite observations of the association between conditions is given in Figure 4.78 (after aurora and the local convection for positive IMF (Sandholt, Jacobsen, Lybekk, Egeland, Bythrow and Hardy 1989a)). This figure also doc uments the relationship between aurora and the IMF cusp region field-aligned currents (FACs) (see (Erlandson, Zanetti, Acuña, Eriksson, Eliasson, Boehm and Blomberg 1994, Watanabe, Iijima and Rich 1996)). Each auroral form is accompanied by a pair of oppositely directed FACs. We note the sharp equatorward boundary of aurora and currents.

4.11.3 Dayside Auroras in Relation to Particle Precipitation and Boundary Layer Plasma Sources General background

Statistical studies of particle precipitation onto the dayside ionosphere have been provided by (Newell and Meng 1988, Newell and Meng 1994) and by (Kremser and Lundin 1990). These studies are based on observations from satellites in polar orbit. Newell and Meng distinguish between the following main precipitation categories, referring to plasma source regions: central plasma sheet (CPS), boundary plasma sheet (BPS), lowlatitude boundary layer (LLBL), cusp, and mantle. The regimes cusp/mantle and part of the LLBL may be located on open field lines (connected to the solar wind and thereby have more or less direct access to solar wind plasma) whereas BPS and CPS refer to field lines which

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are generally closed within the magnetosphere. In this categorization the open LLBL/cusp is the region of most direct access of solar wind plasma. (Lockwood and Smith 1992, Lockwood and Smith 1994) have pointed out that during southward-oriented IMF, the LLBL and cusp/mantle may represent different phases of the time-history of precipitation along tailward moving open field lines/flux tubes, i.e. LLBL and mantle precipitations corresponding to newly opened and old open flux, respectively. Such relationships have been derived from the study of energy versus latitude dispersion of the precipitating protons and its control by IMF orientation (Lockwood and Smith 1992, Lockwood and Smith 1994, Woch and Lundin 1992).

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Examples of LLBL precipitation located on closed field lines has been reported by (Newell and Meng 1998)). This precipitation shows the characteristics of magnetosheath plasma with protons lacking the energy versus latitude dispersion mentioned above. It presumably corres ponds to the high-altitude observations of a specific LLBL plasma prevailing during northward IMF orientation, as discussed by (Le, Russell and Gosling 1994, Le et al. 1996). Kremser and Lundin (1990) use the category “cusp proper” to describe the precipitation of unaccelerated magnetosheath plasma. This precipitation corresponds to our observations of the “midday gap” aurora, i.e. cusp region auroras with a clear minimum of green line emission (see cases 1 and 6). This auroral condition may be referred to as the “ground-state” cusp. In the present study particle precipitation data (obtained from satellites in polar orbit) which can be directly related to ground-based auroral observations were shown in Cases 4, 6, 7, and 8. In the discussion below we will separate these combined ground-satellite observations north (>0) and south (<0) cases. in north characteristics

Here we focus on the particle precipitation characteristics of the type 2 cusp aurora during northward IMF orientation. This aurora often shows a sharp poleward boundary, which is subject to a sequence of intensifications, the so-called poleward boundary intensifications (PBIs). Case 8 (0756-0801 UT) represents a strongly north condition. A sharp onset of electron/proton precipitation is observed when the satellite crossed the poleward boundary of the cusp aurora from north to south. The ion dispersion signature at the poleward boundary of the cusp aurora is consistent with the observed component of equatorward (“reverse”) plasma con vection (see also (Øieroset et al. 1997a)). The energies and fluxes of the particle precipitation are typical for plasma of magnetosheath origin (Newell and Meng 1988). Slightly equatorward of the poleward boundary, the particle precipitation shows the characteristics of the closed LLBL (Newell and Meng 1998). In our view this case illustrates capture of magnetosheath plasma by the magnetosphere via lobe reconnection in both hemispheres (Sandholt, Farrugia, Cowley, Denig, Lester and Lybekk 1999a, Song and Russell 1992). In line with this reasoning we suggest that the two regions of low-altitude precipitation, that is 1) the cusp poleward boundary with “reverse” energy-latitude dispersion of ion precipitation, and 2) the patch of dispersionless ion precipitation at slightly lower latitudes, correspond to the outer and inner layers of the high-altitude LLBL plasma reported by (Le et al. 1994, Le et al. 1996). The Case 6 observations of the optical aurora and particle precipitation in the cusp region can be discussed in terms of the particle classification of (Kremser and Lundin 1990). Accord ing to Kremser and Lundin, the particle precipitation in the cusp region can be distinguished into three categories, which we refer to as A, B, and C in Figure 4.48. These are called cusp proper (B), cusp/cleft (C) and mantle (A). Region B, located within 1000-1330 MLT, is characterized by the absence of accelerated electrons. Region C (cusp/cleft), extending equatorward of the cusp, within 0800-1400 MLT, contains field-aligned electron acceleration. A is located poleward of the cusp/cleft and contains accelerated electrons. The corresponding plasma source regions are the exterior cusp (B), the entry layer/LLBL (C) and the plasma mantle (A), as indicated in Figure 4.50. These particle categories may correspond to the three basic cusp region auroral forms observed during the interval 0700-0800 UT (1000-1100 MLT) on Nov. 22, 1995 (Case 6; Figure 4.41). south characteristics Here we focus on the particle precipitation characteristics of the type 1a and 1b cusp auroras during southward IMF orientation. The type 1a aurora shows a sharp equatorward boundary,

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which is often subject to quasi-periodic brightenings, the so-called equatorward boundary intensifications (EBIs). EBIs are often followed by PMAFs (see Figures 4.24 and 4.27). Figure 4.30 shows the geometry of ground and satellite observations for the interval 0730-0738 UT on January 3, 1995 (Case 4). The satellite crossed over a PMAF in the vicinity of its equatorward boundary near 1030 MLT, as indicated in the figure. The ion precipitation in this region shows a sharp low-energy cutoff at 500 eV (Figure 4.29). This low-energy cutoff is uniquely related to the type 1a form (see also Figure 4.55). It is not seen when the satellite crosses over the auroral types 3 and 5, during this same pass. In our view, the type 1b auroral form marked in Figure 4.1, which is characterized by the lack of poleward expansion (compare form 1a/PMAFs), represents the initial phase of the auroral brightening/expansion events which later give rise to PMAFs in the region of poleward convection in the throat region, after westward or eastward expansions (see Figure 4.77). Our observations support the view that PMAFs reflect the temporal-spatial evolution of plasma transfer from the magnetosheath to the magnetosphere-ionosphere system accompa nying magnetopause flux transfer events (Russell and Elphic 1978). The initial stage of the plasma transfer events (in the vicinity of the magnetopause X line) corresponds at low alti tude to the precipitation category called open LLBL (see (Newell and Meng 1998)) and the brightening phase of PMAFs (equatorward boundary intensifications). This phase is followed by cusp/mantle precipitation, as the reconnected flux tube evolves over the polar magneto sphere (Sandholt et al. 1993). The time-history of the plasma transfer/precipitation event is reflected in the poleward (and east-west) expanding auroral forms (PMAFs). The association between flux transfer events (FTEs) and PMAFs is also supported by the very similar IMF local time distribution of these two phenomena. A statistical study of the IMF regulation of the FTE occurrence pattern has been reported by (Kawano and Russell 1997). That pattern is consistent with the IMF related prenoon-postnoon asymmetry of PMAFs documented in Figure 4.74. The PMAFs observed during positive are observed to expand in the westward (dawnward) direction, while the negative cases move eastward (duskward). This is also the case when positive events are observed postnoon (example: Jan. 12, 1988; see (Sandholt et al. 1990)), and when negative events are observed prenoon (see Case 8; interval 0630-0700 UT; Figure 4.58). The statistical prenoon-postnoon asymmetry in PMAF occurrence corresponds to the similar asymmetry observed in mantle precipitation (Watanabe et al. 1996).

4.11.4

Cusp Aurora in Relation to Solar Wind Density/Dynamic Pressure

The dayside aurora in the cusp region is strongly sensitive to variations in the solar wind density/dynamic pressure. One example was shown in Case 2. A factor 2 enhancement of the solar wind density (reaching recorded by spacecraft Wind during 0713-0808 UT on Nov. 30, 1997, gave rise to a large enhancement of the red line intensity in the cusp region (Figure 4.12). Another example of the strong sensitivity of the cusp aurora to enhancements in the solar wind density/dynamic pressure is given below. Figure 4.79 shows the correspondence between component (upper panel) and the latitude of the type 1 cusp aurora variations in the IMF (second panel) on Jan. 10, 1993. The latter is derived from MSP observations in Ny Ålesund (see below). The lower panel of Figure 4.79 shows the H-component magnetic deflection recorded at station Nord on Greenland (81°MLAT). The figure illustrates that the latitude of component. Here we focus on the auroral the cusp aurora is strongly regulated by the IMF behaviour around 1000 UT. At this time, the auroral equatorward boundary was displaced equatorward, in response to an abrupt increase in the solar wind density/dynamic pressure

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(see insert in Figure 4.79). This displacement was also accompanied by enhanced magnetic deflections (DPY convection current) at stations on Svalbard (Hopen) and Greenland (NRD). In the present case, IMF was negative throughout the whole interval (Sandholt, Farrugia, Burlaga, Holtet, Moen, Lybekk, Jacobsen, Opsvik, Egeland, Lepping, Lazarus, Hansen, Brekke and Friis-Christensen 1994). A regular sequence of PMAFs was observed during the interval 0900-1130 UT (1200-1330 MLT). This is the typical local time sector of PMAF oc conditions (see Figure 4.1). MSP observations of this aucurrence during negative IMF roral activity are shown in Figure 4.80. We note the regular sequence of auroral brightenings/PMAFs. The equatorward boundary moved equatorward during 0900-1100 UT. The sixteen brightenings in this interval correspond to a mean event recurrence time of 7.5 min. The intensification and equatorward excursion of auroral luminosity at ~1005 UT are clearly seen in Figure 4.80. The brightening sequence ended at 1130 UT, when aurora faded and

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moved poleward in response to a northward turning of the IMF (see Figure 4.79). The strong auroral brightening equatorward of the pre-existing auroral equatorward bound ary at 1005 UT is the response to the enhancement of the plasma density/the solar wind dynamic pressure. The geometry of the auroral observations in relation to a model plasma convection/FAC pattern for this case is shown in Figure 4.81. A more detailed presentation of this case is given in (Sandholt et al. 1994). The observations on January 10, 1993, support the idea that the magnetopause recon nection process, including the reconnection rate, may be modulated by solar wind dynamic pressure. According to (Song and Lysak 2000) localized reconnection depends on the dir ection of the IMF as well as the solar wind dynamic pressure at the magnetopause. In the present case, the causal relationship may be like this: enhanced solar wind dynamic pressure enhanced reconnection rate - enhanced magnetospheric erosion/equatorward displacement of the open-closed field line boundary/cusp aurora. Such a relationship has been schematically illustrated in Figure 5 of (Mende, Klumpar, Fuselier and Anderson 1998) and also discussed by (Prikryl, Greenwald, Sofko, Villain, Ziesolleck and Friis-Christensen 1998).

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Chapter 5

Morphology of Polar-Cap Sun-Aligned Arcs In this chapter we present observational data, and adopt an inductive reasoning approach to our discussions of its significance. These data will never go out of date. Any theory seeking to explain the fundamental processes leading to the observed properties and morphology of sun-aligned arcs must be consistent with observations such as these.

5.1

Introduction

Our focus is very strongly on the nature of the sun-aligned arcs that are present in the polar cap about half the time, and which characterize the northward IMF polar cap. We do not explore the large-scale northward IMF convection of the polar cap (Heelis 1984, Heppner and Maynard 1987, Hairston and Heelis 1995, Reiff and Heelis 1994). Nor do we explore the body of research on visual-intense (kRs) sun-aligned arcs (Mawson 1916, Ismail, Wallis and Cogger 1977, Lassen and Danielsen 1978, Gussenhoven 1982, Burke et al. 1982, Frank, Craven, Gurnett, Shawhan, Weimer, Burch, Winningham, Chappell, Waite, Heelis, Maynard, Sugiura, Peterson and Shelley 1986). Rather, we focus on sub-visual intensity arcs (Weber and Buchau 1981, Carlson, Wickwar, Weber, Buchau, Moore and Whiting 1984, Carlson et al. 1988, Robinson, Vondrak and Friis-Christensen 1987, Mende, Doolittle, Robinson, Vondrak and Rich 1988, Valladares and Carlson 1991). Prior to the 1980s, the plasma velocity structure in the polar cap was described in the lit erature as turbulent and noisy. Subsequent work with the sub-visual intensity sun-aligned arcs changed that widely held perception forever. The research of the 1980s and 1990s showed that polar cap plasma behavior was far from random, and instead exhibited a highly anisotropic and highly ordered plasma velocity structure oriented in the earth-sun direction (Carlson et al. 1988). We draw upon our own unpublished observations, as well as previously published ones, by others and ourselves, to illustrate key manifestations of the physical processes coupling solar wind to consequences in the magnetosphere-ionosphere-thermosphere system. We present a wide range of different types of data available to inspire, test, and validate any present or future theory seeking to explain the phenomena under study. The theory that ultimately successfully explains the key phenomena at work, will in turn incorporate a wide range of elemental component processes. We distinguish between the individual physical processes, which can be isolated, and the interactive set of processes that [ 167 ]

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collectively add up to a comprehensive theoretical understanding of geophysical phenomena. By discussing the data presented here, we lay the groundwork for independent evaluation of those elemental building blocks. Chapter 6 collects these elements within the context of the current theory. We separate the discussion of theory into a separate chapter, because, as understanding improves, theory will evolve to follow understanding.

5.1.1

Why this Interest in Sun-Aligned Arcs?

We have known that sun-aligned arcs exist in the polar cap since the early 1900s, but interest in understanding them took a sudden leap in the early 1980s. It is important to note why. The reasons are both scientific and practical. Knowledge up to the early 1980s of the morphology of aurora, as seen optically, is as shown

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in Figures 5.1 and 5.2, taken from a most comprehensive review by Akasofu (1976). The polar cap (latitudes above ~75°) is seen as blank. Polar-cap arcs were thought to occur only a few percent of the time, and thus to be an intriguing puzzle but not central to the understanding of the fundamental nature of polar magnetosphere-ionosphere-thermosphere physics. In stark contrast, during the 1980s it came to be understood that polar cap sun-aligned arcs exist about 50% of the time, not just occasionally. In fact, after extended periods of northward IMF, the polar cap can become filled with sun-aligned arcs. Thus, they represent the normal state of the northward IMF central polar cap, not an infrequent curiosity. As such they take on much more significance than previously recognized. The practical societal impact of these phenomena was also recognized during the same time period. They introduced dynamic variability, not only at optical wavelengths (from the UV through the visible wavelength range, and no doubt in the IR as well, were such data available), but at radio wavelengths. The effects span the full range of radio wavelengths, from kHz (VLF auroral hiss) through GHz (scintillation of communications and GPS signals). Thus, we are also interested in sun-aligned arcs because they compromise the viability of radio signals at high latitudes, which in turn affects the ability of vessels at sea, aircraft, and satellites to maintain continuity of communication and GPS navigation. That is, the phenomena can determine whether we can hear, talk, and find our way at high latitudes.

5.1.2

Presentation Plan

In Section 5.2 we outline the historical context, by presenting some of the data that led to the realization that the optical nature of the polar sky was not predominantly benign as thought for most of the history of polar research, but richly dynamic in character. Included are optical, radio, and radar observations. We order these data within the framework of previous understanding of other known phenomena of the polar space environment. We will show that optical and radio wavelength signatures are a useful tool for distinguishing between two distinctly different physical states of the polar upper atmosphere. In Section 5.3 we show that the simple hypothesis, that sun-aligned arcs are Ohm’s-law arcs, is readily verifiable, and that from this validated hypothesis a great many properties of the arcs can be directly derived from additional observations. Next we examine observations from different complementary experimental techniques, each of which offers unique capabilities for extending understanding of the nature of sun-aligned arcs. Section 5.4 establishes detailed internal properties of arcs from satellite in situ data, giving unique insight into the particle and electrical structure of the arcs. Given this base from which to proceed, Section 5.5 next extends understanding to larger spatial and parameter space, with insights into the thermal structure and energetics of the arcs, including the role of Poynting flux. Section 5.6 shows unique findings derivable from a rocket penetration of a sun-aligned arc, including wave particle interaction physics. After the detailed study of individual arcs, we present in Section 5.7 statistical studies of large numbers of arcs. To be successful, any theory of the arcs must not only explain features within an arc, but also the dominant behavior of a large number of arcs, in terms of statistical likelihood of their gross behavioral morphology. Such statistical probabilities of behavior can often make or break a proposed theory. In the remaining sections of this chapter, we present focussed studies establishing additional arc properties.

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5.2

Morphology of Polar-Cap Sun-Aligned Arcs

Historical Background and IMF Context

Here we describe the historical context within which subvisual polar cap optical emissions were first seen, the initial impact of this discovery, and the shared role of ground-based and satellite images since that time. We describe the related discovery that the north-south sign of the IMF can be determined from ground-based measurements of subvisual intensity polar cap optical emissions and radio signals. This discrimination of the sense of the IMF is based on the two distinctly different morphological states in which the polar cap is found by such optical and radio sensors.

5.2.1 Historical Context At the start of the 1980s, the accepted view of the polar region was as in Figure 5.2 (Akasofu 1976). This representation of optical emission features characteristic of polar regions sum marized all of the generally-known morphological features at the time in a single figure. This view has served the field well as a frame of reference for polar research for decades, and still today continues to do so for many purposes. The central polar cap is shown blank, because no optical emissions were thought to exist there, excepting the very rare sightings of features called sun-aligned arcs. Sun-aligned arcs were first seen to occur deep within the polar cap, in the blank area of Figure 5.2 roughly poleward of 75° MLAT, since early in the 20th century (Mawson 1916, Mawson 1925). More recently, they became more widely recognized, studied, and noted to occur when the interplanetary magnetic field (IMF) is northward (Berkey, Cogger, Ismail and Kamide 1976, Ismail et al. 1977, Lassen and Danielsen 1978), as, for example, in Figure 5.3. These and more recent studies (Gussenhoven 1982), had all described these sun-aligned arcs as present in the polar cap only a few percent of the time. Lassen and Danielsen (1978) documented strong correlation of >1 kR sun-aligned arc sight ings with northward IMF conditions. Murphree and Cogger (1982) brought attention to the presence at times of a teardrop-shaped (see Figure 5.4) optical polar cap boundary, inter preted as the boundary between open and closed high-latitude field lines. This finding led Meng (1981) to suggest that polar cap arcs could be manifestations of a very expanded au

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roral zone. Observations by Frank et al. (1986), from satellites sufficiently high in altitude to see the entire earth, called attention to rare (couple of percent of the time) sun-aligned arcs clearly connected noon-to-midnight across the polar cap, and named them theta aurora because of their similarity to the Greek letter (Figure 5.5). The data from this near-earth satellite has a limited field of view of the polar cap, and ~1 kR sensitivity with no spectral discrimination (white light), but better spatial resolution than more distant “all earth” imaging-satellite images. The data are collected by scanning the optical sensor perpendicular across the satellite track, forming an image as a series of successive adjacent strips. In order for this “broom-sweep” image to map transpolar arcs over 20 degrees of latitude, as in this figure, the arc must be stable for at least 5 minutes, because the satellite moves over its ground track at ~7 km/s. We now know from ground-based data that stability duration typically well exceeds this, as, for example, in Figure 5.7. Murphree and Cogger (1982) has shown what has been named a “teardrop” polar boundary shape. The shape of this ionospheric signature has been attributed to a clear boundary between closed and open magnetic-field lines, the latter being confined within the dark teardrop-shaped polar region. Interpretations of images such as this have led to questions about where the auroral oval ends, and polar cap arcs begin.

5.2.2

First Sighting of Sub-Visual Sun-Aligned Arc Aurora

In 1981, a new instrumental capability was deployed to the polar region, an ASIP (all-sky imaging photometer) with image-enhancement capability and narrow wave length optical fil ters for selected atmospheric emission lines. The ASIP literally saw the polar cap with new eyes, and what it saw sent polar cap physics in entirely new directions. These studies are still being pursued today, with ever-new frontiers of science yet ahead. The ASIP repeatedly saw polar cap F-layer aurora (Weber and Buchau 1981).

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In that seminal set of early data, two entirely different shapes of features were seen: a diffuse quasi-oval patch of light filling a significant part of the ASIP field of view, and a relatively narrow straight arc across the entire field of view. In data taken during any particular time within this observing campaign, we see optical features with either one or the other of these two distinctly different characters. Each of these two types of features behaves differently with time. The speed of drift is ~1 km/s for the patch, ~100 m/s for the arc. The direction of drift is always from noon toward midnight for the patch, from dawn toward dusk or vice-versa for the arc. The patch always appears from the noon or cusp direction; the arc from the dawn or dusk edge of the field of view, or the dawn or dusk oval edge, or emerging from the midnight oval, or fading into view within the field of view. The patch always drifts out of the field of view toward the midnight oval; the arc drifts out of the field of view toward dawn or dusk, sometimes with slowing or reversals, or fades from view, usually fading from the noonward tip to the opposite tip. The patches are typically ~100 R intensity at 630.0 nm; the arcs are from threshold of detectability (10s of R) to > kR. These patches were entirely unanticipated and were truly a new discovery. The sun-aligned arcs, present a significant fraction of the time (~half the time), even though weak, were not anticipated, either. The little that was known about sun-aligned arcs was

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that they could be seen a few percent of the time, and were believed to exhibit Ohm’s-law properties. When these very rare and intense aurora were seen from satellites imaging the entire polar cap, they were called theta aurora (Frank et al. 1986). Study of these two classes of features, each of very different character, soon revealed that the physics driving their creation and their behavior were likewise fundamentally different. Section 5.2.5 shows how these two distinctly different and contrasting states have become a ground-based surrogate indicator of the north-south sense of the IMF, another unexpected discovery. Study of each of the features has generated two new research subcommunities within the space sciences, each still vibrantly robust at the turn of the 21st century (Basu and Valladares 1999). Section 5.2.3 sketches these sub-disciplines, often referred to as the study of polar cap patches, and polar cap arcs. Thereafter, the text focuses entirely on polar cap sun-aligned arcs, the primary topic of this chapter. These findings, and the techniques leading to them, have also launched research initiatives in the scientific community [CEDAR HLPS; GEM Pilot Program (Carlson and Basu 1990)]

5.2.3

The Two States of the Polar Cap

It was already well known by the early 1980s that the polar ionosphere and upper atmosphere alternate between two states, depending on whether the IMF is southward or northward. This alternation is due to the very different degree of coupling (interconnection geometry) between the solar wind and the magnetosphere, for opposite signs of the Z component of the IMF What was not known at that time was that ground-based optical and radio measurements in the polar cap could determine which of these two states prevailed. This section summarizes the salient optical and radio signatures that characterize the southward and northward states. The state for negative (southward) is characterized by ~100–1000 km islands of en hanced F-region plasma, surrounded by lower-density plasma. In midwinter, the overhead ionosphere can switch sharply between plasma densities and values (Weber, Moore, Sharber, Livingston, Winningham and Reinisch 1984). This excursion is literally from night to day (from typical electron densities found near midnight and near midday in fully sunlit latitudes). These islands are called patches, after their appearance in 630.0 nm imagers which discovered them (Weber and Buchau 1981). They move antisunward across the polar cap, at 1 km/s from near noon toward midnight (Weber, Klobuchar, Buchau, Carlson, Livingston, de la Beaujardiere, McCready, Moore and Bishop 1986), consistent with a normal two-cell convection pattern (Heelis, Lowell and Spiro 1982, Heelis 1984). A representative example of a patch is seen in Figure 5.6 (Weber et al. 1984). The upper sequence of frames shows 630.0 nm intensity images overhead. The bright form moving from noon (bottom of first frame) toward midnight (top of fifth frame) is the signature of the high-density plasma “patch”. That it is associated with high-density ionospheric plasma is verified by its coincidence with approaching, then passing overhead, then receding Dopplershifted HF echos, as seen on the lower frames of the figure. Approaching plasma, as indicated by upward Doppler shift, is in the lower set of ionogram panels; receding plasma is in the upper set of ionogram frames. The enhanced plasma is of an ionospheric of ~9 MHz, the background ~3 MHz. The 630.0 nm emission is due to the chemically-stored energy released by dissociative recombination of the ionospheric plasma, which first excites the lowest excitation state of atomic oxygen upon recombination, then emits a 630.0 nm photon when the excited state relaxes back to its ground state. The 9 MHz ionosphere has ~ten times the recombination rate in electron-ion pairs per unit time, thus about ten times the atomic oxygen excitation rate as the 3 MHz, and thus is ~ten times as bright in the 630.0 nm emission. Hence the bright patch. It is not aurora, it is just standard atomic oxygen recombination red line airglow from nighttime ionospheric recombination.

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The plasma within these enhanced plasma patches is highly structured over scale sizes spanning at least 0.1 to tens of km, producing severe scintillation (Basu, Basu, MacKenzie, Fougere, Coley, Maynard, Winningham, Sugiura, Hanson and Hoegy 1988). These electrondensity irregularities are produced by a plasma instability, the gradient drift instability, with the horizontal electrical force acting across the density gradients. The patches are less obvious in summer when the patches, still present, are of lower contrast against the higher background sunlit F-region polar plasma (Buchau and Reinisch 1991). They are also far less structured, because the conducting E-region plasma largely shorts out the instability process. It is thought that these patches exit the polar cap near midnight, to become the “blobs” (Vickrey, Rino and Potemra 1988, Robinson, Tsunoda, Vickrey and Guerin 1985) often seen on winter nights, in the post-sunset subauroral return-flow region. This exit from the polar cap, and distortion of the boundary about the set of incompressible magnetic flux tubes within the initial patch, has not yet been directly confirmed experimentally. Despite much progress on determining the source, evolution, and ultimate fate of this polar plasma, we remain challenged to establish the dominant mechanism(s) which chop mid-latitude plasma, entering the dayside polar cap, into such islands, called patches (Carlson 1994, Bullet and Dandekar 1999). There is another state, characterized by arcs. Sample observations, shown in Figure 5.7,

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show a sun-aligned arc emerging from the dawn edge of the auroral oval and moving duskward toward the central polar cap. The motion is tracked at equal 5-minute intervals per frame over a one-hour period. The arc moves steadily, persisting nearly an hour before finally starting to fade. The bright edge at the left of the frame is the poleward edge of the polar cap (oriented sun-aligned) at dawn. The two bright spots to the upper right of each frame are an artifact (two lights on buildings on the horizon in the distance). The field of view is about 155° of an overhead hemisphere in this fish-eye view. Figure 5.8 shows a pair of such images of a sun-aligned arc taken during a period of northward IMF. Here, observations from two stations within the nighttime polar cap show that single discrete emissions can extend very large distances (>2000 km for the assumed 250 km of this 630.0 nm emission) in the noon-midnight direction. These data give the appearance of a theta aurora (Frank et al. 1986), but one should recall that these emissions are not of visible intensity, are not produced by very energetic electrons (i.e., not produced by and would not be detected with 1980s satellite imagery. We also note that the emission features are quite discrete, and that their location within the polar cap, as defined by the lack of neighboring auroral glow is easily confirmed. The antisunward edge of the arc is imaged from Svalbard (~75° MLT), and is seen here at its most antisunward extreme to connect to the auroral oval near midnight. The more sunward image is from Thule, Greenland, and shows the sunward extension of the same arc. The time sequence of images of this arc (not shown here) shows the arc segments observed in the two separate imagers to track one another during the time the arc was visible in both imagers. The connection to the midnight oval is often far more dynamic than deeper into the polar cap. Figure 5.9 shows a sequence of frames, this time at one-minute intervals, of the same field of view, with about a 1000 km overhead span for the 630 nm assumed emission altitude of 250 km. Here we see the onset of a sun-aligned arc at the poleward edge of the midnight auroral oval. Complementary measurements by other ground-based and satellite sensors have allowed determination that the sun-aligned arcs are excited by sun-aligned sheets of soft precipitating electrons, which mark shear lines of transpolar ionospheric plasma flow. Section 5.3 demon strates how this conclusion was arrived at. Here, we set the stage by summarizing the findings to be examined in detail below. The fact that the arcs are the optical signature of electronimpact excitation of the upper atmosphere by suprathermal electron-current carriers, means that the sun-aligned arcs are true aurora. Their low intensity is due to weak fluxes of soft, low-energy electrons, order hundreds of eV. The state of the polar cap for positive is characterized by considerable structure along the dawn/dusk direction in its plasma velocities, and weak stable sun-aligned arcs seen in optical emissions. Studies such as outlined below have led them to be known now as markers of highly anisotropic transpolar velocity gradients (Carlson et al. 1988). Antisunward plasma flow, decreasing (or even reversing) along a dawn-to-dusk direction, requires a vertical up ward current to maintain divergence-free current (Burke et al. 1982) across the arc. The arc emissions are impact-excited by the downgoing current-carrying electrons. Near the central part of the polar cap, the arcs drift duskward (dawnward) for positive (negative) IMF at a non-uniform speed ~0.1–0.2 km/s (Weber et al. 1984, Frank et al. 1986, Valladares and Carlson 1991). The F-region plasma above these arcs is also structured, in this case driven by a different class of plasma instability, a shear-driven instability (Basu et al.; 1984, 1988, 1990b). Scintillation is strong over these arcs only during sunspot maximum. During sunspot min imum, the F-region densities are much lower and the scintillation much weaker. Above the optical signature of the arcs, current sheets must then extend along the earth-sun direction, over the quasi-transpolar length of the arc, reaching upwards into the magnetosphere to as far as the electron current source. Much has been learned about the near-earth electrodynamics,

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thermal structure, and energetics of these arcs (Valladares and Carlsen 1991). Major chal lenges ahead include understanding the three-dimensional electric circuit of these arcs, their relation to magnetospheric topology, and the detailed dependence of their dynamics on the solar wind. This framework for viewing and studying the polar cap in terms of two states is summarized in Figure 5.10 (Carlson 1994).

5.2.4

Ground-Based Signatures of Southward vs. Northward IMF

The polar ionosphere is characterized by quite different properties of the overhead sky seen by an all-sky image-intensified photometer (ASIP), a digital ionosonde (digisonde), or a total

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electron content (TEC) polarimeter receiver of signals from a satellite radio beacon. Thus, such ground-based measurements can serve as surrogate indicators of the north-south sense of the IMP. Figure 5.11 demonstrates the signatures of the southward vs. northward IMF conditions of the polar cap, in the upper vs. lower portions, respectively, of an ASIP, a digital ionosonde, and a TEC polarimeter. The data are all overhead Thule, Greenland, in the central polar cap, during local winter. The ASIP shows an antisunward drifting patch vs. a dawn-dusk drifting sun-aligned arc. The digisonde shows sporadic strong enhancements of F-region peak electron values, upto ~9 MHz) above a background-low nighttime fluctuating density (large about an approximate mean of 3 MHz. The TEC data shows a series of TEC enhancements above a low nighttime value fluctuating about an average value near 5 TEC units. Strongly enhanced scintillation (Basu, Basu, Weber and Bishop 1990b) due to plasma irregularities of 0.1 to tens of km scale size, coincides in time and space with both the plasma-density enhancements (patches) seen during southward IMF by each of these three diagnostics, and sun-aligned arcs during northward IMF sunspot maximum conditions. In Figure 5.12a we see the long-standing polar network of such diagnostics, established by the AF Geophysics Laboratory (now AFRL, HAFB) in cooperation with the University

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5.2 Historical Background and IMF Context

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Morphology of Polar-Cap Sun-Aligned Arcs

of Oslo and the Danish Meteorological Institute. This network can track transpolar patch motion and arc extent. It can define a frame of reference for patches, arcs, and other global and localized studies of polar features and phenomena. Figure 5.12b can be used to translate position within any ASIP image presented here into distance from the station along the curved earth, for an assumed altitude of emission of 250 km.

5.2.5

Improved Sensitivity

Why did perception of the frequency of occurrence of sun-aligned arcs change so dramatically during the 1980s? Because of development of improved photometric imaging sensitivity. Before 1980, the threshold of sensitivity of optical imagers (ground-based all-sky imagers, and satellite-based imagers) was about a kR, comparable to the threshold of the dark-adapted eye for visual observation by scientists in the field. With a kR, threshold, sun-aligned arcs were reported a few percent of the time. All-sky imaging photometers (ASIPs) achieved sensitivities of tens of Rayleighs (R) through use of image intensifiers. The ASIPs have observed sun-aligned arcs deep within the polar cap, initially at Thule, Greenland (Weber and Buchau 1981), and subsequently more extensively at Qaanaaq and Søndre Strømfjord, Greenland and Svalbard (Carlson et al., 1984, 1988). With a threshold of tens of R, polar cap sun-aligned arcs were reported about 50% of the time. We can then explain consistency between the pre- and post-1980 view of polar cap arcs, at least at optical wavelengths, in terms of instrument sensitivity. The greater the sensitivity of the sensor, the more sun-aligned arcs we see, up to the background noise limit. After extended periods of northward IMF, the polar cap, as viewed through ASIPs, can become filled with sun-aligned arcs, some of which can grow to visible intensity, that is > 1kR. Weak arcs are attributed to weak fluxes of soft (order 100 eV) particles. Less often, coincident with the relatively stronger 630.0 nm arcs, sun-aligned arcs are also seen in the 427.8 nm images, with ratios of 630.0 nm to 427.8 nm exceeding 10 to 1, and 427.8 nm emissions approaching, but below, the kR level (Carlson et al., 1984). These are also attributed to weak fluxes of relatively soft electrons (order hundreds of eV). Still less often (a few percent of the time), occasional intense sun-aligned arcs are seen, producing significant E-region ionization. These more intense arcs are attributed to more intense fluxes of harder particles (~keV). Collectively, the data indicate that with steadily increasing intensity of emission (from tens of R to kR), and hardness of electron flux (from to ~1keV), the probability of occurrence steadily decreases. Persistence in our ASIP fields of view typically exceeds 10 minutes, and spatial extent typically exceeds 1000 km (filling the diameter of the ASIP field of view for F-region optical emissions), since very rarely is a sun-aligned arc seen to terminate part way across an image.

5.2.6

Improved Physical Insights

We characterize sun-aligned arcs as stable in time, extended in space, and largely parallel to the direction of a line from the earth to the sun. We will see below that incoherent scatter radars can “image” arcs also, as long as the arcs are sufficiently slowly varying, and can measure many key physical properties to determine arc signatures. Dramatic views of the polar cap from the DE satellite have called attention to bright, wide, long-lived sun-aligned arcs that connect from the day to the nightside auroral oval, as in Figure 5.5. These so-called theta arcs, described in considerable detail (Frank et al. 1986), were also seen a very small fraction of the time. The small fraction of the time that they were observed can readily be attributed to the high intensity/low probability correlation.

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However, Frank et al. (1986) also measured strong ion fluxes coincident with the strong electron fluxes producing the optical signature of the arc. This measurement was recognized as an energetic-particle population characteristic of plasma-sheet particles. This discovery led to speculation about the possible mechanisms that might be responsible for the arcs, speculations that posed highly unusual magnetospheric topological conditions. Later we shall see that reconnection of open field lines near noon and/or midnight can lead to sun-aligned arcs. There is strong evidence from ISR data that these connections directly force narrow channels of antisunward plasma flow, surrounded by channels of return sunward plasma flow by continuity. Can small-scale plasma-flow gradients have associated electric fields of scale too small to extend as far as the distant magnetospheric boundaries? Such views have led to considerable speculation as to how the associated current sheets map to distant magnetospheric boundaries (see Chapter 2 and Section 6.1). There is a large literature on this subject, and this question continues to be an active area of research. What is clear from the large body of ground-based observations of sun-aligned arcs, is that such intense and broad optical features (and current sheets) follow from extended periods of north ward IMF. These intense arcs must both mark and trace to important boundaries that, over time, form and organize within the magnetosphere. As we proceed, keep in mind the distinc tion between dayside and nightside auroral character, as underscored by Akasofu (1976) in Figure 5.2. It has been common in the literature to lump together all arcs that fulfill the sun-aligned optical signature definition. However, categorizing by optical signature alone should be viewed as too simplistic an approach at present. We need to understand better the underlying driv ing physical processes producing these arcs, and thereby determine how many importantly different classes of arcs there may be. Until then, we will have outstanding questions about, for example, whether intense arcs represent major magnetospheric boundaries, while weak arcs represent more localized vari ations in plasma flow, and which might be trans-magnetospheric or trans-polar. Observing the temporal evolution of arcs as they form and decay should be of significant value to such discrimination, as well as, of course, measurement of many physical parameters associated with the arc (Rodriguez, Valladares, Fukui and Gallagher Jr. 1997). The high-altitude satellite, effectively hovering near apogee for hours, has the special ad vantage of seeing the entire polar cap (Figure 5.5) with meaningful temporal coverage. The ground-based images, on the other hand, offer the advantage of seeing spatial structure with good spatial resolution (Figure 5.9). This resolution will be crucial to some studies. For example, the “bright spot” reported by satellites can be a blurred image of a set of highlystructured intense arcs. Note the pixel size in Figures 5.9 and 5.5, and that the arc separations between the arc pairs caught in a 10-second frame as they jet out of the midnight auroral arc, sunward into the polar cap, have been seen to be ~10km. Also note that transpolar coverage can be achieved by a chain of ground-based imagers.

5.3

Arc Electrodynamics

To understand the nature of sun-aligned arcs, we must first establish unambiguously that they are Ohm’s Law arcs. To document this axiom, we cite here results from both ground-based and satellite observations. We shall see that from this simple start, by addition of ever more comprehensive data sets, there unfolds an enormously richer understanding of the physics around the arcs, as well as very distant processes driving the arcs.

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5.3.1

Morphology of Polar-Cap Sun-Aligned Arcs

Ohm’s-Law Arcs

We begin by illustrating, in Figure 5.13, a pair of ionospheric conditions for which application of well-known electrodynamics leads to an optical arc signature along a plasma-velocity bound ary line (Reiff and Burch 1985). Let us go through the electrodynamics of this illustration step by step to emphasize a few subtle aspects of these simple Ohm’s Law arcs. Figure 5.13 shows uniform conductivity across the plasma-velocity shear boundary. A plasma velocity reversal (left-hand side) or velocity gradient (right-hand side) across the boundary line, and the equivalent horizontal electric-field differential, must produce a ho rizontal Pedersen current convergence at the boundary, in the absence of a Birkeland current component. The electrodynamics requirement of a divergence-free current state, then, must lead to Birkeland currents of whatever magnitude necessary to maintain zero current diver gence. Up to this point in the description, the only difference between the left-hand shear reversal and the right-hand shear differential flow is the rest-frame velocity from which the flow is viewed. If the rest frame of the observer were the only difference, though, the physics of the electrodynamics would be the same for both cases. In fact, the physics is different, because it is the rest frame of the neutral gas that determines the currents. It is essential to operate in the rest frame of the moving neutral atmosphere. Its velocity must be known. The importance

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of the neutral gas rest frame may be understood in part from the following facts. Recall that it is the collisions between the charged and neutral-gas particles, and in particular, different collision frequencies (mobilities) of the ions vs. electrons with the neutral-gas particles, which produce a finite conductivity and Pedersen current perpendicular to the velocity shear boundary. The sense of the velocity difference across the boundary determines whether it tends to drive a horizontal convergence or divergence, and thus requires a vertically upward or down ward current to maintain a divergence-free state. In Figure 5.13, the sense leads to an upward current, presumably carried into the ionosphere by a flux of precipitating suprathermal elec trons. Note that if a series of velocity differentials occurs, alternatively reversing (plasma altern ately speeding up then slowing), the upward current sheets will alternate with intervening downward current sheets along velocity-difference boundary lines. Escaping thermal electrons arc presumed to carry these downward current sheets. This simple arc electrodynamics, as discussed by Lyons (1980), is in fact the electrodynamics that pertains to (stable in time, extensive in sunward direction) sun-aligned arcs (Carlson et al. 1988). We should note that, as indicated on the left-hand side of Figure 5.13, a change in conductivity alone across the boundary could lead to this same circuitry, in the absence of a velocity shear. We have found no examples of a sharp conductivity gradient alone, in the absence of a coincident sharp velocity gradient, creating sun-aligned arcs. Since the current carriers of the Birkeland current are suprathermal electrons with energies of tens of eV or greater, they should produce impact excitation of optical emissions, providing the optical signature of the arc. They by the same token will be expected to produce impact ionization, thereby enhancing the conductivity within the arc, and modifying the distribution of currents that flow within the arc itself. This feedback effect must be allowed for in any detailed self-consistent treatment of the circuitry of the arc. Here, our main point is that the sun-aligned arcs are visual markers of velocity shear lines. The lines of sharp velocity differentials must have a specific sense, namely greater antisunward velocity on the dawn side than the dusk side of a polar cap sun-aligned arc. Thus, one is to look at sun-aligned arcs with new eyes (literally with ASIPs, and figuratively with this simple arc electrodynamics in mind) as regards polar ionospheric convection. This view is simply as indicated in the cartoon Figure 5.14.

5.3.2

Verification and Calibration Using Satellite Data

Coincident satellite in situ and ground-based ASIP data used to verify this finding are shown in Figure 5.15 (Carlson et al. 1988). The two brightest sun-aligned arcs are labeled ARC A and ARC B. They are seen as two clearly visible bright streaks in the image (630.0 nm sunaligned arcs) in Figure 5.15a. They coincide with the magnetic field-aligned projection of the two strongest energetic electron-flux enhancements within the polar cap in Figure 5.15b. The auroral electron flux ends abruptly at the poleward edge at about 79° magnetic invariant latit ude (ILAT). The two strongest electron flux enhancements, peaking near 85° and 86.5° ILAT, are deep within the polar cap, as determined by the in situ particle population measurements (and consistent with the ASIP data). This was also the assumption made by Burke et al. (1982). However, they had no images to confirm the assumption that the electrodynamic signatures were in fact sun-aligned arcs. Our main focus here, however, is the gradient of the plasma drift-velocity component in the sunward/antisunward direction, shown in Figure 5.15d. The two sun-aligned arcs coincide in space and time with the steep gradient in antisunward velocity. The 630.0 nmelectron impact component, and simultaneously-measured incident electron-flux enhancement,

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both begin where the antisunward velocity begins to decrease (in the earth-sun frame of reference), and end where the antisunward velocity component stops decreasing (and may begin to increase again). This spatial coincidence applies to both arcs clearly visible in the ASIP. This correspondence between velocity gradient and enhanced electron flux is seen for several other electron flux enhancements in Figure 5.15b within the polar cap. We might reasonably speculate from this similar electrodynamic correlation that these features might also be sunaligned arcs. However, without simultaneous ASIP-data we do not know the horizontal extent of these electrodynamic features across the one-dimensional sub-orbital satellite track, and cannot know that they are sun-aligned arcs.

5.3.3

Incoherent-Scatter Radar Verification and Calibration

If it were possible to measure electric fields, or plasma flow patterns, over a large horizontal area, such measurements could provide an important independent verification of this descrip tion, as well as an important diagnostic tool. In fact, there is a way to map horizontal two-dimensional plasma flow fields, by using incoherent-scatter radar (ISR). An example of such data collected by the Søndre Strømfjord ISR is shown in Figure 5.16. These data were collected by scanning the azimuth of the radar in a full circle at a fixed 45-degree elevation, and then projecting the data on this conical surface down onto a horizontal plane. Wherever the electron density is sufficiently great to give useful radar echoes, data are obtained; where no data are shown, the electron density is below the lowest density contour shown The contours are of electron density in uniform steps of 0.2 from 0.4 to The radial lines are line-of-sight ionospheric plasma velocities, for the most part away from the direction of the sun, with a scale as shown in this figure. The circular feet of the radial lines identify the location at which the velocity component was measured. The chemical lifetime of ionization below 200 km is so short that for this stable electron density feature, observed to persist for over ten minutes in a stable configuration and location, we must recognize this as a region of stable ongoing production of ionization, by a flux of energetic particles. Thus, we must further conclude that this stable feature would be seen in optical impact excitation as well. If the simple electrodynamics we have described applies to this arc, we can examine the flow vector data to determine where we expect to find the incoming flux of enhanced ionizing energetic particles, i.e., the enhanced electron-density contours below 200km altitude. This means going through the plasma-flow vector data, point by point, to identify where successive vectors show a slowing of antisunward velocity, vs. constant or increasing velocity in going in the direction from dawn toward dusk in the earth-sun frame of reference. We find quite precise spatial and temporal coincidence of arcs with electric field and ve locity gradients of the right sign (see Figures 5.16 and 5.25). We, then, must also check for quantitative agreement with the magnitude of the current and its continuity. Calculations of horizontal conductivity and currents, and vertical currents, must also be done to establish con firmation. The results of these calculations (Carlson et al. 1988) quantitatively substantiate these findings. Based on a large number of arcs so investigated, we conclude that stable, spatially extended, sun-aligned arcs in the polar cap (and numerous other arcs as well) have the simple arc electrodynamics illustrated in Figures 5.13 and 5.14. With the starting-point assumption of Ohm’s law arc electrodynamics now validated, we next move on to detailed case studies that build to a great deal of additional knowledge.

5.3 Arc Electrodynamics

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5.4 Studying Arc Events Using Satellite Overflights

5.4

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Studying Arc Events Using Satellite Overflights

Early application of satellite data to the study of electrodynamic signatures associated with sun-aligned arcs contributed significantly to establishing understanding of the character of electrodynamic structures associated with polar cap arcs. Interpretation of experimental data by Saflekos and Potemra (1978) and theoretical work by Chiu and Cornwall (1980) were early contributions. Further experimental studies, while valuable, were limited to assuming the presence of sun-aligned arcs in the absence of imaging, or, when imagers were used, limited to kR arc events viewed as applicable to abnormal conditions present only a few percent of the time (Burke et al., 1979, 1982). Once it was learned that sun-aligned arcs were present half the time, and characterized northward IMF polar cap conditions, more systematic study was planned. In the following section we will demontrate that ASIP, combined with satellite data, can be a valuable dia gnostic tool for studies of the polar cap and its plasma circulation for that half of the time that the IMF is northward. Such studies are particularly valuable because northward IMF conditions are much more poorly understood than southward IMF conditions. In January 1982, simultaneous operations were scheduled for the NASA-operated Dynamics Explorer satellite, DE-2, and the AFGL-operated 155° field-of-view ASIP located at Thule, Greenland, geographic coordinates 76.5°N, 68.6°W (Weber and Buchau 1981, Weber et al. 1984). The findings, reported by Carlson et al. (1988), provide the basis for the discussion in this section.

5.4.1

Supporting ASIP Images

The photometer imaged emissions at 630.0 nm, from atomic oxygen and 427.8 nm, from with time interval of one image every 30s. Note that a near-earth satellite moves at about 7km per second. The imaging photometers have a complete dynamic range from 50 R to several kR, with an adjustable gain that permits a dynamic range of about a factor of 20 in any given image. For an assumed emission height of 250km, the useful image diameter is about 1200km. In addition to bright auroral features produced by precipitating electrons with energies in excess of 1 keV, the sensitivity of the ASIP allows detection of emissions produced by precipitating electrons with characteristic energies < 500 eV and energy fluxes Coincident observations by a zenith-looking Ebert-Fastie spectrometer allow absolute calibration of the ASIP images when they viewed the same emission feature. We refer back now to Figure 5.15 for data gathered after a prolonged period (~6 hours) of northward IMF. The thin lines show the orientation of the dawn-dusk and noon-midnight period meridians. The 630.0 nm emissions are elongated along the noon-midnight direction. We shall now examine the electrodynamic properties of this feature in detail.

5.4.2

In Situ Measurements by Dynamics Explorer, DE-2, Satellite

Simultaneous observations of the energetic plasma environment and the thermal ion drift velocity from DE-2 enable us to investigate the electrodynamic properties of these arcs in some detail. The low-altitude plasma instrument (LAPI) (Winningham, Burch, Eaker, Blevins and Hoffman 1981) is an energetic-particle detector that measures the flux of electrons and ions in the energy range 5 eV to 40 keV and at various pitch angles. The component of the horizontal ion drift velocity perpendicular to the satellite track is measured by the ion drift meter (Heelis, Hanson, Lippincott, Zuccaro, Harmon, Holt, Doherty and Power 1981) to a precision of better than 25 m/s. The second horizontal component of velocity is unfortunately not available during this period. Nevertheless, the electrostatic potential distribution along the satellite track can be approximately derived from the one available drift component (assuming

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the other is zero), and the orientation of the satellite track relative to the sun-aligned arcs allows the derivation of convective flow contours with some confidence. For the detailed comparison of the optical and satellite measurements, the position of the satellite at 750 km was mapped along magnetic field lines to 250km altitude, and projected into the 630.0 nm image of Figure 5.15. Two sun-aligned arcs are readily apparent in the ASIP field of view: one discrete feature, almost in the zenith, that decreases in intensity as it projects in the sunward direction; and a broader, more diffuse arc located approximately along the noon-midnight meridian. Images of the same features over the period 0630 to 0705 UT show their temporal as well as spatial coherence over this time period. Only minor variations occur in intensity and structure. Figure 5.17 shows the LAPI energy-time spectrogram of electron energy flux along the full high-latitude satellite trajectory during the ASIP observation period surrounding Figure 5.15. Energy fluxes of electrons and ions at pitch angles near 0° and 45°, respectively, are shown. The postmidnight auroral zone is identified by keV electron precipitation, from 0644:20 to 0649:00 UT. Beyond this boundary, the polar cap is identified by the presence of polar rain precipitation with a peak flux near 100 eV. Within the polar cap are clearly identifiable regions, between 0650:45 and 0652:00 UT, responsible for the polar cap arc emissions seen in the ASIP image. The spatial extent of the ASIP image covers the satellite trajectory time (0649:30 UT to 0652:30 UT) for a distance of about 1200km. The large-scale feature on the spectrogram located between 0654:00 and 0658:00 UT is the daysidc cusp precipitation region. Note that ion precipitation is seen coincident with the nightside and dayside auroral region, but not coincident with the several (about 9) narrow electron flux signatures isolated within the polar cap (about 0649:30 through 0653:40). This absence might at first sight seem to be taken as evidence that these features had no ion precipitation, in contrast to the Frank et al. (1986) observation of large ion fluxes coincident with electron fluxes in theta aurora. The data do not support this conclusion. Proportionate scaling-back of both electron and ion fluxes by the same factor, from theta aurora magnitudes to the magnitudes seen here for the electron fluxes, would put the corresponding ion fluxes in this case below threshold for detection by the LAPI instrument (Winningham et al. 1981). Figure 5.15 shows key parameters derived from the energetic electron distribution function along with the horizontal ion drift velocity from the ion drift meter. Also plotted is the electrostatic potential distribution along the satellite track, obtained by integrating the crosstrack horizontal ion-drift velocity. Note that increases in the particle flux with energies near 100 eV, which are visible between 0650:45 and 0652:00UT, are more clearly resolved in the top panel as increases in the total electron energy flux above the background steady polar rain. Comparison with the optical image in Figure 5.15 shows that only the more energetic precipitation regions (arcs A and B), which exceed produce arcs that appear in the ASIP image. Careful examination of the full sequence of images also showed a narrow arc, which was excited by the flux increase at 0650:45 UT, and which faded at its sunward edge, retreating antisunward. The precipitation flux of electrons had turned off, and the visible arc was fading with the 630.0 nm lifetime of about a minute (the Einstein coefficient lifetime but including quenching effects), so the ASIP was seeing the “fossil” of the arc. Later we shall note that arcs often are seen to turn off by extinguishing from the sunward end toward the antisunward end. Arc extinction has been studied by Rodriguez et al. (1997). In the theory section it will be noted that this can be a logical consequence of a southward turning of the IMF.

5.4 Studying Arc Events Using Satellite Overflights

5.4.3

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In Situ Signatures

The orientation of the satellite track with respect to the noon-midnight meridian enables us to easily distinguish flow with a sunward component from flow with an antisunward component. These flow directions are indicated in the ion drift panel. We notice immediately that the horizontal ion drift signature could be quite easily characterized as irregular, or structured, or even turbulent. In fact, we shall see later that there is considerable order within this apparent turbulence. At this stage, we simply note that a gradient in the horizontal ion velocity is always observed across each observation of a particle flux enhancement. Across arcs A and B, the gradient involves a change in direction of the velocity perpen dicular to the arc components in the cross-track ion drift that is measured. The uniformity of the emission feature along its length suggests that the ionospheric conductivity, particle precipitation, and ion velocity gradients are similarly uniform. The flow velocity along the arc

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Morphology of Polar-Cap Sun-Aligned Arcs

changes by about 400 m/s (20 mV/m) across the arc, which is about 160 km in width. The electric-field gradient is therefore If we assume that the plasma flow across the arc (dawn-dusk direction) is constant, and the Hall conductivity is uniform, we find that every electron-precipitation peak corresponds to a repeatable signature. The signature is a divergence of horizontal current giving an upward Birkeland current, an electric potential dip (divergence of E < 0), and a gradient of plasma flow across the arc, which decreases from the dawn to the dusk side. For every arc, the gradients in the ion-drift velocity are always in a manner that requires the divergence of E < 0 and a corresponding upward field-aligned current. For the arc A that is visible in the image, we have calculated the ionospheric conductivity produced by the precipitating electrons, also using the numerical code described by Weber et al. (1989). Such a procedure yields values of 0.6 mho and 0.3 mho for the Pedersen and Hall conductivity, respectively. In arc A, ion velocities and current imply a field-aligned current of if the Pedersen conductivity is 0.5 mho, as indicated in panel (c) of Figure 5.15. These values agree well with the field-aligned current carried by the precipitating electrons and the conductivity that they produce.

5.4.4

Geophysical Noise in Arc Detection

Note particularly that every arc detected by the ASIP had signatures in several in situ meas ured parameters. Which of these were most sensitive? The ASIP sees only two clearly defined arcs in the window 0650:50 to 0651:55 UT. Any other arcs present are lost in the background 630.0 nm geophysical noise. This optical background is induced by both polar rain and iono spheric recombination airglow. Near 0650:50 and 0651:20 and :25 are three more in-situ signatures that could be sun-aligned arcs. We cannot say, because without an image we can not know for sure if they extend significantly more in the sun-aligned direction that they do in the dawn-dusk direction (strongly anistropic in shape). However, within the context of a great body of data it would appear that these, too, are likely sun-aligned arc in character. If that were so, then the particle- and cross-drift plasma velocity, and dips in electric po tential, arc more sensitive detectors that the optical images in 630.0 nm, against background geophysical fluctuations, i.e., geophysical noise. Note likewise the cross-track fluctuations in plasma drift velocity of Figure 5.18. If one could establish that these dawn-dusk fluctuations were all strongly anistropic in direction with respect to earth-sun alignment, this could be an interesting tool for study of the energy spectrum of polar plasma “stirring”.

5.4.5

Small-Scale Flow Reversals

The calculated electrostatic potential distribution along the satellite track, shown in the bot tom panel of Figure 5.15d and e, can be used to generate a convection pattern associated with these sun-aligned arcs. The completion of convection paths at distances away from the satellite track can usually only be based on cumulative experience from many satellite passes and from theoretical models. In this case, however, we are able to make additional use of the simul taneous image taken by the all-sky camera. Wo have noted that significant velocity gradients are associated with discrete emissions that extend throughout the ASIP field of view in the noon-midnight direction. The coherence of the emissions in two dimensions implies a similar coherence on the same spatial scales in the ion velocity and associated convection pattern. Using this information, the derived convection pattern for this pass is shown in Figure 5.18. The cells labeled I and II may be identified as those usually expected from dayside merging for a southward IMF. The relative cell sizes and their geometry are also consistent with models and observations of this process for the negative component of the IMF that exists for

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these data (Crooker 1979, Heelis 1984). These cells are shown by dashed lines, since none of their flow contours pass through the ASIP field of view, and only our prior experience is used in completing the closed loops. Within the large dawnside convection cell, the existence of at least three discrete convection cells, or large-scale “fingers”, is suggested. These cells all circulate anti-clockwise in the same sense as the dawnside cell II. They have been labeled III, IV, and V. We note that all these cells have flow paths that pass through the ASIP field of view. The boundaries between the cells mark locations where divergence of E < 0, and where the discrete emissions and upward field-aligned currents are observed. The arcs A and B are shown by the shaded regions B and D. Note also that the electrostatic potential distribution shown in the bottom panel of Figure 5.15 indicates that each of these boundaries corresponds to a local minimum in the potential. Finally, it should be pointed out that there exists some ambiguity in the interpretation of possible convection trajectories on the dayside of the ASIP field of view. We have illustrated two possibilities (shown by dashed and dotted lines) that allow for self-contained individual circulation cells, or for a single large cell with multiple bifurcations or fingers. This ambiguity exists because the illustrated flow lines in cells III, IV, and V are all on the same potential. If the emissions are contained within a narrow self-contained convection cell, then the flow velocity across the arc must change direction at some point along the emission. Such gradients may exist and be responsible for the changes in luminosity of the features. As noted before in the calculation of electrodynamic quantities, we have assumed that the plasma flow across the arc is constant, and additionally that it is comparable in magnitude to that of, or smaller than, the adjacent flow parallel to the arc. Also note that no flow across the arcs is shown in the Figure 5.18, although we have assumed that a constant flow velocity might exist. The convective representation simply implies that the flow parallel to the arc is larger than the flow across the arc. If the flow across the arc, in the direction perpendicular to the arc, is the same as that of the apparent dawn-to-dusk motion of the arc, then the arc represents an adiaroic line as described by Siscoe and Huang (1985).

5.4.6

Small-Scale and Large-Scale Flow

We will consider one further case study of this nature, to illustrate a meaningfully different background polar convection pattern, within which the same approach leads to arc structure of the very same nature. The same analysis procedure was applied to a DE-2 satellite overpass ten orbits later on the same day, leading to the results shown in Figure 5.19. Figure 5.19 shows large-scale convection paths that are consistent with these discrete emis sion features. Within the ASIP field of view, plasma flow contour lines are defined by the observations. Outside the ASIP field of view, plasma flow contour lines can only be speculat ive, but must be consistent with the satellite data. The flow contours are connected between the ASIP viewing region and the satellite pass by preserving the flow characteristics seen by the satellite. In this period of strongly northward IMF, cells I and II may result from viscous interaction at the magnetopause. At very high latitudes, clockwise circulation in cell III is associated with magnetic flux tubes that thread the magnetotail lobes. The large-scale cir culation in this cell is consistent with the positive Y component of the IMF existing at this time. Notice also that the high-latitude circulation is reversed from the previous observation, component. reflecting a reversal in sign of the IMF These data demonstrate the potential for such studies inherent in combined ASIP and satellite in situ overflight data. It will become clearer later, in the theory chapter, that any trans-polar satellite pass must be penetrating sun-aligned arcs half of its flight time across the polar cap. There is a wealth of data.

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5.5

Studying Arcs with Incoherent-Scatter Radar (ISR)

In this section, we illustrate the power of ISR data for sun-aligned arc studies, provided that the observing mode is tailored for this purpose. While the key examples presented here deal with an intense arc, the focus is on techniques and principles learned that apply to sun-aligned arcs in general. This section logically divides into two halves. The first half, Sections 5.5.1 through 5.5.5, treats ISR imaging and directly-observed parameters. Sections 5.5.6 through 5.5.10 treat the many physical parameters that have been derived from those directly-observed ones, based on physical laws, relationships, and models.

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To establish the presence of a sun-aligned arc, instead of assuming its presence, requires a two-dimensional map. It is possible to produce such a map with an incoherent-scatter radar, using conical scans projected to a horizontal plane. In this section, we describe the basis for incoherent-scatter radar mapping, and the findings from its application (Valladares and Carlson 1991, Carlson 1996).

5.5.1 Imaging Entering the 21st century, there are two incoherent-scatter radars capable of mapping sunaligned arcs, one in Greenland, and one in Svalbard. The Søndre Strømfjord incoherent-scatter radar (66.99°N, 50.95°W, 74° invariant latitude), in operation since 1982, is capable of probing

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the nighttime polar cap and examining the dynamics of polar sun-aligned arcs (Carlson et al. 1984, Robinson et al. 1987, Mende et al. 1988). The other is the EISCAT Svalbard radar (ESR) near Longyearbyen, in operation since winter 1998, at about the same MLAT. While it is difficult, if not impossible, to recognize sun-aligned arcs with conventional ISR observing modes in the absence of coincident optical imaging data, it is possible to design a radar mode that can unambiguously recognize them. Such a mode is possible for the following reasons. First, electric fields map along magnetic field lines above altitudes where ion-neutral collisions affect ion mobilities. In addition, the arcs are relatively slowly changing. And finally, the arcs are characterized by a particular geometry for their velocity and for their electron-density distribution. We describe the design here, illustrated in Figure 5.20. The design is based on what we know of arc morphology from optical imagery. The essence of the design is simple. We want to map the electron-density profile and velocity field over an area, with the area large compared to the arc, and with time resolution short compared to the time for the arc to pass through the ISR field of view. Signal to noise (S/N) considerations determine look elevation and area of coverage, and limit azimuth scan rates and cycle time resolution. We generally need and ~ 10 000 samples for 1 to a few percent statistical accuracy of measured parameters. At inter-pulse-periods (IPP) of ~10ms typical of past ISR data, this requires ~100 seconds. New

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ISR techniques can bring this down to ~10 seconds or less. Variability in the motion and lifetime of sun-aligned arcs (Weber et al. 1989) also imposes some restrictions. For this reason, the azimuth sweeps (AZ) at two different speeds. One AZ scan is rapid, to obtain a snapshot of the electron density inside fast moving arcs. Two slow AZ scans sweep 90° in 2 min, probing slower-moving arcs and measuring the line-of-sight (LOS) velocity and the number density with smaller error bars. An EL scan completes the volume scanned by the ISR, shown schematically in Figure 5.20. In general, the intersection of a vertical plane with a conical surface is a parabola. At these high latitudes, B within ~10° of vertical, and for sun-aligned arcs with features narrow relative to the radar range, arcs are repeatedly seen as just that on the raw radar scan. Signatures are far more rich however, as will unfold in the rest of this section.

5.5.2 Mapping The technique of horizontal-projection mapping is illustrated in this section using data taken with the Søndre Strømfjord ISR during February/March 1987. Good horizontal trans-arc spatial resolution was also achieved (15km at 150km of altitude when pointed across the arc, and ultimately limited to the 2.5km radar beam width smeared over the integration time when pointed along the arc). Figure 5.16 shows electron densities within, and line-of-sight (LOS) plasma velocities across the arc, as mapped by the ISR. The ISR was scanning at a fixed elevation of 35 degrees, through 180 degrees of azimuth, from directly away to directly toward the sun. The plasma properties mapped by the ISR are projected onto the ground in this figure, as mapped from 0231–0239 UT. The velocity decreases from 1 km/s antisunward on the eastward edge of the arc to 0.2 km/s sunward on its westward edge. To better see how an ISR can image sun-aligned arcs, consider the following consequences of aeronomical sciences. First, this detection of a sun-aligned ridge of significant ionization much below 200km is equivalent to detecting an optical sun-aligned arc. This equivalence is because the chemical lifetime of ions below 180 to 200km in the ionosphere is very short. Much below 200km, the primary ions are molecular. They recombine rapidly with thermal electrons by dissociative recombination at a rate of This chemical reac tion leads to chemical lifetimes of seconds to minutes. Consequently, ionization below 200 km is a direct indication of freshly-produced ionization. If the ionization is oriented sun-aligned, its production source is sun-aligned. The only dark polar cap source of ionization production is precipitating electrons, so this ionization below 200km indicates a sun-aligned sheet of elec tron precipitation of energy greater than 100 eV. This sheet must produce a sun-aligned optical emission closely co-located with magnetic field lines through the ionization enhancement below 200 km. Note that this ionization below 200 km is in sharp contrast to F-region ionization, for which direct recombination of electrons with atomic oxygen ions is times slower, with chemical lifetimes of hours. Recombination typically requires the ionization to fall under gravity to and for the atomic oxygen ions to form and altitudes where there is sufficient which then can rapidly recombine with electrons, by the dissociative recombination just discussed. The presence of electron densities much exceeding at altitudes below 180km, is greater than can be maintained by downward transport of ionization, and must be a direct indication of presently-ongoing direct production. This ongoing production of fresh ioniza tion, attributed to a particle precipitation flux existing at the time of observation, must then be accompanied by not only impact ionization, but impact excitation of optically-emitting states. Since the relative cross-sections for electron-impact production of ionization and vari ous optically-emitting excited states are well known, one can calculate values from these

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relationships. For production rates leading to ion densities below 180–200 km, optical omission intensities would be produced that would be detectable by ASIPs (sensitivity threshold tens of R). Since the pertinent recombination rate is the product of the number of electrons per available to recombine, the square of the electron density times the number of ions per (fourth power is a measure of the precipitating-electron flux producing the ionizatioii (av erage ~36eV per ionization). The sun-aligned ridge of electron density exceeding below 180km in Figure 5.16 is found to be ~1 kR as observed during the period of Fig ure 5.23a,b (~100 times the few tens of R threshold for detection). As separate confirmation, the pair of sun-aligned arcs imaged from both the ground-based ASIP and the Polar Bear satellite shown in Figure 5.23b was the very same arc imaged here by the ISR in Figure 5.16. Thus the ISR scan, as a stand-alone sensor, establishes the existence of an arc, seen to be sun-aligned over 600km extent (the 630.0 nm ASIP extends this limit to a 1000km extent). The electron density well below 200 km offers an estimate of the intensity that should be ob served, and the altitude of the peak electron density provides an estimate of the characteristic energy of the precipitating particles. That the particles are electrons must be deduced from the context of many sets of coincident ISR, ASIP, and satellite data (Carlson et al. 1988). The electrodynamics of the arc will be discussed later. This arc penetrated to the E-layer, with enhanced at altitudes as low as 120km, and was detected near local midnight. Its southernmost region was observed to merge with the poleward edge of the auroral oval during the early part of the arc transit. At the time of the experiment, the background density in the polar cap F region was a value well below the lowest density-contour level shown in Figure 5.16 or any similar figures presented in this section. We shall next view this arc with another observing mode, which proves startlingly valuable in allowing extraction of highly illuminating properties about the thermal and energetic nature of the arcs, and even smoking-gun evidence as to the direct driver of at least intense sun-aligned arc formation.

5.5.3

Deriving Cross Sections

For a stable arc, moving slowly and steadily, another ISR observing mode can be very valuable. It looks directly up the magnetic field line so as to give true instantaneous altitude profiles within a sun-aligned arc. Such time-continuous observations during the full passage of an overhead arc have allowed detailed diagnosis of the properties of a transverse section of an arc, as we will now discuss. Let us illustrate this observing mode and findings by examining Figures 5.21 and 5.22, obtained between 0204 and 0222 UT on February 26, 1987, as the arc passed overhead with the antenna parked in the direction up B, the magnetic field. An integration time of 1 minute was selected in order to provide temperature error bars smaller than 10% at a 200 krn altitude. The arc motion across the radar beam cannot be assumed constant. However, from ISR scans before and after, and coincident ASIP data, we know that the sun-aligned arc moved westward (toward dusk) at a relatively steady 100 m/s in the earth co-rotating frame. This same arc is shown at earlier and later times in other figures in this section. It is found from ISR and ASIP data to have rather stable electron density and width for over half an hour as it drifts dawn-to-dusk. Vertical profiles of and are superimposed on the density contours in Figures 5.21 and 5.22, respectively. In the center of the arc, the ion temperature is 500 K in the E-layer and sharply increases in the dawnside edge of the arc, reaching about 700 K at F-region heights.

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1300 K, and exceeding in both the E- and F-layer altitudes. The electron temperature shows a pattern with a significant difference, with enhancements of 500 K, colocated with regions of higher bottom-side electron densities (more intense precipitation). This behavior is reflected ratio is observed to decrease from > 1.5 to 0.8 near in Figures 5.21 and 5.22 where the contour, drifted across the dawn edge boundary. The sun-aligned arc, defined by the the radar beam in 10 minutes. Taking the average westward speed of 100m/s in the earth frame, leads to a width of about 60 km for the sun-aligned arc, in rather good agreement with the contours from the rapid EL scan, cut across the arc, as per Figure 5.20. The maximum density was at 130 km of altitude. Important physical insights from these data will be discussed below, when we review parameters derived from the data, vs. the directly-observed parameters presented here. There is consistency of arc pair features with atomic vs. molecular impact emissions and ionization. The Polar Bear imaged the arc pair in the atomic oxygen line 135.6 nm UV emission (Figure 5.23b). The ASIP imager showed the arc pair (Figure 5.23a) in the F-region 630.0 nm emission, but only one arc in the E-region 427.8 nm emission, demonstrating that only the dawnward arc contained sufficiently energetic electrons to penetrate to E-region heights. The ISR profiles showed exactly the same situation, two F-region and one E-region arc. Because the UV images from the Polar Bear satellite are atomic oxygen and thus F-region emission lines, they also saw a pair of arcs, as expected. The persistence of the arc pair features indicates long-term stability of the source of the arc. In Figure 5.23 a and b, east is to the right, and the atomic oxygen 630.0 nm emission, due to soft electron precipitation, shows a pair of arcs, one on either side of overhead the co-located ISR. The 427.8 nm emission, due to harder electron precipitation, appears only to the west of the ISR, indicating that the weaker trailing arc (both were moving westward during this event) was of a softer electron flux. The stronger arc was also visible to the naked eye. In Figure 5.23b, the same arc (pair) is seen in the atomic oxygen 135.6 nm UV emission, as viewed from the Polar Bear satellite, 26 minutes later when the same arc pair had drifted a few hundred km duskward. The edge of the yellow area marks the 0.3 kR contour. Continuous observation of the arc with the ASIP used to gather the data of Figure 5.23a, as well as the Søndre Strøinfjord ISR, showed the arc to move relatively continuously westward. The UV is not visible from the ground, as this wavelength is absorbed in the atmosphere well above the earth’s surface. The EL scans through the arc pair show the relative stability over the 45-minute interval 0202–0247UT, February 26, 1987 (Figure 5.24), during which the ISR was able to track this arc by successive EL scans as it drifted to the west (duskward). Note the geometry of the field-aligned cross-section of the horizontal arc line intersecting the conical scan surface: (1) greatest electron-density contours, which here are below 200km altitude (short-lived E-region plasma due to ongoing production of ionization of a live arc) are horizontal cigar-shaped; (2) while the lesser electron-density contours above 200 km (long lived F-region plasma transported horizontally, largely along the are axis, from upstream of the E-region arc below) are wall-shaped.

5.5.4

Mesoscale Dynamics

Successive images such as described in Section 5.5.2 can provide, in effect, movies of the electrodynamics and energetics of arc dynamics, for changes on the time scale of the radar scan repetition rate. The sun-aligned arc pair 150km west of overhead in Figure 5.25 merges into a single wide are by the time it is revisited by the ISR. That is, the return current sheet dividing the arc pair is displaced to outside the arc. Figure 5.25 clearly shows a sun-aligned arc 150km to the west of overhead the ISR. Closer

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inspection shows additional electron-density enhancements along a line 70 km west of the ISR. This sun-aligned arc is so close to overhead that it appears as two ovals as it pierces the ISR conical scan surface. Note that the penetration of a small horizontal cylinder, near the vertical axis of a cone, yields a matched pair of ovals, vs. the elongated slice near the side of a cone. This pair of electron-density enhancements lies along a line also pointing toward the sun. Simultaneous ASIP images confirm a double sun-aligned arc. The dawnward edges abut relatively rapid antisunward flow. Their width coincides with decreasing antisunward plasma flow, as Ohm’s law electrodynamics would demand. They are manifestations of the production of thermal plasma and optical emissions by the Birkeland current carriers, the energetic electrons producing impact ionization and excitation. Now consider that this arc pair has a double reversal of plasma flow velocity coincident with the sub-200 km electron-density enhancements for this double sun-aligned arc. Going from the dawn to the dusk side of the arc pair, the plasma flows away from the sun, then toward it, then away again, then almost zero flow, and finally away from the sun once more. The onset of decreasing antisunward plasma flow is on the dawn edge of each of the two arcs, as measured by the sub-200km electron-density enhancement, and as anticipated from Ohm’s law. It follows, then, that the 30km gap between the two upward current sheets must be filled with a downward current sheet, presumably carried by an upward flux of thermal electrons. In general, return currents from the incoming electrons producing the sun-aligned current sheets cannot be seen by the ISR or the ASIP. However, satellite-borne magnetometers of sufficient

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sensitivity should see them. Data from the next AZ scan of the ISR are significant to arc dynamics. The central return current sheet that is required by the sunward channel of plasma flow to separate the pair of antisunward flow areas dawnward of the pair of electron density enhancements, was gone seven minutes later. The pair of arcs were replaced by a single arc the width of the sum of the two arcs, that is, with the gap filled in. This merging of two thin arcs to one thick arc was observed with both the ISR and the ASIP.

5.5.5

Directly-Observed ISR Parameters

ISR directly-observed parameters are electron density, electron and ion temperature, and plasma line-of-sight velocities. The electron densities show persistent sun-aligned ridges of enhancements. Contours of constant electron density are sun-aligned over mesoscale distances of 500–1000 km for altitudes between 100 and 200km. Electron temperatures are enhanced along magnetic field lines above these enhanced ridges of ionization, consistent with the ex pectation of heating by a (sheet) flux of incident energetic electrons. On the dawnside of the sun-aligned arc, there is a channel of high ion temperature, exceed ing that of the electron gas. This channel of ion heating coincides with maximum antisunward plasma flow speeds. Moving dawn to dusk into the arc, the ion temperature falls to equilibrium with the neutral particle gas, and only the electron gas has significantly enhanced temperat ure. Antisunward velocity gradients are coaligned with the sun-aligned contours of enhanced electron density below 200km altitude. That is, where the antisunward plasma flow velocity decreases in going from dawn toward dusk, ionization is enhanced; when the dawn-to-dusk plasma velocity gradient has the opposite sense (increases dawn to dusk), or is constant, there is no enhanced ionization below 200 km. Convergent electric fields produce converging Peder sen currents, whose continuity is maintained across the arc by incident energetic electrons.

5.5.6

Derived Parameters

Parameters derived from ISR observations are Pedersen and Hall conductivity, horizontal and Birkeland currents, E-field maps, and Poynting and particle flux. The conductivities are derived from electron-density maps, combined with model neutral densities, using estimates of molecular ion composition with their different collision cross sections. We estimate steadystate currents within the arc from the plasma density and velocity data. E-fleld maps have already been discussed, as have particle flux estimates. Poynting flux has historically been difficult to measure, so we discuss this measurement in greater detail in the section below. An example of all these derived parameters is illustrated in Figure 5.26 for the arc shown in Figures 5.21 and 5.22. From altitude profiles of electron density, and knowledge of the neutral atmospheric density profile, we can calculate the two components of the conductivity that controls the current flow in the presence of electric fields. It is common to use a model for the neutral atmospheric density, a practice adequate for most purposes requiring E-region conductivity calculations. For the extremely low electron densities, and thus low conductivity, sometimes found in the nighttime polar cap, the net overhead conductivity may depend significantly on the F-region electron density and thus conductivity. For this situation, models for the neutral atmospheric density become much more problematic, because of the large variability of F-region neutral densities, and thus the difficulty of adequately modeling F-region conductivities within the poorly-understood polar cap thermosphere. E-region conductivity and ion mobility are sufficiently rich and complex in their details, that the reader is referred to Rishbeth and Garriott (1969) for a thorough treatment of the

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general case, and to Valladares and Carlson (1991) for specific application to polar cap arc conditions. Valladares and Carlson describe how E-region electron densities and relatively reliable neutral-density model densities were used to derive conductivity profiles across polar cap sun-aligned arcs. These conductivity profiles arc then used with electric field vectors, derived from plasma velocity vectors, to calculate horizontal and then Birkeland currents. The Birkeland currents were calculated by two independent means. First, the electron density contours of a cross section of the arc (found to match the density contours several hundred kilometers upstream and downstream and on earlier and later measurements) were used to estimate the steady-state production rate of the arc, and hence the upward current above the arc carried by the estimated incident energetic electron flux. In the second method, the horizontal electric-field gradient across the arc was derived from the plasma-velocity gradient across the arc. This electric field, applied to the ionospheric con ductivity (derived from observed electron density profiles and a standard model atmosphere), led to derived Pedersen and Hall currents. Based on arc symmetry upstream and downstream, transarc gradients in calculated Pedersen currents were then used to derive (from the hori zontal divergence of currents across the arc) the variation of Birkeland currents across the arc. The upward Birkeland currents derived from these two different approaches were comBecause downward parable, and both peaked near the center of the arc at about Birkeland currents (presumably carried by upgoing thermal electrons) can be calculated only where electron densities are large enough to give a useful ISR signal, complete current map ping cannot be done by this technique. However, this approach gave a rather comprehensive estimate of all but this ionospheric current system in and near the observed stable sun-aligned arcs. This estimate should extrapolate well to such arcs in general. A sample actual data set is illustrated in Figure 5.26.

5.5.7

Non-Particle Heating

We can perform three independent calculations of the energy associated with the sun-aligned arc by non-particle heating, by calculating Poynting flux, ion gas heating rate, and ion gas cooling rate. While this non-particle energy input is distributed across the full arc, it is strongest into the dawn edge. The three calculations are motivated by recognition that energy external to the ionosphere (ultimately mechanical energy in the solar wind) is carried down into the ionosphere along magnetic field lines as electromagnetic energy or Poynting flux. This energy is dissipated as frictional heating of the ions dragged through the neutral atmosphere, or Joule heating. Heat, initially deposited mostly in the ion gas, is rapidly passed onto the neutral gas. The basic idea is that we first calculate the electromagnetic or Poynting flux energy incident on the top of the ionosphere. Next, we calculate the dissipation of this energy into the ionospheric ion gas by frictional drag or current Joule heating. Then, finally we calculate the energy lost by this ion gas to the ultimate energy sink, the neutral upper atmospheric gas. Each of these calculations is based on independently-measured parameters, so each of the derived values is an independent calculation of the same energy flux, just seen at different stages of its passage from the solar wind/magnetosphere above to the neutral atmosphere below. Whether these independently-derived values are equal to each other is an extremely rigid test of the validity of the answer. Alternately, it endorses the premise of the calculation. The first calculation follows the rationale of a direct dE × dB Poynting flux calculation. We note that the sun-aligned arc changes much more slowly along than across its axis, and is very stable over times large compared to an Alfvén bounce period. Neglecting the spatial and time derivatives then, we take a cross section transverse to the arc, and slice it into

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a series of adjacent differential elements. We look at the electric field differential (dawn to dusk reduction) and Pedersen current differential across each element. Remember that the electric field is in the measured neutral-wind rest frame. The current is then crossed against the electric field strength to derive the first estimate of energy, plotted in Figure 5.26j and labelled Poynting flux. Relative to an idealized measurement of Poynting flux, this calculation has its shortcomings. Yet we feel it is useful, partly to underscore the real energy source, and partly because of the good quantitative agreement between the found from this calculation and from the following two. The Poynting flux calculation in Chapter 6 is for the net downward energy flux into the arc. In quasi steady state, the net energy flowing into the arc must be balanced by an equal rate of energy flowing out of the arc, mostly to the thermosphere, by collisions of plasma particles with the neutral particles within which they are immersed. True, some of the energy is lost to photons escaping to the ground and space. These photons are critically important to our ability to remotely sense the arcs. However, these photons carry off so small a fraction of the net energy into the arc, that leaving this term out of the energy balance introduces less error than the measurement uncertainty. The second calculation is based on the additional measurement of the ion gas temperature. Given the measured value of the plasma velocity and the neutral atmospheric velocity, we can calculate the rate at which heat should be going into the ion gas, due to the relative motion of the ions through the neutral gas (Figure 5.26i). This calculation leads to the second estimate of energy flow, and is verified by the good agreement between the calculated and the actually observed ion temperature. Note that this calculation is performed only in the edge of the arc, where there is negligible electron-gas heating by precipitating particle flux. The third calculation deals with the heat loss from the ion gas to the neutrals. It is based on the measured ion gas temperature and the neutral gas temperature. Because of the cooling-rate cross-section dependence on ion composition, which we do not measure, we show two values for the extremes of100% versus 100% This third energy estimate agrees well for the ion thought to be more likely dominant, and poorly for the less likely dominant ion. Note the consistency of this with directly measured ion composition in Figure 5.32, for another strong sun-aligned arc. It is important to note a critical step in the above argument before proceeding. We note that can be derived in two complementary independent ways, one (theoretically) from measured ion and neutral particle velocities, the other directly from the radar-observed spectral shape. Note that ion velocities are derived from the spectral center of gravity, that is, the Doppler shift, not the spectral shape. Our calculations of the differential temperature based on the ion and neutral velocities show good agreement with the observed enhancements (see Figures 5.26 and 5.27). The Fabry-Perot also measured enhanced neutral temperatures, possibly co-located with the region of larger ion temperatures (Niciejewski, Mehwether Jr., McCormac, Hecht, Christensen, Sivjee, Strickland, Swenson, Monde, Vallance Jones, Gattinger, Carlson and Valladares 1989). The observed ion heating can be explained solely on the basis of a quantitative analysis of relative to the observed neutral atmospheric rest the large observed plasma flow velocity frame U. Since all three of these parameters are observed, not modeled, we are thus able to confirm by measurement, not merely assume, that these ion temperatures are explained solely on the basis of this heating mechanism, within only small statistical uncertainty. It is this use and that allows us to validate calculation of the rate of heat of the directly-observed flowing into the thermosphere on the basis of a steady-state ion thermal balance argument.

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5.5.8

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Polar Thermospheric Thermal Balance

We have applied an approximate energy balance calculation to these data. We reasoned that it was logical to expect a magnetospheric energy source to drive the high-speed ionospheric plasma flow through thermospheric frictional-drag loading. We emphasize that essentially all this energy dissipation should be ultimately deposited in the neutral atmosphere below the enhanced ionospheric densities in the arc. Minimal energy is lost to the optical emissions, important though they be to diagnosing the arcs. It was possible to estimate this latter quantity in two ways: (1) from the steady-state rate of heat going into the ion gas; and (2) from the steady-state rate of heat lost from the ion gas. The former is derived from the conversion of ordered ion gas energy due to coherent motion under the action of the driving electric field, to disordered energy (heat) as the ions suffer randomizing collisions with neutral particles. This energy loss is proportional to the square of the difference between the ion and the neutral gas bulk velocity (Stubbe and Chandra 1971) (see Chapter 6). The latter is derived from the observed difference between the neutral atmospheric and ion gas temperatures. Uncertainty in the ion composition leads to greater uncertainty in this latter estimate than in the former. However, both of these methods give a value for the energy deposition rate into the neutral atmosphere of a few ergs comparable to the Poynting flux into this region. These three independent calculations thus lead to a simple but reassuringly self-consistent interpretation of the data. The ability to reproduce very similar values for the energy, by calculating it at different points in its flow, using completely independent measured paramet ers, gives us considerable confidence in both the techniques and the results. The comparison is summarized concisely in Figure 5.27 for the February 26 data. For comparison purposes, like values derived for an independent data set on another day, March 1, 1987, are shown in Figure 5.28. Collectively, these calculations establish a nominal value for an energy deposition rate into the polar upper atmosphere due to sun-aligned arc processes. Let us now consider the geophysical significance of this magnitude for the heating rate. The Poynting flux and energetic-particle flux deposit energy to the neutral atmosphere near and above 120 km of several ergs over the sun-aligned arc of width of order 100 km. This energy deposition compares to about for a global mean thermospheric EUV heating rate. Parts of the arc experienced 10 times this reference average, and the average across the arc was three or four times this reference. Given that the arc is of order 100 km wide and persists for a few hours, it is a significant local heat source. It can be an important heat source even when averaged over its motion across the dark polar cap (of order 10% to tens of percent of the global mean EUV) and thus comparable to a presently missing polar thermospheric heating component (Carlson and Crowley 1989). While the February 26 arc discussed (Figure 5.27) is unusually intense, the polar cap under northward IMF conditions typically has several stable sun-aligned arcs, each of a tenth or more of this visual, particleprecipitation intensity, and spanning ~2000 km polar cap width, with temporal persistence of order an hour or more. Estimates based on measured particle-precipitation energy flux alone under these conditions may underestimate the thermospheric heating rate due to stable sun-aligned arcs by a factor of 3 or more. Note that for the March 1 sun-aligned arc (Figure 5.28), the particle precipitation energy is about a sixth that for the February 26 case (Figure 5.27), although both arcs have comparable incident Poynting flux. The Poynting flux into the arc is directly driven by the solar wind interaction with the ionosphere, via the magnetosphere, respectively functioning as the input energy, the load, and the active transmission media. The particle flux, representing the current system, is the response of the coupled ionosphere/magnetosphere to the driving force, and will be different for different ionospheric loading conditions.

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5.5.9 Variability Along Arcs Sun-aligned arcs can have gradients, bright spots, and other spatial structures along their length, which are of significance to the physics of their formation. We have stressed taking advantage of the stability of the arc, in time and in space along its axial (earth-sun) dimension, to optimally diagnose its character. Ultimately, temporal and spatial variations must occur and become large enough to be of interest during one series of arc observations. During the 40 minutes of observation of the February 26, 1987 arc, it drifted from overhead (0212 UT) to 100 km west (0225 UT) to 180km west (0234UT). A composite image of the arc was constructed from the many conical and planar cross sections of the arc to estimate departures from idealized constancy along its axial length. This analysis indicates a velocity gradient along the arc axis, with penetration of sunward-drifting plasma only part way into the polar cap. The two-dimensional cross section (Figure 5.29) suggests that plasma, from where the arc connects to the auroral oval, flows sunward with partial entry into the polar cap, then deflects clockwise with rotational flow back to antisunward, finally exiting again from the polar cap. Farther into the polar cap along the visual arc, there is slowing of antisunward plasma flow, but not a reversal to sunward. This flow is sketched schematically in Figure 5.30. This interpretation of ISR data is very similar to the interpretation of some AE-C satellite data by Hoffman, Heelis and Prasad (1985) under similar geophysical conditions. It is clear from Figure 5.30 that the net velocity difference from the dawnside to the duskside of the arc is greater in the velocity reversal region than the merely velocity reduction region. Thus, we conclude that the Pedersen current divergence, and balancing Birkeland currents, lead to extra energetic particle input within the tongue of sunward moving plasma, relative to the energetic particle input in the region of simply a velocity gradient deeper in the polar cap. Furthermore, while Hall currents along the dawn edge of an arc of constant properties along its axial length simply continue into the polar cap, those Hall currents in the region of partial penetration of sunward flow penetrate only partially into the polar cap, and then must rotate westward near their region of maximum poleward penetration. These effects can combine to produce the “bright spot” or extra particle precipitation often seen near where the sun-aligned arc connects to the nightside auroral oval.

5.5.10

Summary of ISR studies

Finally, we condense the essential character we have found as typical of the electrodynamic, thermal, and energetic properties of these sun-aligned arcs into the illustration of Figure 5.31. This character is based on observation of strong sun-aligned arcs. The extent to which these findings characterize the much more common weak polar cap arcs, ubiquitous for northward IMF conditions, is presumed for the electrodynamic and electron-gas thermal character. How ever, in the absence of high E-region electron densities, ion gas heating should not be presumed the same. The qualitative features of this cross section should apply to sun-aligned arcs, and even Ohm’s law arcs in general. Within the center of the arc, where is enhanced below 200km, must be enhanced by the incoming flux of impact-ionizing electrons. These precipitating electrons must also carry an incoming particle energy flux, and an outgoing Birkeland current, to one side, while there must be a return (incoming) current on the other side. The Pedersen currents must decrease from dawn to dusk, as the antisunward plasma drifts must decrease from dawn to dusk. The plasma drifts may simply decrease, or reverse as in this illustration. The ion temperature will be enhanced where the difference between the plasma and neutral atmospheric velocities is sufficiently great, and ion-neutral collision frequencies significant. The ion temperature enhancement depends on the thermospheric winds across the polar cap, and the extent to which plasma drifts may have strong sunward flow on the duskside of the arc.

5.5 Studying Arcs with Incoherent-Scatter Radar (ISR)

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will generally be enhanced on the dawnside, and occasionally on the duskside. Likewise, the Poynting flux typically peaks on the dawnside, but peaks on the duskside if there are strong sunward flows on the duskside. The magnitude of the Poynting flux is many for relatively intense sun-aligned arcs, exceeding the particle energy, and is smaller by a factor to be defined for more common weaker arcs. In summary, then, we have shown that the cross sections of the plasma-density and thermal character across stable sun-aligned arcs can be measured. They show that the electron gas temperature is enhanced over the arc, supporting the expectation that the arc is produced by a sheet of incoming energetic electrons. Along the high-velocity dawnside of the incoming en ergetic electron sheet producing the arc, there is a channel of enhanced ion temperature within which the ion temperature exceeds that of the electron gas. This sun-aligned channel, within which heat flows from the ion gas to the electron gas, coincides with the high antisunward

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velocity channel along the dawn edge of the sun-aligned region of enhanced electron density. In addition, if there is also return sunward flow at sufficiently high speed, ion heating is also found on the duskside of the arc. The ion-gas heating can be explained solely on the basis of ion frictional drag of high plasma velocity in the thermosphere rest frame. Persistent strong Joule heating, driven by Poynting flux down into a sun-aligned arc, well exceeds the particle energy flux. This rate can be estimated by three independent calculations based on a horizontal current differential, a velocity differential, and a temperature differential. The magnitude of this heat flux, several ergs is of significance to the trans-polar thermospheric energy budget. The techniques illustrated here apply to a broad class of arcs beyond only sun-aligned arcs. These same techniques have been used to study dawn (Carlson 1990) and dusk arcs, dayside and nightside arcs, and auroral hooks (Valladares and Carlson 1991), to mention a few.

5.6

Studying Arcs with Rockets

There are certain parameters, scale sizes, coincident data sets, and experimental tests of theory that can only be obtained from rockets. Here we present the only data we know of that measure

5.6 Studying Arcs with Rockets

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the profile of ion composition across sun-aligned arcs and their background. Two other unique measurements within sun-aligned arcs are also shown. A rocket payload was launched into a polar cap F-layer aurora to investigate the ener getic particle, plasma, and electric-circuit parameters of a sun-aligned arc (Weber et al. 1989). The rocket was launched from Søndre Strømfjord, on March 15, 1985, at 0205:52 UT (ap proximately midnight corrected geomagnetic local time). Instrumentation measured in situ the following parameters: energetic electron flux, ion composition and density fluctuations, electron density and temperature, electron density fluctuations, and ac and dc electric fields. Near apogee (429 km altitude), the rocket traversed a rapidly-moving sun-aligned F-layer arc. Real-time airborne and ground-based ASIP measurements tracked the location and motion of the aurora. These data, particularly that from the ASIP-equipped aircraft in the vicinity of the trajectory, were used to determine the proper geophysical situation for pre-determined launch criteria. Comparison of the in situ measurements with remote optical measurements showed that the arc was produced by fluxes of soft electrons (<1 keV) having a spectrum peaking near 300 eV. Field-aligned potentials in the arc were inferred from the electron spectra to have a maximum value of approximately 300V. A parent population of pre-accelerated electrons characteristic of the boundary plasma sheet, or magnetosheath, was inferred from the electron spectral shape. Electric-field components along and across the arc showed sunward flow within the arc, and duskward drift of the arc, consistent with the drift direction and speed determined from optical imaging. That is, the arc was drifting duskward under the influence of the convection electric field. Within the arc the thermal plasma density decreased moderately, despite the presence of precipitating electron fluxes, reflecting the balance of electron impact ionization, chemistry, and transport. The field-aligned currents associated with the arc, as calculated from measured electric fields and computed conductivity, agreed very well in magnitude and location with that carried by the energetic electron flux. These comparisons were consistent with satellite and ISR studies. Three new and unique findings followed from the rocket capability as well: ion composition, lower hybrid frequency, and plasma instabilities. We draw particular attention to them here.

5.6.1

Ion Composition

For the first time, ion composition was measured within the arc (Weber et al. 1989). It is shown here in Figure 5.32. The arcs are marked by onset of strong structuring of significant enhancement of decrease of and some ion concentration changes. In particular, the concentration of and decreases but that of shows negligible change.

5.6.2

Lower Hybrid Frequency

The ac electric field and ion mass spectrometer measurements allowed the first direct com parison of the low-frequency cutoff in the wave spectra with the local lower hybrid frequency (Weber et al. 1989). There is good agreement at high altitudes, and disagreement at low altitudes could be explained by propagation effects.

5.6.3

Plasma Instabilities

In addition, ionospheric plasma density irregularity and electric field fluctuations (Figure 5.33) gave evidence for different plasma irregularity-generation mechanisms on the dawnside vs. duskside of the arc. On the duskside, moderate density fluctuations are not accompanied by significant electric field fluctuations, suggestive of an interchange process (Tsunoda 1988).

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5.7 Statistical Behavior of Sun-Aligned Arcs

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On the dawnside, the higher levels of both parameters are consistent with a velocity-shear instability (Basu et al. 1988).

5.7

Statistical Behavior of Sun-Aligned Arcs

Statistical studies are needed to complement case studies. Case studies can be used to establish either the existence of, or consistency with the existence of, a physical process. Often, however, statistical studies may be needed to establish whether the process is dominant or important, rather than occasional or incidental. Further, some hypothesized processes may be proven or disproven by statistical studies. In this section, the statistics of dusk vs. dawn hemisphere sun-aligned arcs, and the inconsistency of some models of polar convection now in common use, are but two examples of the impact of statistical studies on our understanding of the polar cap.

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5.7.1

Morphology of Polar-Cap Sun-Aligned Arcs

Methodology for Statistical Studies

To accomplish a meaningful statistical study requires that many conditions be met. For example, the data must be adequately noise free. That is, a large enough number of samples must be available to establish all important parameter dependencies. It is further essential to process the data in a sufficiently rigorous mariner as to eliminate bias and distortion, which can be caused by, for example, instrumental effects, coordinate systems, and data smoothing. Also implicit is unambiguous recognition of the signature of the features under study. A clean arc identification is essential. It must not be confused with auroral hooks, scattered-light artifacts, other auroral boundary features, or the like. Figure 5.34 shows a system of polar cap sun-aligned arcs observed over Qaanaaq on Feb ruary 19, 1987. This figure is typical of the data set used in the discussion below. To date, the study by Valladares et al. (1994) is the only one meeting the criteria we note here. Thus, the statistical findings presented here, and analysis procedures recommended for any similar study, are based on that study. Essential to any statistical study is availability of a data set large enough to be statistically significant, with small enough statistical error bars and absence of bias to lead to meaningful results. The studies reported here are based on the results of Valladares et al. (1994), who used over 2700 sun-aligned arc observations by ASIPs, all within the central polar cap. Figure 5.35 shows the distribution of number of arc events vs. two key physical parameters derived from the data set. The Gaussian-like distribution (law of large numbers) lends assurance that the body of data is sufficiently large and representative of a wide range of conditions that conclusions from its study can be meaningful. Similar distributions were examined vs. each of the three components of IMF, giving the same high degree of assurance.

5.7 Statistical Behavior of Sun-Aligned Arcs

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Figure 5.35 shows statistical analysis of three different polar cap arc morphological para meters: (1) the presence of at least one arc in the ASIP field of view; (2) the deviation of the arc orientation (offset angle) from the sun-earth direction; and (3) the dusk-dawn body motion of the arc. The data are presented in the form of histograms, with an arrow to identify the center of gravity. To illustrate, consider Figure 5.35, which shows the distribution of the number of arcs as a function of the arc orientation (labeled offset angle in Figure 5.35a), and as a function of the dusk-dawn arc motion (Figure 5.35b). The center of gravity of each histo gram is indicated by the arrows and the initials CG. A positive offset angle means a dawnward (clockwise) rotation of the arc border closest to the sun. The distribution of Figure 5.35a has a shape that approaches a Gaussian curve. The mean value is equal to 5°, with largest offset values of -45° and 40° for counterclockwise and clockwise rotations, respectively. The arc ve locity is defined positive for dawnward displacements and negative for duskward translations. The velocity distribution of Figure 5.35b has a center of gravity equal to -21m/s. However, a significant number of arcs possess duskward velocities exceeding 240 m/s. This tail in the velocity distribution displaces the center of gravity toward more negative values. During the winter of 1986 – 87, the Qaanaaq ASIP operated between the months of October and February. A total of 850 hours of film were recorded. Owing to cloudy sky conditions or to the lack of IMF information, this number of hours was reduced to 223 hours. Further exclusion of events that show polar cap patches, or no aurora, reduced the data set to only 46.4 hours of red-line emissions collected on 20 different days. The intensity of the polar cap arcs observed during this winter varied between 50 R and 1 kR. Blue-line (427.8 nm) arcs coincided

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Morphology of Polar-Cap Sun-Aligned Arcs

with only 36% of the red-line (630.0 nm) arcs observed. Most arc-image pairs were taken every 2 minutes. The total number of arc frames used in this study is 2722. Figure 5.12 shows the location and the field of view of the ASIP established and deployed in the arctic region by AFGL (now AFRL/Hanscom). The ASIPs remain at Qaanaaq, Nord, and Ny Ålesund. While restricted to nighttime moonless clear skies, they can provide simultaneous measurements of the polar cap and auroral oval regions. The ASIPs provide photometrically imaged emissions at 630.0 nm, and 427.8 nm, with time interval of one image every two minutes or 30 s depending on the setting. They have a complete dynamic range from 50 R to several kR with an adjustable gain that permits a dynamic range of about a factor of 20 in any given image. Their wide-angle field of view is 155°. For an assumed emission height of 250km, the useful image diameter is about 1200km, as indicated by the circles in Figure 5.12. The fish-eye lens gives best spatial resolution within the few hundred km nearest to overhead, with increasingly compressed spatial coverage nearing the edge of its field of view. In addition to bright auroral features produced by precipitating electrons with energies in excess of 1 keV, the sensitivity of the ASIP allows detection of emissions produced by precipitating electrons with characteristic energies <500eV and energy fluxes The proximity of Qaanaaq to the magnetic north pole gives a very special capability to maintain the entire field of view (155°) of the ASIP inside the polar cap region at almost any local time.

5.7.2

Coordinates for Sun-Aligned Arc Statistics

To image and track arc motion, not all required coordinate transformations are obvious, or simple. Clearly the arcs must be field-flattened to correct for the fish-eye distortion. This field flattening then can be put on geographic coordinates. Since sun-aligned arcs are ordered by geophysical magnetic fields, these maps then need to be transformed into magnetic coordinates. Motion and orientation, however, must be referenced to earth-sun direction, with a rotation of both the ground station, the magnetic pole, and the arc within that moving rest frame. Finally, the arc has a degree of undulation in shape, which must be averaged to get a meaningful orientation and speed in the earth-sun rest frame. Figure 5.36 summarizes the key issues that must be addressed before one can begin to derive meaningful drift speeds, orientations, and statistics. Figure 5.36 shows a triple of arcs in three different coordinate systems (Figure 5.36a, b, c) and in Figure 5.36d, the arc motion, in the dusk-dawn direction, of selected points along the arcs. Geographic north is to the top and west to the left of the plot (270°). It is assumed that the emissions originate at 250 km altitude. Figure 5.36c depicts the arcs in a corrected geomagnetic coordinate system based on the IGRF 1980 model. In this system the arcs approximate straight lines almost parallel to the noon-midnight meridian; all three arcs have a small offset angle. A careful examination of the arcs indicates that there are small undulations along the arc alignment. These structures resemble the twists and folds that have been observed on auroral arcs, such as Hallinan and Davies (1970) suggested were associated with Kelvin-Helmholtz instabilities. Figure 5.36d shows the dusk-to-dawn velocities derived by comparing two con secutive frames. The length of the arrow is proportional to the dusk-to-dawn velocity of the arc. The arc motion was determined in the following fashion: for each individual point along the arc, a line was traced in the dawn-dusk direction (almost perpendicular to the arc align ment) until it intersected the location of the arc in the next 2-minute frame. This displacement in geographic coordinates, divided by the time interval between the frames, gave the velocity along the dawn-dusk direction. This velocity was subsequently corrected to take into account

5.7 Statistical Behavior of Sun-Aligned Arcs

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Morphology of Polar-Cap Sun-Aligned Arcs

the earth’s co-rotation both at Qaanaaq and at the magnetic north pole. This method is subject to minor errors due to the assumption of a fixed altitude of the emissions and a thin emitting layer, and use of the arc center to represent the arc location. Figure 5.36d shows that the two most poleward arcs have constant velocities, suggesting that these arcs move uniformly with little change in shape or direction of alignment. The most equatorward arc moves differently. Such non-uniform arc motion has been found to be present in a large number of the polar cap arcs. The variable arc motion (motion of the optical emission feature nominally transverse to its length) is caused by undulations that provide additional displacement to certain parts of the arc. Consequently, if the dusk-dawn velocity is computed taking a simple average of all the displacements calculated along the arc, then the resultant velocity will be contaminated by the undulations. Techniques to address all these issues are summarized in Valladares et al. (1994), as are many statistical properties of sun-aligned arcs. Having underscored the need for rigorous analysis and validation such as done by Valladares et al. (1994), and noting that they applied it to the 2700 arcs they studied, we next summarize the most salient statistical properties of sun-aligned arcs from their findings.

5.7.3 Detecting Arcs Figures 5.38 and 5.37 present the probability of detecting one or more arcs within the central polar cap, sorted by the three components of the IMF Histograms of the distribution of the number of minutes that the Qaanaaq imager was on operation during the winter of 1986-87, with sky conditions optimum for clear detection of weak arcs, well represented IMF conditions, with many hundreds of minutes of each IMF component divided into 1 nT bins over roughly + or – 5 nT on the histogram. Histograms were formed by dividing the total number of minutes that the ASIP detected one or more polar cap arcs by the number of minutes that the imager was operating. Overall, examination of the histograms shows that the probability of observing at least one polar cap arc at the Qaanaaq station is about 20%, and seems to be independent of the magnitude and sign of and We will explain shortly why it is 20% and not 50%.

As expected,

shows very different statistics. The probability of observing an arc is larger than 20% when is positive. This result agrees with the findings of Berkey et al. northward (1976), and Lassen and Danielsen (1978). They found good correlation between conditions and the occurrence of polar cap arcs. However, the larger database provides extra information. The probability of observing an arc decreases nearly linearly with increasingly negative to practically zero when Polar cap arcs are sometimes seen during southward conditions, but the probability of occurrence decreases linearly as becomes south conditions originated before more negative. All the polar cap arcs observed during the IMF turned south and remained detectable, in some cases, for tens of minutes after changed sign. This is about the time that is needed for polar cap arcs, formed under extended conditions, to flush out of the polar cap after a southward turning. The probability of observing a polar cap arc in the dawn vs. dusk hemispheres is shown in Figures 5.38 vs. 5.37. The prominent feature of these figures is the difference between hemispheres in the average value of the arc occurrence probability. It is about 40% for arcs located in the dawnside and only 10% for the duskside. This factor of four difference may be due to a distinct intensity of the aurora between hemispheres. If the duskside arcs are fainter than the dawnside arcs, then they could more easily fall below the ASIP sensitivity threshold, and the statistics would be greatly diminished. In this scenario, the statistics for the duskside would be scaled down with respect to its dawnside counterpart. The histogram of Figure 5.38 shows a trend different to the and histograms. For values larger than -5 nT, the dawnside distribution increases linearly, reaching values close to 50% at +7nT.

5.7 Statistical Behavior of Sun-Aligned Arcs

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5.7 Statistical Behavior of Sun-Aligned Arcs

5.7.4

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Arc Orientation

To measure the arc offset angle, or arc orientation, the digitized images were transformed to a corrected geomagnetic (CG) coordinate system. In this frame, a line was fitted to the arc, and the angle between this line and the noon-midnight meridian was calculated and included in our statistics, when the arc was 30° or more above the horizon and the fitting procedure indicated a small deviation error. Figure 5.39 shows the location of the polar cap arcs that were observed during the winter of 1986–1987, portrayed in a CG latitude versus CG local time polar frame. Since the arc extension varies from less than 100km up to a few thousand kilometers (typical of transpolar arcs), it usually spans more than one hour of MLT. The center of the arc was taken to indicate the location of the arc. The information about the arc orientation is color-coded according to offset angle with positive (dawnward or clockwise) rotations of 20° or more in red, while negative (duskward or counterclockwise) angles equal to -20° or less are in blue. Angles in between follow the color scale displayed in the upper right corner. It is evident that a larger number of arcs were observed in the 0000–0600 and 0600–1200 (CG) MLT quadrants, and that a minimum in the arc population was detected in the 1200-1800 quadrant. This effect is partially due to the uneven local time sampling through the winter months, but also to the lesser probability for arcs to be seen in the duskside, as it is shown in Figure 5.37. The imager operated fewer hours in the 1200–1800 quadrant than in any of the other hour sectors. Figure 5.39 shows that most of the arcs of the 0600–1200 quadrant have a negative offset angle, and are seen in blue or light blue, contrary to the arcs of the 1200–1800 quadrant, which are mostly depicted in red, orange, and yellow. The other two quadrants, 1800–2400 and 0000–0600 CGLT, possess almost equal numbers of positive and negative rotations. There is a large number of arcs in the 0600–1200 quadrant between 80° and 85° CG latitude. A more quantitative summary of the arc orientation, and its in Table 5.1 of arc offset angles in degrees vs. UT.

dependence, is presented

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5.7.5

Morphology of Polar-Cap Sun-Aligned Arcs

Dawn-Dusk Velocity

The average dawn-dusk velocity of all arcs, divided for each CGLT quadrant, varies between + and -40 m/s. The dawnside averages -25m/s (duskward) velocity. The duskside averages +40 m/s (dawnward) velocity. More dawnside arcs than duskside arcs implies that an observer in space will more commonly observe arcs moving dawn-to-dusk than in the opposite direction. The dependence is summarized in Table 5.2 for dawn-dusk velocity in m/s vs. UT. Statistical significance is reduced for the 1200 – 1800 CGLT quadrant, but the other three

5.8 Particle and Emission Spectra

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segments indicate a dependency of the dusk-dawn arc motion. The dawnside histograms ( positive) show an average dawn-dusk motion equal to -120m/s, while for 1800–2400 CGLT the velocity is near zero. The negative conditions mirror the conditions for reverses the hemisphere in which the rapidlypositive. In other words, the polarity of moving arcs are found, as well as the direction of motion of the polar cap arcs.

5.7.6

Arc Motion vs. IMF

Figure 5.40 shows the variability of both the arc velocity and the arc location vs. the value of in steps of 1 or 2 nT. Arcs with dawnward velocities are shown as red arrows; those with duskward velocities are shown as blue arrows. The arrow length gives the magnitude of the (upper left dusk-dawn velocity. Starting with the plot corresponding to corner), the arcs moving dawnward (red) are seen to be located in the dusk hemisphere and covering a large part of the dawn hemisphere. They extend up to 82° of latitude in the dawn hemisphere. The velocity of the dawnward moving arcs shows a large variability, changing with the location of the arc in the polar cap. It maximizes at about 200 m/s in the far left side of the subframe, then diminishes near the pole, and approaches 0 m/s at the extreme dawn side of the plot. The arcs slow when they reach a boundary where the motion of the arcs changes from a dawnward to a duskward motion. Examining the blue arrows in this frame indicates that most of the duskward moving arcs are located at the dawnside and equatorward of 84° in a region where almost no dawnward moving arcs are observed. Similar to the red arrows, the magnitudes of the velocities are small near the boundary and larger further away from this line. A velocity boundary divides dawnward-moving arcs (located to the left) from duskward transiting arcs (located to the right). The other subframes of Figure 5.40 (left to right and top to bottom) show that as becomes more positive, the population of dawnward-moving arcs is greatly reduced in the dawn hemisphere and is replaced by duskward-moving arcs. Concur positive. For positive values, rently, the velocity boundary is shifted in the direction of the location of duskward-moving arcs covers not only the dawn hemisphere, but also extends into the dusk hemisphere. These dawn-dusk drift directions are summarized schematically in Chapter 6, Figure 6.13.

5.7.7

Arc Lifetime

The average number of arcs per image that was observed by the Qaanaaq’s ASIP during the winter of 1986–1987 was 2. The average lifetime of the polar cap arcs used in this study was 38 minutes. The longest-lasting arc within the ASIP field of view was 3 hours and 46 minutes. For very extended periods of positive (northward), longer lifetimes would be expected for the existence of sun-aligned arcs within the polar cap, but of course to remain within the same transitions ~1200km field of view of an ASIP, the motion must be very limited. Typical reversal for a duration of about 10–15 minutes, and a single-step were a square wave reversal extending for at least 25 minutes.

5.8

Particle and Emission Spectra

Earlier in this chapter we presented findings from a rocket passage through a sun-aligned arc near its connection to the midnight auroral oval. The energy spectra of the electron flux producing that sun-aligned arc, and the spectra of precipitating-electron fluxes into the polar cap, near but outside that arc, are shown here in Figure 5.41. The characteristic enhancement of electron fluxes for energies < 1 keV is clear in this comparison of within vs. outside the arc.

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5.8.1 Satellite Study of Electron Precipitation Energy Spectra Data such as these need to be complemented by a statistically large set of measurements of such fluxes. In fact, such a larger, statistically-significant data set has been collected, by Obara et al. (1996), using hundreds of Akebono satellite measurements of precipitating-electron flux into verified sun-aligned arcs. Satellites passing dawn-dusk within the polar cap must have up to a 50% chance of passing through a sheet of sun-aligned arc-producing electrons, so why is

5.8 Particle and Emission Spectra

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Morphology of Polar-Cap Sun-Aligned Arcs

it hard to collect such data? Several processes could produce spikes in electron fluxes within the polar cap. Looking for spikes in electron flux that coincide with electric field gradients of the Ohm’s law sign could be a further consistency test, but would still leave room for many phenomena. Without coincident two-dimensional imaging data, to confirm coincident sun-aligned arcs, doubt remains. Obara et al. (1996) have coordinated Akebono (EXOS D) satellite overflights of ground based ASIPs for a large number of passes. The coincidence with magnetic field-line tracing to sun-aligned arcs in the ionosphere below, and coincident electricfield gradients of the expected sign, validate the electron spectra they collected as indeed the electron energy spectra sought. Figure 5.42 shows the average energy of precipitating electrons found by Obara et al. (1996). It shows that after examination of hundreds of cases, the energies are below 1 keV for all but a few of the sun-aligned arc events, just as the visual sightings have found, and that typical average energies of the precipitating-electron fluxes are near 200 eV. They found a shift toward lower energies for the dayside sun-aligned arcs with respect to the nightside arcs. The average energy was defined as the integral of the energy flux divided by the integral of the number flux. Obara et al. (1996) further found that the average energies in both day- and night-side were in the range 100–400eV, and on close inspection, most electrons in the spectra had energies less than 200 eV. This led them to suggest that the magnetosheath or magnetospheric mantle appeared to be the likely source region of the precipitating electrons into sun-aligned arcs. They also point out, by use of their statistically large electron-energy spectral data base, validated as to connection with polar cap sun-aligned arcs, the connection to the polar rain and polar showers discovered by satellite-borne particle spectrometers in polar orbit (Winningham and Heikkila 1974). Polar rain is a relatively uniform low-energy and number-flux background of electron particle precipitation into the polar cap, and is enhanced during southward IMF. Polar shower is a stronger energy and number flux of electrons within a limited spatial region, and is seen during northward IMF. Obara et al. relates the latter to passage through sunaligned arcs.

5.8.2 Spectroscopic Measurements Given the energy spectrum of a precipitating electron flux into the upper atmosphere, one can apply principles as discussed in Chapter 3 to calculate the expected emission spectra. To our knowledge there is only one published set of spectroscopic measurements of the emission from within a sun-aligned arc (Niciejewski et al. 1989). We reproduce these measurements here in Figure 5.43, for the molecular nitrogen ion emission at 427.8 nm and the atomic oxygen emissions at 557.7, 630.0, and 844.6 nm. Such measurements can shed light on energy flow and poorly understood atmospheric composition in the polar cap. This data set includes the same February 26, 1987 sun-aligned arc pair discussed in detail in Section 5.5 and also shown in Figure 5.23 and 5.24.

5.9 Response Time and Dynamics Sun-aligned arcs appear and disappear with IMF reversals. For steady northward IMF, they are also very dynamic near connection to the midnight auroral oval.

5.9.1

Response Time

Reversals

Polar cap ionospheric convection depends dramatically on whether the solar-wind magnetic field is more nearly parallel or antiparallel to the sub-solar magnetic field of the earth. When

5.9 Response Time and Dynamics

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the fields are antiparallel (the IMF is southward), there is a well-ordered two-coll polar iono spheric convection pattern, responding to strong large-scale ordered magnetospheric convec tion driven by solar-wind coupling. When the fields are antiparallel, although solar-wind coupling still drives convection, the convection becomes very anisotropic (Heelis et al. 1982). Convection flows are ordered over scale sizes of thousands of km in the earth-sun direction, but tens of km in the dawn-dusk direction. These strong differences between how the solar wind couples to plasma circulation (convection) in the magnetosphere and polar ionosphere for parallel vs. antiparallel IMF orientation lead to dramatic differences in the character of the polar ionosphere. One can say it is in one of two states. The magnetic reconnection view has the best success at predicting many of the contrasting properties of these two states (Heelis 1984). Contrasts between these two states is sufficiently dramatic that ground-based observations typically can identify the sign of the (north/south) component of the IMF vector if stable for 100–15 minutes (Weber and Buchau 1981). Often this is also true of the (dawn/dusk) component of IMF, if stable for at least 5 minutes. The character of the southward and northward IMF state of the polar ionosphere has been well observed by satellites (Buchau, Weber, Anderson, Carlson, Moore, Reinisch and Livingston 1985, Crowley 1996, Carlson and Crowley 1989) and ground-based instrumentation

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Morphology of Polar-Cap Sun-Aligned Arcs

(Basu, Basu, Chaturvedi and Bryant 1994, Clauer and Friis-Christensen 1988). The southward IMF state is relatively well defined and understood (Buchau et al. 1985, Anderson, Buchau and Heelis 1988), while the northward IMF state is not (Weber et al. 1984, Burke et al. 1982). Understanding of some aspects of the coupling between the polar ionosphere and solar wind is illuminated by studying the transient response of the polar ionosphere to sharp changes in IMF (Valladares and Carlson 1991). Table 5.3 summarizes the time delays between reversals of the sign of the IMF and component, and response of observed properties in or near the polar cap. Rodriguez et al. (1997) have studied the decay of arcs within the polar cap. Clauer and Friis-Christensen (1988) have studied currents during strong northward IMF conditions, and associated response times.

5.9.2

Simultaneous Arcs and Patches

If our reconnection model for how arcs and patches behave is valid, one should be able to see arcs and patches simultaneously in the same ASIP field of view after a clear IMF reversal from southward to northward, but not for the case of northward to southward. It takes a finite amount of time for patches to traverse the polar cap once launched into it. It is natural to look to find whether this expectation is validated by observation.

5.9 Response Time and Dynamics

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Valladares, Sheehan, Carlson and Bullet (1988) have reported the results of a search for coincident sun-aligned arcs and patches. Figure 5.44 shows one example of a sun-aligned arc and antisunward-moving patch simultaneously observed in a single ASIP field of view on December 28, 1991. The top row of Figure 5.44 displays four images obtained between 2218:32 and 2242:32 UT on December 18, 1991. The bottom row shows an artist’s representation of the patch and arc emissions recorded in the four images of the top row. The motion of patch 2 is in the antisunward direction (toward geographic east). This patch almost disappears near the northern edge of the imager field of view in the image corresponding to 2242:32UT. The image of 2234:32 UT shows the first clear view of a polar cap arc near the southwestern horizon (duskside). The arc is sun-aligned and drifts toward the dawnside of the image. Also indicated in the schematics of the bottom row are the approximate horizontal distances in the east-west direction. Note that the spatial distortion produced by the camera fish-eye lens is axially symmetric. This work provided the first experimental evidence that polar cap patches and sun-aligned arcs can exist simultaneously inside the ~ 1000 km field of view of an ASIP that is located deep within the polar cap. It also interpreted these observations within the context of a model of the operative mechanism explaining the IMF and time dependencies of the conditions under which coexistence is observed. The interpretation leads to the expectation that similar-sensitivity images of the entire polar cap would show this to be a common occurrence. Key is that all these simultaneous patches-with-arcs were restricted to near and shortly after the time the IMF changed from a negative to a positive orientation. Also, where ob servation allowed tracking, the patches continued to move antisunward, apparently unaffected by the emergence and the motion of the polar cap arcs. In the cases presented, the last patches

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observed had progressed to near the midnight sector by the time the first arc was seen. As it takes approximately an hour for a patch to travel from the dayside to the midnight oval, this is consistent with the suggestion that all the patches presented here were created well before the arcs appeared within the Qaanaaq field of view. The patches did not have time to exit the polar cap before the time the arcs had developed. Observed arc delays approximately an hour were upper limits on the time delay between IMF positive turning and arc appearance. This is borne out by the limited coverage that a single camera can provide. Hairston and Heelis (1995) found 1/2 to 3/4 hour time lags for DMSP to show establishment of a new polar convection signature in the ionosphere after the IMF turned from southward to northward. Note that these changes represent a large-scale convection pattern establishment, vs. presum ably quicker initiation of individual transport signatures, such as patch and arc events. The latter should be and are seen near the cusp (reconnection) region within minutes. Cowley and Lockwood (1992) have discussed how the polar convection is maintained by the combined action of two reconnection-driven components. One occurs at the dayside, where the solar wind comes in contact with the magnetopause; the other occurs in the tail. During northward conditions, the plasma convection may occur by combined lobe stirring due to reconnection between the northward IMF and open tail lobe lines, and continuing tail reconnection. Polar-cap arcs could follow from any of these reconnection processes, with field lines tracing from the polar cap arcs to regions in the tail. Of course, field lines threading polar cap patches are on open field lines also extending toward the magnetotail.

5.9.3

Arcs at the Auroral Oval Boundary

We have mentioned that sun-aligned arcs near the auroral boundary show more dynamic variations than those in the central polar cap. Contrast, for example, the behavior of the arcs of Figure 5.34 over the central polar cap with arcs in Figure 5.45 through 5.48, which shows rapid dynamics of arcs where they connect to the poleward edge of the midnight auroral oval. These arcs were discussed in some detail earlier. It is one of these arcs through which a rocket was fired. That arc emerged from the polarward edge of the midnight oval, but with a somewhat sporadic trajectory. This is a rather common phenomenon (see Figures 5.6 and 5.45). Hoffman et al. (1985) have published measurements by the AE-C satellite through a sunaligned arc near its connection to the midnight auroral oval in the southern hemisphere, confirming the presence of the sun-aligned arc by a near-coincident DMSP pass. It was thus a bright (>1 kR) arc. They measured a peak energy flux of and an average electron energy at about 0.9 keV. Both are about as expected from the arc brightness. This is in very sharp contrast to the very weak arc intensity, electron energy spectra, and electron energy flux into the arc studied by the rocket in Section 5.6. The latter fall in the category of the more common class of weak arcs. Hoffman et al. (1985) found their precipitating electrons producing the arc to have no clear monoenergetic beam, and to have coincident ion precipitation at energies of a few keV. They found a very sharp ion convection reversal coincident with the particle precipitation, and embedded within a region of constant antisunward flow. The IMF was nearly zero, consistent with seeing a sun-aligned arc. The electron flux was consistent with Ohm’s law arc electrodynamics. As to the current carriers, they noted their data were consistent with a source for the precipitating particles at altitudes near 5 to 8 earth radii on field lines containing the plasma sheet boundary layer. Observations of theta aurora near the central polar cap are also attributed to a plasma-sheet particle population. This bright arc observation of Hoffman et al. (1985) is then consistent with other data for the class of very bright sun-aligned arcs, which are generally considered to have plasma sheet-like precipitating particle populations.

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They also presented schematic diagrams of the electrodynamic parameters associated with the sun-aligned arc, and of a possible global high-latitude convection pattern consistent with their observations, reproduced here as Figures 5.46 and 5.47. Compare this convection with the convection flow of Figure 5.29, deduced by the two-dimensional mapping of the ISR for the very similar intensity and energy deposition case discussed in Section 5.5. The consistency is striking, given the total independence of the two data sets and analyses. Berg et al. (1994) fired a rocket from Søndre Strømfjord a month after the Weber et al. (1989) rocket discussed in Section 5.6. That rocket was also fired through a sun-aligned arc pair erupting from the poleward edge of the polar cap, but this time the arc brightness exceeded 4kR. Berg et al. (1994) quite reasonably suggest that these arcs are associated with a contracting polar cap, as indeed they have resulted from a southward to northward IMF turning, which must lead to a contracting polar cap. One might say it was of the class of intense kR, E-region, theta-arc-like optical and energy character. Its dynamic motion near the midnight polar boundary (illustrated in Figure 5.48), and its Ohm’s law properties were as all the arcs discussed here. The connection of a sun-aligned arc to the noon-midday oval region would nonetheless involve quite different physical and topological connections. The onset of such a connection can be hypothesized by reconnection theory. To our knowledge, the first observation ever published of the onset of sun-aligned arc connection to the midday aurora is the one presented here in Chapter 4. The optical data also show the behavior that would be expected from reconnection models.

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Chapter 6

Theory of Polar-Cap Sun-Aligned Arcs In this chapter we place primary emphasis on theory, and adopt a deductive approach to its discussion. This presentation contrasts with the discussion of morphological data in Chapter 5.

6.1

Introduction

We focus first on theory at the fundamental, or elemental building-block level. Each element should be both valid in itself, and believed to be relevant and of significance to the topic at hand. We must then of course show why the theory is believed to apply to sun-aligned arcs, so some reference to data is necessary. Finally, we will proceed to a discussion of a set of interlocking theories, which collectively are thought to represent processes dominating the behavior and character of sun-aligned arc morphology.

6.1.1

Elements of Theory and Processes

By discussing the theories and processes within the context of deductive analysis, we can capture large classes of predictable behavior that of necessity must be exhibited in the mor phology of the arcs and their derived physical properties. This success in prediction is taken as validation of at least consistency of the theory with observation, if not confirmation. The processes presented here are those which have survived extensive comparison and predictive testing, with the exception of a few, clearly designated as speculative.

6.1.2

Energy-Flow Framework

Let us begin here with the end in mind, that is, let us start with a description of the gross character of the large-scale forces at work, which lead to the ubiquitous presence of sun-aligned arcs in the polar cap. Where does the energy come from to drive them? In what form(s) is the energy conveyed or transmitted, and in what form(s) is it ultimately dissipated? We begin with these bigger-picture questions before proceeding to the building-block processes intended to lead to a more comprehensive description of the arcs. Recall that the solar atmosphere, too hot for the sun’s gravitational field to contain, is con tinually escaping the sun. This escaping atmosphere, too hot for electrons to remain bound to nuclei, is a fully-ionized plasma carrying its own magnetic field. Earth‘s magnetic field represents an obstacle to this “solar wind”. At that sunward distance from the earth where [ 235 ]

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Theory of Polar-Cap Sun-Aligned Arcs

the earth’s magnetic field pressure approximates the opposing plasma pressure of the solar wind (~10 earth radii toward the sun), the solar wind flows around the earth’s magneto– sphere (~50 earth radii diameter cylinder, extending downstream beyond the distance to the moon). Mechanical (kinetic) energy of the solar-wind plasma is converted to electrical energy by the interaction, carried downward along magnetic fields into the ionosphere in the form of energetic electrons and ions (current carriers) and Poynting flux. A magnetic reconnection interpretation of the interaction at the magnetopause best predicts observed behavior of iono spheric circulation, and a great many other properties of earth’s space environment, and is thus employed here (see Chapter 2). As the electrical energy is conveyed into the high-latitude ionosphere and upper atmo sphere, the Poynting flux manifests itself there as Joule or Ohmic heating. The particle flux leads directly to production of secondary electrons; thermal plasma ionization, which can be thought of as chemically-stored energy; heat into the thermal electron gas; and excited states relaxing via optical emissions, which escape optically thin but redeposit energy in optically thick regions of the upper atmosphere. Ultimately the energy is carried away in the form of photons, Alfven waves, electromagnetic emissions and plasma instabilities, “polar wind” light and heavy ions, currents driven by polar thermosphere “flywheel” effects, but mostly in the form of downward heat conduction to the lower thermosphere heat sink. Above 110 km in the thermosphere, near auroral latitudes, the rate of heat deposition tracing to solar wind-driven processes at times well exceeds the global These solar wind-driven processes average solar UV/EUV heating rate of have global impact on the ionosphere and thermosphere. The associated upward-current sheets are carried by precipitating electron fluxes (~ 0.1 – – 1 keV), which excite optical emissions, produce ionization, and heat ambient electrons within the magnetic flux tubes containing the current. Among the manifestations of this energy deposition are sun-aligned arcs, whose properties allow a study of the nature of this energy transfer from far- to near-earth space, a study we will now begin. Let us give a sense of scale to the particle energy in a strong sun-aligned arc. For a kR intensity, visible to an early satellite, or visible to the dark-adapted human eye, the particle or approximately energy input to the atmosphere must be about For an arc about 10 km wide and 1 000 km long, approximately are needed, comparable to the power capacity of a large power plant. Most strong sun-aligned arcs are more like 50–100 km wide, and > 2 000km long. Only about 1 percent of the particle input to the atmosphere is used to produce visible light.

6.2

Anatomy of a Polar-Cap Sun-Aligned Arc

The essential electrodynamics and thermal and energetic properties of sun-aligned arcs at ionospheric altitudes are summarized in Figure 6.1. All of these characteristics should apply to strong sun-aligned arcs. The basic electrodynamics and electron gas properties should apply as well to weak polar cap arcs, ubiquitous for northward IMF conditions. Weak arcs will, however, have much lower E-region electron densities, which will reflect in differences in Poynting flux, ion heating, and ratios of Hall-to-Pedersen conductivities.

6.2.1

Ionospheric Altitudes

Within the center of all such arcs, whether or not is enhanced below 200 km, its electron gas temperature must be enhanced by the incoming flux of precipitating electron current carriers. These electron fluxes must also carry an incoming particle energy flux, and an outgoing Birkeland current, to one side, while there must be a return (incoming) current on

6.2 Anatomy of a Polar-Cap Sun-Aligned Arc

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the other side. The Pedersen currents must decrease from dawn to dusk, as the antisunward plasma drifts must decrease from dawn to dusk. The plasma drifts may simply decrease, or reverse as in this illustration (Figure 6.1). The ion temperature will be enhanced where the difference between the plasma and neutral atmospheric velocities is sufficiently great, and ion-neutral collision frequencies significant. This collision frequency consequence depends on the thermospheric winds across the polar cap, and the extent to which plasma drifts may have strong sunward flow on the duskside of the arc. will generally be enhanced on the dawnside, and occasionally on the duskside. Likewise, a Poynting flux present would typically peak on the dawnside, but there can be peaks on the duskside if there are strong sunward flows on the duskside. The magnitude of the Poynting flux is many for relatively intense sun-aligned arcs, exceeding the particle energy, and smaller by a factor to be defined for more common, weaker arcs. Figure 6.1 (see also Figure 5.31) represents strong sun-aligned features over at least me soscale horizontal areas, on the order of for plasma densities, velocities, and temperatures of stable (minutes vs. seconds) large-scale (many km) plasma features, such as stable sun-aligned arcs. It illustrates that the electron-gas temperature is enhanced over the arc, as the arc is produced by a sheet of incoming energetic electrons. Along the high-velocity dawnside of the incoming energetic electron sheet (producing the arc), there is a channel of enhanced ion temperature, within which the ion temperature exceeds that of the electron

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gas. This sun-aligned channel, within which heat flows from the ion gas to the electron gas, coincides with the high antisunward velocity channel along the dawn edge of the sunaligned region of enhanced electron density. In addition, if there is also return sunward flow at sufficiently high speed, ion heating is also found on the duskside of the arc. The ion-gas heating can be explained solely on the basis of ion frictional drag of high plasma velocity in the thermosphere rest frame. Persistent strong Joule heating, driven by Poynting flux down into a sun-aligned arc, well exceeds the particle energy flux. This rate can be estimated by three independent calculations based on a horizontal current differential, a velocity differential, is of and a temperature differential. The magnitude of this heat flux, several significance to the trans-polar thermospheric energy budget.

6.2.2

Near-Earth Altitudes

Figure 6.2 shows extension of sun-aligned arc properties upward along magnetic field lines to at least a few earth radii. The trans-polar arc signature is illustrated here as, in fact, a current sheet of precipitating electrons, with a return current volume for current continuity. This figure helps demonstrate that any trans-polar satellite pass must of necessity penetrate sun-aligned arcs half the time it is passing through the polar cap. This near-earth threedimensional view can be shown with some good degree of confidence, as magnetic field lines and sun-aligned surfaces are extended upwards a modest distance.

6.2.3

Out to the Solar Wind Interface

Figures 6.4, 6.5, 6.6 show some of the magnetospheric topological extensions that have been suggested to explain how the ionospheric arc features may connect to the outer reaches of the magnetosphere as it interfaces with the solar wind. Figures 6.4 through 6.8 and 2.9 are for northward IMF, and should be viewed within the context of the discussion in Chapter 2. Figures 6.4, 6.5 and 6.6 look downstream, away from the sun. These cartoons are speculative, and are offered by various authors to spur better understanding of how the arcs are indeed formed, driven, and finally dissipated. Determining what the topology really is, is a very active area of research at this time. Figures 6.7, 6.8 and 2.9 look upstream to reconnection to solar wind sunward of the earth. Figure 6.7 shows the classic solar-wind reconnection geometry as proposed by Reiff and Burch (1985). Figure 6.8 shows another view of IMF reconnection. Figure 2.9 begins to illustrate the range of IMF conditions under which the Cowley and Lockwood (1992) views are being tested.

6.3

Electrodynamics of Sun-Aligned Arcs

Sun-aligned arcs are a natural consequence of the nature of the electrodynamics of plasma flow in the polar regions. Here we discuss the key considerations for understanding these processes.

6.3.1

Ohm’s-Law Arcs

Having withstood the trial of repeated tests, as described in Chapter 5, the electrodynamics hypothesized in Figure 5.13 is accepted now as established; sun-aligned arcs are in the class of auroral arcs referred to as Ohm’s law arcs. That is to say, the current associated with the arc is that given by a simple relationship of where E is the electric field driving the auroral current, I is the current, and R is the electrical resistivity of the arc. While this simple equation is a good approximation for a purely F region arc, for more energetic arcs, with E

6.3 Electrodynamics of Sun-Aligned Arcs

[ 239 ]

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6.3 Electrodynamics of Sun-Aligned Arcs

[ 241]

region ionization, R must more rigorously be replaced by the reciprocal of the conductivity tensor (Rishbeth and Garriott 1969). Any electric field E in the polar cap will of necessity have associated with it a convection of polar plasma, given by the standard formula of plasma velocity where B is earth’s magnetic field. Regions in the ionosphere where drive upward field-aligned currents to maintain current continuity, while regions where drive downward field-aligned currents.

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6.3 Electrodynamics of Sun-Aligned Arcs

[ 243 ]

[ 244 ]

Theory of Polar-Cap Sun-Aligned Arcs

Since electrons are far more mobile than ions, we anticipate that they are the primary current carriers. And, in fact, current densities observed in downward field-aligned currents can readily be carried by ionospheric electrons, but not by ions (Lyons 1980). Satellite passes through sun-aligned arcs also see electron current carriers, but not ion current carriers. Upward currents carried by high-altitude electrons flowing into the ionosphere are also limited to current density values less than those observed in intense auroral arcs, unless field-aligned electric potential differences develop. Here we are dealing with polar cap arcs, which for the most part are relatively weak, so we shall not discuss this complication, but rather refer the interested reader to review articles that deal with such potentials (also referenced as “double layers” in the literature). Experiments have been proposed to look for such E-parallel acceleration regions above mature sun-aligned arcs, but satellites have not yet achieved the orbits/timing/sensitivity required to test for these acceleration regions above sun-aligned arcs. For southward IMF, polar ionospheric plasma convection follows an anti-sunward flow pattern near the central polar cap, with return flow equatorward of the auroral region. For northward IMF, in addition to anti-sunward flow within the polar cap, there are also plasma flow reversals, leading to channels of return sunward plasma flow within the polar cap. Consequently, polar ionospheric convection patterns for northward IMF should lead to conditions that could produce optical emissions by electron impact (incoming electrons for (but not for outgoing current carriers) along lines of plasma flow reversal for A simple electrodynamics situation that leads to an optical arc signature along such a boundary line is illustrated in Figure 5.13 (Carlson 1990). Consider initially uniform conductivity across the boundary. A plasma velocity reversal (left-hand side) or velocity gradient (right-hand side) across the boundary line, and the equi valent horizontal electric-field differential, would produce a horizontal Pedersen current con vergence at the boundary, in the absence of a vertical current component. Thus, magnetic field-aligned current will flow, with whatever magnitude is required to maintain a divergencefree current state.

6.3 Electrodynamics of Sun-Aligned Arcs

6.3.2

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Neutral Gas Rest Frame

Up to this point in our description, the only difference between the left-hand shear reversal and the right-hand shear differential flow is the rest-frame velocity from which the flow is viewed. Significantly, it is the rest frame of the neutral gas that determines the currents. The need to operate in the neutral gas rest frame may be understood in part by recalling that it is collisions between the charged and neutral gas particles, and in particular different collision frequencies (mobilities) of the ions vs. electrons with the neutral gas particles, which produce a finite conductivity and a Pedersen current perpendicular to the velocity shear boundary. The sense of the velocity difference across the boundary determines whether it tends to drive a horizontal convergence or divergence, and thus require a vertically upward or down ward current to maintain a divergence-free state. In Figure 5.13, the sense leads to an upward current, presumably carried into the ionosphere by a flux of precipitating suprathermal elec trons. Note that if a series of velocity differentials occur, alternatively reversing (plasma altern ately speeding up, then slowing), the upward current sheets will alternate with interveningdownward current sheets along velocity difference boundary lines. Escaping thermal electrons will presumably carry these downward current sheets. This simple arc electrodynamics, as discussed by Lyons (1980), is in fact the electrodynam ics that pertains to (stable in time, extensive in sunward direction) sun-aligned arcs (Carlson et al. 1988). We should note that, as indicated on the left-hand side of Figure 6.1, a change in conductivity alone across the boundary could lead to this same circuitry, in the absence of a velocity shear. However, we have found no examples of this conductivity change alone creating sun-aligned arcs.

6.3.3

Electric Fields, Plasma Drift, and Currents

Electric fields in the F-region trace very well along magnetic fields, and can be mapped by their quantitative relationship with plasma flow. However, the F-region relationship does not extrapolate to the E-region, because of the high collision frequency there between ions and neutrals. Much below 200 km, collisions of ions with neutral atmospheric particles begin to deflect ion horizontal motion relative to electron horizontal motion, so the relative difference in horizontal motion, or drift, begins to create a new current component in addition to the Birkeland current. By an altitude of 140 km, this difference in direction is so great that the ions will drift on average at a 45° angle to The electrons are still locked to motion as given in the F-region by the electric field direction. Below 120km, ions collide so frequently with neutral particles that they are dragged along locked in the neutral gas. At and below 80km, even electrons collide so frequently with neutral particles that they, too, are locked to the neutral gas, and a relative difference in drift, hence current, no longer is found. Auroral currents (j = current density, )

flow, where provides current carriers, primarily above 90km, is a conductivity tensor, and is the net ion or electron velocity, which depends on collision frequency with neutral particles deflecting their motion from simple direct control by the electric field E alone, and is the number density ions Wherever ions drift relative to electrons (usually primarily below 140km), E and lead to a current with components that are perpendicular to B and parallel to E (Pedersen current), perpendicular to both B and E (Hall current), and parallel

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to B (Birkeland current). Birkeland currents are, of course, generated not only by processes locally in the E-region. can be calculated from the divergence of the field-aligned The field-aligned current horizontal current. This is a Birkeland current in that it is parallel to B, but not in the usual sense in which the term Birkeland current is used, with reference to currents into and out of the primary auroral region. We assume that quantities do not vary along the arc (the Y coordinate) direction, and therefore is computed from the divergence of the cross-arc current, where

and hence

Positive values of are downward, parallel to the earth’s magnetic field. The microscopic conductivities and are calculated as a function of position across the arc from the precipitating electron fluxes, and then integrated to arrive at the height integrated Pedersen and and Hall conductivities, Birkeland currents out of the topside ionosphere are carried by energetic electrons that penetrate to the altitude of their unity optical depth. This electron penetration builds up an excess charge, producing a displacement of charge, or polarization field, which drives a hori zontal current. The electric fields extend down along magnetic fields deeply enough to reach conductivity adequate to drive that horizontal current needed to neutralize the polarization field with the displacement current. For all practical purposes, this occurs instantaneously. Note that for very low E-region conductivity, F-region conductivity can become important. Return currents into the ionosphere are presumably carried by ambient electrons, highly mo bile along the magnetic field line, and of a density easily able to accommodate demands for return-current continuity.

6.3.4

Self-Consistent Calculation of Arc Conductivities

Since the current carriers of the vertical (actually, magnetic field-aligned) current are suprathermal electrons with energies of hundreds of eV or greater, they should produce impact excitation of optical emissions, providing the optical signature of the arc. By the same token, they are expected to produce impact ionization, thereby enhancing the conductivity within the arc, and modifying the distribution of currents that flow within the arc itself. This feedback effect must be allowed for in any detailed self-consistent treatment of the circuitry of the arc. This feedback effect merits a future, more detailed study.

6.3.5

A New Research Tool

Sun-aligned arcs are visual markers of velocity shear lines, that is, lines of sharp velocity differentials, of a specific sense: greater antisunward velocity on the dawn side than the dusk side of a polar cap sun-aligned arc. Sun-aligned arcs can map important two-dimensional properties of polar ionospheric convection. This view is as indicated in Figure 5.13, 5.14 and 5.18.

6.4 Current Sheets

6.3.6

[ 247 ]

Chemical Lifetime

The chemical lifetime of ionization below 200km is so short that any stable electron density feature, observed to persist for over ten minutes in a stable configuration and location below 200 km, must be recognized as a region of stable ongoing production of ionization. In a sunaligned arc, this ongoing production is by a flux of energetic particles. Thus, we must further conclude that this stable feature would be seen in optical emission as well, in the form of a stable auroral arc.

6.4

Current Sheets

We have just discussed how the polar cap sun-aligned arcs are associated with flow lines of polar plasma flow, in particular, gradients of plasma flow from rapid antisunward to the dawn side to slower or even reversed (sunward) polar plasma flow to the dusk side. The electric circuitry of the arcs involves current carriers in the form of sun-aligned sheets of outgoing current, carried by sun-aligned sheets of incoming electrons. These current carriers typically have energies on the order of 100s of eV for weaker arcs, and near 1 keV for the strongest (about 2% of the time) arcs. These energies are great enough to produce both optical emissions and ionization (electron-ion pairs). The basic character and underlying general principles of particle-impact excitation of op tical emissions, and production of ionization, have been discussed in Chapter 3. Here we discuss the application of these principles to sun-aligned arc production. We then apply these same principles to the production of ionization later in this chapter.

6.4.1

Optical Emission in and Near Sun-Aligned Arcs

After we distinguish the sun-aligned arc emission from emissions of 630.0 nm from other pro cesses, we must go on to distinguish between emission rate and excitation rate. This distinc tion is necessary to the application of observations to quantitative physical understanding of the processes. Of course, quantitative comparison introduces the requirement for absolute calibration of emission intensities in standard units (R), referenced to international intensitycalibration standards. The following paragraphs discuss background emission, arc emission produced by particle precipitation, and excitation vs. emission after quenching. In the F-region, weak sun-aligned arcs must be separated from a background of other optical emission. For 630.0 nm, this background can be both dissociative-recombination emission and polar-rain and emission. The latter is simply a broad, diffuse emission due to widespread softparticle precipitation, and has been seen to fill a hemisphere of the dark sky up to a (usually sun-aligned) boundary between a brighter and darker area of the polar sky. The former is a calculable 630.0 nm intensity due to polar rain (see Chapter 5, section 5.8) and the background ambient F-region ionization that is recombining as it passes overhead. It is typically on the order of tens to a hundred R, very comparable to sun-aligned arc intensities. Weak sun-aligned arcs are produced by soft-electron precipitation that stops at F-region altitudes and has only detectable 630.0 nm emission. It comes from altitudes so high that there is relatively little quenching deactivation, and is a relatively good indicator of incident flux of electrons of ~0.1keV. Increasingly intense sun-aligned arcs also contain increasingly more hard electrons that penetrate to E-region altitudes, and thereby excite additional molecular emissions. The 630.0 nm emission then becomes increasingly quenched, and emission rates can be significantly lower than excitation rates. The E-region molecular emission intensities then, however, become good measures of the electron-particle energy flux at energies >0.5keV. Recall Chapter 3, section 3.1.2, which defines the close relationship between 391.4nm (or

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427.8 nm) intensities and electron energy fluxes, and E-region maximum electron-density values for the quasi steady-state conditions achieved within sun-aligned arcs. These relationships apply to quantities much below 200 km, where chemical equilibrium applies. The weak sun-aligned arcs are excited by particles of energies <1 keV, which do not pen etrate more deeply into the atmosphere than adequate to excite the atomic species of the atmosphere (which “float” atop the denser molecular species). Atomic oxygen is the domin ant atomic species for earth at the altitudes (~300km) sufficiently dense to stop the ~100 eV electrons. At much greater altitudes (above ~1000km), still lighter species dominate (hydro gen with significant helium at times). We will thus first discuss atomic-oxygen emissions. For the more rare (~2% of the time) intense (~ kR) sun-aligned arcs, the electron energies can exceed a kV. These more energetic electrons penetrate to well below 200 km into an atmo sphere dominated by molecular species (notably and and thus also excite emission lines characteristic of the common auroral zones. Note that there is an important distinction between the red line and the green or blue line (630.0nm vs. 557.7 and 777.4nm). Different than the others, the red-line emission rate is less than its excitation rate. Quenching by collisions with other atmospheric constituents substantially reduces the number of 630.0 nm-photons emitted below the number of state goes into states excited. The 1.96 eV used by the oxygen atom in exciting the vibrationally exciting the local atmospheric gases, instead of radiating from the atmosphere. This affects the apparent location both vertically (low altitude excitation does not lead to low altitude emission) and horizontally (a horizontal motion of, say, 1 km/s common at polar latitudes moves 30 to 90km for a residence time of 30 to 90s). Sun-aligned arcs typically drift dawn-dusk at ~200m/s, and thus may optically appear dawnward (or duskward) by ~10km of where the incoming exciting electrons excite the state. The emission time is also a statistical distribution of many different times (averaging the times noted here), so the 630 nm arc width observed for this red-line emission will be spread wider than the thickness of the exciting electron sheet. For the 557.7 nm emission, the lifetime is ~0.8s, so these considerations are minimal for green-line emission.

6.4.2

Thermal Excitation of Arc Emission

Direct particle-precipitation impact excitation leading to 630.0 nm emission is not expected much above 300km. The reason is simply because precipitating electron energies observed must penetrate to lower altitudes than 300 km before the cumulative number of atomic oxygen atoms they pass builds up to a great enough cross-section to collisionally stop the electron flux. However, soft electrons will lose energy to the ambient background ionospheric electrons by Coulomb collisions, as they precipitate into the upper ionosphere. In time, these electrons are heated to a temperature for which their cooling rate just balances their heating rate by the precipitating electron flux. As will be discussed later in the section on electron thermal balance, for sufficiently great electron densities, this electron-gas cooling is dominated by electron-ion collisions, and electrons remain in relatively good thermal contact with the ion gas. However, if the F-region ionization above the sun-aligned arc is sufficiently weak at its Fregion maximum, or at a sufficiently great altitude, the upper F-region electron gas is insulated from the lower ionosphere/atmosphere to which it must ultimately cool. Then the electron gas temperature builds up to a temperature sufficiently great that its cooling rate by downward electron heat conduction can flush out the heat. This conductivity cooling rate requires a much higher peak electron-gas temperature before it can become sufficiently effective to balance the heat input. (Note the heat input has not changed here, only the cooling mechanism.) Once the downward heat conduction is the only available cooling process available, electron

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gas temperatures may become sufficiently great (>3000K) that they produces thermal red line emission (Figure 6.9). This physical process (insulation) has already been applied with success to explaining previously unexplained behavior of both stable auroral-red (SAR) arcs on the edge of the auroral region (Kozyra et al. 1990), and artificial aurora produced by high-power HF transmitters (Carlson, Wickwar and Mantas 1982, Mantas and Carlson 1996). This thermal excitation mechanism can only operate on emission lines for which the excitation energy threshold is low (e.g., for atomic oxygen the 630.0nm threshold is very near 2eV, and the 557.7nm threshold is very near 4eV). Thermal excitation, while important to thermal-balance calculations, may be crucially im portant to the mapping of processes, particularly in the cusp. Thermal excitation comes from altitudes in the 300–500 km range, in contrast to particle excitation, which comes from altitudes near the 225–275 km range. Oblique-angle observations of red-line bound aries can be very significantly misinterpreted, if their assumed altitude is in error by so great an amount (Lockwood, Carlson and Sandholt 1993). Triangulation eliminates such ambiguity, but is a rare observational luxury, given cost and land accessibility for hospitable observing locations. Yet, strong sun-aligned arcs and cusp red-line emissions are without question some times thermally excited. This area of investigation is almost unexplored to date.

6.4.3

IR and UV Auroral Emissions

Molecules also can store energy in the form of vibrational energy (along the molecular axis) and/or rotational energy (along a transverse axis). Because of the close spacing of vibrationalenergy levels, auroral emissions from molecules have bandwidths of nanometers, whereas

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atomic-line bandwidths are on the order of 0.1 nm or less. These emissions are of import ance for many physical and practical matters, but will not be treated in detail here, as we limit discussion to more readily-observable emissions. IR emissions must be made from space, and as they are in effect “heat emissions”, they are seen effectively only with very special and expensive cooled sensors. UV emission are also mostly seen from space, as they, too, do not penetrate to the ground. Sun-aligned arcs have been studied extensively by satellites ob serving in the UV (Strickland, Bishop, Evans, Majeed, Shen, Cox, Link and Huffman 1999), as discussed in Chapter 5, Figure 5.23 giving global- or quasi-global-scale views of their behavior and properties.

6.4.4

Hydrogen Emissions

Hydrogen optical emissions can be particularly valuable in the study of polar cap arcs (as well as cusp, aurora, and magnetospheric configurations more generally), because their presence is a strong indicator of the source region from which they come. For example, hydrogen ions flowing in alongside of electrons are common in aurora, as signatures of source particles coming from the plasma sheet. Hydrogen ions are found in theta aurora. We suggest that they are present in all strong sun-aligned arcs, and absent from weak sun-aligned arcs. However, observational answers vs. speculation are needed. Recall that protons travel along helical paths spiraling around the magnetic lines of force with a given pitch angle. Before giving off a photon however, they alternate, through charge exchange, between charged and neutral states. While missing an electron (charged), they follow the initial helical path, but while in possession of an electron (neutral hydrogen particle) they move in straight-line paths. Thus, even a narrow beam of protons disperses into a broad swath of emission by the time of the inelastic collision that leads to photon emissions or proton aurora signatures.

6.5

The Polar Ionosphere

The polar ionosphere has properties significantly different from those of the ionosphere at subauroral latitudes (Rishbeth and Garriott 1969). For the midlatitude ionosphere, the dom inant ionizing source is solar radiation 1, and the dominant heating source is likewise solar radiation. Transport is usually dominated by neutral winds and downward diffusion due to gravity. In disturbed auroral regions, the dominant ionizing source is particle precipitation, and the dominant heating source is particle precipitation (for electrons), and neutral particles, Poynting flux, or Joule heating (for ions). The dominant transport is driven by electric fields. While we would have to say that the character of the polar ionosphere is significantly more like the auroral character than the midlatitude, there are significant qualifications. Its Eregion ionospheric densities and conductivities are dominated by local particle production, as for aurora, while its F-region ionospheric densities are dominated by plasma transport in through the dayside sector from upper midlatitudes, where the plasma is largely produced by mid-latitude processes. The transport is dominated by solar wind-driven forces. For polar (winter) night, the electron thermal balance is dominated by polar particle fluxes, while the ion thermal balance is domimated by solar wind-driven Poynting flux.

6.5.1

Polar Ionization

Ionization in the polar cap has lifetimes of hours above 250 km, but minutes to seconds below 180km. 1

Extreme-ultraviolet [EUV] and UV rays and X-rays

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The ionosphere much below 200 km is composed primarily of molecular ions. They are relatively short-lived, recombining rapidly with thermal electrons (dissociative recombination at a rate of Thus, any significant electron density seen in the nighttime polar (or any) region much below 200 km in altitude is a measure of production of ionization at the time and place it is seen. The recombination rate is the product of the number of electrons per unit volume times the number of ions per unit volume Because is a measure of precipitating auroral particle energy flux. At 36 eV per ion pair produced (see Section 3.1.2), in quasi steady-state, production and recombination rates are balanced, thereby relating to particle energy flux through the effective recombination rate. The basic nature of electron and proton impact excitation of optical emissions has been treated in Chapter 3. Here we do wish to draw attention, however, to the fact that there exist very useful relationships between a downward flux of energetic electrons and several associ ated consequences, including the amount of optical emission produced at various wavelengths, ionization produced, and energy deposited in the upper atmosphere. These very useful re lationships are summarized in Section 3.1.2, which allows useful estimates of some of these parameters from other observables. While molecular ions dominate in the E-region, in the F-region, by contrast, atomic ions dominate. Recombination of atomic ions with electrons is greatly slower than recombination of molecular ions, by a factor of about These ions are generally lost through the following two-step process:

or

This recombination-rate limiting process, a chemical reaction with neutral molecules to form a molecular ion (by charge exchange), is followed by rapid dissociative recombination. These reactions explain the dominance of below the F-region. The polar ionospheric morphology described in Chapter 5 is due to the combination of this chemistry with the polar transport of the F-region plasma.

6.5.2 Polar Plasma Motion The earth’s magnetic field greatly influences the motion of plasma in the ionosphere. For this reason, many ionospheric properties are better described in geomagnetic coordinates than in geographic coordinates. If collisions with neutral particles are sufficiently infrequent, electrons and ions will move together in the presence of an electric field (E) frozen to magnetic field lines (B) in the direction perpendicular to both the electric and magnetic fields at a velocity

where the subscripts refer to electrons (e) and ions (i). This equation applies above 200km. Polar plasma transport dominates the polar F-region ionosphere, because of the long life times of ions at F-region altitudes. Ionized particles in the F-region may survive for many hours before chemically recombining. At auroral and polar latitudes, strong auroral electric fields may move ionospheric plasma at velocities on the order of 1 km/s. Ionization may have

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occurred thousands of kilometers away from the location where ions are observed. As polar ionospheric convection is always observed in the high-latitude ionosphere around the auroral oval, the displacement of ions from their source location in the F-region is a continuing process (Heelis 1988). Figure 6.10, from a model calculation by Crain et al. (1993), well illustrates the altitude dependence on ionospheric transport (in this case calculated for a sun-aligned arc-produced ionization). The lifetime above 300km is hours, decreasing to minutes below 200km. The Utah State University modeling group, under Schunk and Sojka, have been a constant source

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of highly valuable modeling guidance to understanding the physics of sun-aligned arcs (Zhu, Schunk and Soijka 1997). We do not yet understand well the three-dimensional currents in a system of sun-aligned arcs, the distinction between ionospheric flow shears and magnetospheric boundaries, the transient aspects of arc growth and decay, and arc effects on other phenomena. The key out standing issue related to sun-aligned arcs, however, is what can we learn from their properties about northward-IMF solar-wind coupling to the magnetosphere? This issue is closely tied to the topological problem of how processes controlling the ionospheric arcs project to and through the magnetosphere.

6.5.3

The Thermosphere as a Rest Frame for Electrodynamics

Because it is the rest frame of the neutral gas that determines currents, effective electric-field strengths, and even plasma-instability growth rates, an understanding of thermospheric winds in the polar cap is crucial. Transpolar thermospheric winds can vary enormously (km/s). Let us consider how this happens and why it is so important. The collision frequency of ions with neutral particles in the upper thermosphere (F-region altitudes) is determined by the charge exchange rate between the dominant neutral and ion species, respectively atomic oxygen O, and atomic oxygen ions When an electron leaps from an O to an the neutral/ion particle energies are of course likewise exchanged (each becomes the other). Near 300km, the neutral density (predominantly O) is order while the ion density (predominantly ) is order to The average ion collides with an O atom about once per second. If a large electric field is applied to the ion, giving it ordered energy in the direction of the electric field, the charge-exchange collision leaves the resultant ion going in a randomly different direction, thereby changing the ordered energy into disordered energy or heat. This description is simply a particle view of F-region Joule heating. However, this particle view clarifies many aspects of momentum transfer and heating rates. For instance, if an electric field is applied to ions, the once-per-second collision frequency means the entire ion gas will heat up in a few seconds. The change in velocity between the initial O, and the product after the electron exchange, clearly depends on the difference between the mean ion and neutral particle velocity. Thus, the ion heating rate must vary with the square of the velocity difference. This relationship in turn means that the ion heating rate must also vary with the square of the electric field, as seen in the rest frame of the neutral gas. Quantitatively, for mid-latitude ion velocities of order 100 m/s, the ion heating rate is negligible; whereas for high-latitude velocities exceeding 1 km/s, the ion heating exceeds 1000K. If the electric field persists long enough, the mean ion drift velocity will drag the neutrals up to their velocity. How long will this take? For ions and atomic oxygen atoms, each atom will on average experience one collision per 1000 ion collisions. Thus, if the transpolar ionospheric velocity is 1 km/s, the thermosphere will come up to a transpolar velocity of 1 km/s in a few times 1000 seconds, or about half an hour. For an ionospheric density of this would take ten times longer, or about a quarter of a day, by which time the earth has rotated so much that the thermosphere is unaffected. Polar ionospheric plasma densities repeatedly alternate between these two values as patches pass across the polar cap for southward IMF, and also alternate over time scales for which the IMF alternates between northward and southward. Thus, the neutral rest frame can vary enormously, by as much as 1 km/s or more. Ion/neutral-gas velocity differences can vary by 2 km/s, including flywheel effects. These velocity differences can have enormous impact on plasma and thermospheric properties at high latit udes. Major errors in theory or model calculations can result if thermospheric velocities are

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assumed or are taken from climatological models, instead of from measured observations. In short, for electric-field-dependent ion chemical-reaction rates, ion-heating rates, currents in the ionosphere, plasma-instability growth rates, i.e., anything dependent on the difference between ion and neutral gas velocities, the effective electric field is defined in the neutral (thermosphere) rest frame. The (thermosphere) rest frame velocity needs to be measured, not assumed.

6.5.4

Plasma Instabilities

The large auroral and polar cap electric fields, currents, and plasma drifts, relative to the neutral atmospheric velocity, can lead to a variety of plasma instabilities. Plasma instability processes do commonly occur, and produce polar ionospheric plasma structure in the nominal range of 10s of km to meters (Basu, Basu, MacKenzie, Coley, Sharber and Hoegy 1990a). These plasma instabilities cause otherwise relatively smooth plasma to develop inhomogen eous structures, B-aligned, over scale sizes ranging from 0.1 to 10km, through which strong radio scintillations are produced. These auroral scintillations lead to strong amplitude fading and phase fluctuations up to GigaHertz (GHz) frequencies, disrupting VHF (30–300 MHz), UHF (0.3–3 GHz), and GHz communications, and even navigation systems at lower frequen cies. These strong signal amplitude fades are significant not only to physics and engineering, but have societal and economic impact. Magnetic-field-aligned irregularities in the auroral ionospheric plasma, driven by instability processes, also scatter radio waves, especially at HF (3 –30 MHz) and VHF ranges, much as glass rods reflect light where the geometry is similar. This auroral scatter, or clutter, can blind radar tracking, disrupt or improve HF communic ation, and serve as an important geophysical diagnostic tool. Its existence has been known since early in the 1930s, when amateur radio operators discovered that during an aurora, it was possible to receive transmissions from an operator located to the south by directing their antennas toward the north. Wave-particle interactions at auroral latitudes also lead to a vari ety of electromagnetic emissions. The particle energy is converted to electromagnetic energy through wave-particle plasma processes, not all of which are understood. These structures, and the plasma instability processes producing them, are a subject of intense research from both the perspective of understanding the associated plasma physics, and the perspective of their major effects on radio-wave propagation for communications, navigation, and satellite imaging (Basu and Valladares 1999).

6.5.5

General Instability Theory for F-Region Ionosphere

Three fundamentally different mechanisms are presently thought to comprise the set of pro cesses by which cusp irregularities are produced, although it is not known which one(s) dom inate for particular conditions. The gradient-drift plasma-instability mechanism is known to occur for plasma drift, of only the right sign, across a steep plasma-density gradient (perpendicular to the magnetic field at high latitudes) (Keskinen and Ossakow 1983). The theory of this basic gradient-drift process has also been extended to include the effects of high-latitude auroral currents on its quantitative thresholds and behavior. Until 1988, this class of gradient-drift instabilities was accepted, by most of the research community, as the only mechanism of importance to ionospheric F-region irregularity formation in the earth’s ionosphere. Velocity-shear-driven plasma instabilities are now also known to occur for sufficiently severe velocity shears (Basu et al. 1988, Basu et al. 1990a), and are believed to dominate the north ward IMF polar cap. Keskinen, Michell, Fedder, Satyanaryana, Zaleasak and Huba (1988) have applied a Kelvin-Helmholtz instability theory to these latter instabilities, adding the

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refinement of ionospheric-magnetospheric electrical coupling. Tsunoda (1988), in a broad re view paper, has further suggested a “stirring” mechanism, that in effect leads to interchange of low-density plasma flux-tubes with high density. Although not discussed there, we note that a theoretical examination of “stirring” would have to include gradients perpendicular to the magnetic field, of mobility or conductivity driven by particle precipitation, a term not yet included, to our knowledge. Of very special importance to distinguishing between different mechanisms, note that for a given percent plasma-density fluctuation, the percent fluctuation in plasma velocity is severely dependent on whether the mechanism is the velocity-shear in stability or the gradient-drift instability. The fluctuation is 10 times stronger for the velocityshear mechanism than the gradient-drift instability. See Figures 6.11 and 6.12. The proposed “stirring” mechanism is not quantified in such a manner. In all three cases, initially largescale processes produce large-scale (many tens of km to ~1000 km) plasma structures. On the edges of these primary large-scale structures, secondary smaller-scale irregularities form. This cascade to ever smaller-scale sizes ultimately fills in a remarkably broad irregularity spectrum from tens of km, through tens of m, down to the ion-gyro radius. Within the framework of this sketch of the general theory of irregularity formation in the F-region, let us now briefly describe physical findings from the two primary experimental tools for research in this area. These findings are, the large-scale processes driving ionospheric structures, as deduced from scintillation of satellite-borne radio-beacon signals propagating through the ionosphere to ground-based radio receivers; and the small-scale physical processes leading to plasma structuring, deduced from satellite in-situ direct measurements of plasma density and electric-field structure. We will note HF and VHF radar echo-related work as well, in terms of radar-signal backscatter from irregularities.

6.5.6

Ground-Based Scintillation Studies

Basu, Weber, Bullett, Keskinen, MacKenzie, Doherty, Sheehan, Kuenzler, Ning and Bon giolatti (1998) have reviewed plasma structuring in the cusp, using radio-wave scintillation techniques, which are most sensitive to irregularities of scale size 10km to 100m. Scintillation intensity is nominally proportional to the fluctuations in line-of-sight integrated-electron dens ity between the ground-based receiver and the satellite radio source. Consequently, for a given percent fluctuation in plasma density, half the total electron content (TEC) means half the scintillation intensity. Both convective (gradient drift) instabilities associated with patches of plasma entering the polar cap through the cusp for southward IMF conditions, and sheared electric fields (velocity shears), will cause plasma density irregularities at scale sizes of tens of kilometers to tens of meters in the cusp. Velocity shear-generated irregularities of a fixed percent will have 10 times the percent electric field fluctuation amplitude (velocity spread) as for convective-generated irregularities. Note that enhanced electric-field fluctuations, pro duced by any mechanism, will broaden the frequency spectra of HF backscatter echoes as well as produce scintillation. Basu et al. (1998) have shown that frequency spectra a little poleward of the cusp have very special characteristics. For the common case of uniform flow of weak irregularities, the spectra have, (a) narrow bandwidths with a spectral maximum at the Fresnel frequency (equal to the irregularity drift perpendicular to the propagation path, divided by the square root of twice the radio wavelength, times slant range to the irregularity layer); and (b) highfrequency roll-off with power-law indices of about three. Immediately poleward of the cusp however, the spectra are, (a) flat-topped and broadband (as would be the case for the sum of several different uniform velocity spectra each with a different velocity), and (b) have more shallow power law indices of about two. These latter two signatures are interpreted as resulting from irregularities moving with a distributed velocity, and the considerable variation

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of corner frequency and spectral width observed from one spectrum to the next is interpreted as indicating that the velocity dispersion varies considerably with time and space. Such broadband spectra are observed over an extended daytime period, but away from the cusp (pre- and post noon, typically 0900 to 1500 MLT), and are likely linked to the plasma mantle via shear instabilities associated with velocity shears across current sheets associated with discrete aurora there. However, the velocity dispersion signature is most enhanced in the

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cusp, near magnetic noon, consistent with electric field fluctuations reported from DE-1 and DE-2 (Basinska, Burke, Maynard, Hughes, Winningham and Hanson 1992). The extension poleward of the cusp of these irregularities is limited by their lifetime and antisunward velocity. Their lifetime is proportional to the square of the irregularity scale size (perpendicular to the geomagnetic field), divided by the electron perpendicular diffusion coefficient (Basu et al. 1998). At 350km in the dark polar ionosphere, lifetimes are minutes to hours for scale sizes of 100m to 1km, respectively. Thus, Basu et al. (1998) interpret their broad-frequency scintillation spectra immediately poleward of the cusp as due to plasma turbulence or shear instabilities.

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6.5.7

Theory of Polar-Cap Sun-Aligned Arcs

Satellite Direct Measurement of Plasma Irregularities

Kivanic and Heelis (1998) have studied plasma structuring with DE-2 satellite in-situ meas urements, deriving spectra of ion density and ion drift-velocity fluctuations, at 16km intervals along the spacecraft trajectory. They calculated the power spectral index at a scale size of 1.3km, near the spectral peak of plasma-density fluctuations in plasma irregularities in the ionosphere, to produce global maps of the intensity of high-latitude plasma structuring. They found enhanced irregularity structuring in two separate global locations, for both ion density and ion velocity. Recall from the discussion just above, the importance of observing both ion density and velocity structure simultaneously; convective instabilities can then be distin guished from shear-driven instabilities (Basu et al. 1988, Basu et al. 1990a). Away from source regions, Kivanic and Heelis (1997) found convective processes to be the more important of the two in the winter polar ionosphere, at least for southward IMF. (Winter in local darkness has very low E-region conductivity, a condition much more conducive to growth of instabil ities that structure the ionospheric plasma. High sunlit E-region conductivity shorts out the instability-producing electric fields. At night one should not ignore F-region conductivity ef fects.) Kivanic and Heelis (1997) in fact found velocity irregularity amplitudes to be enhanced in the cusp (and also polar cap for northward IMF), regardless of season. The location of the cusp enhancement was 75 to 77.5 degrees latitude and 0800 to 1200 MLT. Their data suggested that the observed plasma-density structure is strongly influenced by velocity-irregularity structure applied to large-density gradients. For southward IMF, they found the peak power in the velocity fluctuations was equatorward of that in the plasma density. The spectral power density of the ion-velocity fluctuations decreased much more rapidly with distance poleward of the cusp than did that of the ion density. This dramatic gradient strongly argues for a simple picture of velocity structure in the cusp (source region for polar patches), driven by velocity structure applied from the magnetosphere. The density structure, produced by this driver, decays with its slow plasma diffusion lifetime (hours for ~1 km scale size), as the patch containing the irregularities convects away from the sun across the polar cap. Primary driving of plasma-velocity structure is only in the cusp, so plasma velocity structure rapidly becomes weak away from the cusp for southward IMF (Basu et al. 1990a). These smaller-scale structures, which grow and feed off the larger scale structures in their production region of the cusp, are presumed to decay much faster than larger-density structures as they drift downstream away from the sun and cusp. Kivanic and Heelis (1997) also found that plasma drift-velocity irregularity amplitudes observed near the cusp are almost three orders of magnitude larger than the accompanying polarization diffusion fields. Thus, their observations directly confirmed that the measured electric field, which operates to structure the plasma density, overwhelms the measured electric field that works to remove the structure through ambipolar diffusion perpendicular to the magnetic field. Thus, from their work, one concludes that, although there are definitely regions within the polar cap where convective (gradient drift) instabilities dominate, plasmadensity structure in the 30 km to 300 m scale-size range near the cusp is directly driven by magnetospheric velocity structure. This argues for “stirring” of plasma flux tubes at the edges of strong gradients of plasma density to produce the density structuring. Any future model of stirring would need to cope with the special challenge of temporal and spatial structure in conductivity due to that in particle precipitation.

6.6

Energy Estimates

Particle energy, momentum, and mass are transferred to the magnetosphere from the solar wind. Consequently, significant energy is conveyed to the ionosphere in the form of particle

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energy and/or as electromagnetic energy. We address the role that the auroral particles play in direct excitation of auroral optical emissions, as well as in ionization of neutral particles. Both mechanisms expend energy.

6.6.1

Particle-Energy Deposition

Arriving at an estimate of the particle heating affords an opportunity to bring out several important points. At E-region altitudes, recombination is proportional to a chemical reaction The production rate multiplied by the number of electrons times the number of ions, or rate of ionization can be estimated by the energy of the particle divided by 36 eV, as discussed in Chapter 3. In a quasi-steady state, the production rate just balances the recombination rate. This equivalence leads to a quantitative estimate of the rate of ionospheric heating by precipitating particles as

where is the effective-altitude recombination coefficient (roughly around 120 km), is the electron density and is the altitude (km). Note that the particle heating-rate is also proportional to a A 3 MHz E-region plasma frequency equates roughly to an electron density of of and 0.5 kR of 391.4 nm and 0.15 kR of 427.8 nm emission for a typical auroral layer. These values are representative of diffuse auroras, which are produced by loss-cone particle precipitation, which is also responsible for an extensive, highly uniform, quasi-equilibrium layer of E-region ionization.

6.6.2

Poynting Flux

However, another form of energy deposition often exceeds the rate of energy input to the auroral and polar regions by particles. The downward Poynting flux from the magnetosphere can be dissipated in the ionosphere-thermosphere and can do mechanical work against j × B forces there. Poynting’s theorem states that the rate of energy flow per unit area is equal to In section 5.5.7 and Figures 5.27 and 5.28 we presented the calculation of the vertical component which was computed using the electric field and the horizontal current of the Poynting flux obtained solely from radar measurements. Following Ampere’s law and the current continuity and (2) the equation and assuming (1) only the field-aligned current sheets contribute to magnitude of the total downward current is equal to the magnitude of the upward (they are considered to be closed by horizontal Pedersen currents), the is nonzero within the arc and given by

where is the average value of the horizontal current measured at the dawn and dusk boundaries of the S-A arc. For satellite or rocket data time may be short compared to Alfvén bounce period, so temporal variation cannot be excluded from those data, whereas the ISR data show steady conditions. We can think of this type of energy input to the ionosphere as starting with mechanical energy in the solar-wind generator that gets conveyed to electromagnetic energy. That electro magnetic energy, in turn, is conveyed down the geomagnetic-field lines (as Poynting flux) into

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the ionosphere, where it is dissipated as Joule heating. Joule heating in the ionosphere is in the ionospheric rest frame. Thus, for an applied electric field, energy is dissipated by the current component parallel to E, that is, the Pedersen current; Hall current (perpendicular to E) is non-dissipative. This relationship leads to a Joule-heating dependence on characteristic energy, because higher-energy particles penetrate more deeply into the atmosphere, where the is greater than the Pedersen conductivity and the currents tend Hall conductivity to be non-dissipative. Joule heating then becomes less important. Lower-energy particles produce ionization at higher altitudes where Pedersen conductivity is greater than Hall con ductivity, and Joule heating is relatively more important. Because the current density equals the electric field (E) times the Pedersen conductivity, the height-integrated Joule heating rate can be expressed as For limited times and locations, where = altitude integral of this heating rate may be many times An important reference for all atmospheric-energy calculations is the globally-averaged energy deposited in sunlit regions by solar UV. It is about above roughly 110 km. This quantity is a valuable reference quantity to use as a measure of the significance of upper-atmospheric energy input under any circumstances. Here, it should be compared with the integral (over altitude) of the rate of Joule heating where must be expressed in the rest frame of the neutral atmosphere as

where is the neutral-wind velocity. The total heating of the lower thermosphere by auroral energy dissipation, with heating rates transiently exceeding that of UV from an overhead sun, will change the temperature, density, composition, and winds of the local thermosphere. In fact, the global-scale thermo spheric and ionospheric responses to such variable heating are subjects of ongoing research.

6.7

Electron and Ion-Gas Thermal Balance

We have discussed the energy input to the ionospheric regions, in the form of particle energy and Poynting flux. This energy is ultimately dissipated, very largely in the ionosphere and upper atmosphere. Here we discuss two major sinks for this energy, the electron gas and the ion gas. These gas temperatures are elevated as long as energy is supplied to them. The flow of energy is typically from the electron to the ion to the neutral-particle gas. However, we will see that on occasion, energy flows from the ion to the electron to the neutral-particle gas. From there, the energy escapes from the local deposition region, either by heat conduction to regions of lower temperature, or escape of photons.

6.7.1

General Considerations for Plasma Thermal Balance

Magnetic field-aligned currents, carried by 0.1 to several keV electrons in aurora, polar cap arcs, and the cusp, will heat the ambient thermal electron gas. For the latter two cases, the precipitating electrons arc soft. For a given energy flux, they produce relatively more secondary electrons at F-region altitudes and greater F-region electron-gas heating rates. For the polar cap arc and cusp soft-electron situation, it is essential to keep in mind that the temperature to which the ambient electrons are heated depends on a thermal balance between well-defined heating and cooling rates. Often it can be the cooling rate of the electron gas in the ionosphere that is the primary variable determining the quasi steady-state electron-gas temperature. The F-region electron

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gas cools by the following mechanisms: downward heat conduction through the electron gas, to sufficiently low altitudes that electron-neutral collisions become frequent enough to bring the electron gas into good thermal contact with the thermosphere (below 200 km); and by collisions locally with the ions. The latter cooling rate is proportional to the number of electrons times the number of ions with which they collide. The ions quickly cool by collisions with the local neutral particles (below ~450 km). The cooling rate of the electron gas is thus proportional to the square of the electron density For the relevant collision cross-sections, when the electron density is much below the electron gas becomes in poor thermal contact with the local ion gas, and can only cool by downward heat conduction, the latter being directly dependent on the altitude gradient of the electron temperature. The electron temperature increases until its altitude gradient is so large that cooling by downward heat conduction can the carry away all the locally-deposited heat. For electron densities much above electron gas is in good thermal contact with the ion gas. It then cools directly to the local ion gas, and thermal balance is achieved for much lower electron temperatures. (The ion gas is clamped closely to the neutral-particle gas temperature by ion-neutral collisions for altitudes below 450km). This strong transition introduces an approximation to a two-position switch, driven by the electron density, between high or low electron-gas temperature. The actual magnitude of the “high” temperature of course depends on the heating rate. For heating rates sufficiently high that the thermal balance is achieved with electron temperatures above 3000 K, a third electron-gas cooling term comes in, excitation of the lowest energyexcited state of atomic oxygen The very strongly temperature-dependent excitation of 630 nm optical emission by thermal electrons becomes appreciable.

6.7.2

Electron Gas Thermal Balance

Under conditions of normal daytime ionospheric production by solar UV and EUV, when the photon energy breaks the neutral particle into an electron and an ion, conservation of momentum (electron and ion mass times velocity) requires that the electron energy (mass times velocity squared) greatly exceed that of the ion. These energetic electrons have a much greater cross-section for cooling to other ambient electrons than to ions or to neutral particles. They heat the electron gas, which then cools at higher altitudes by collisions with ions and by heat conduction, and, at sufficiently low altitudes, by direct electron collisions with the neutral gas. Thus, heat flows from the electron to the ion gas, then from the ion to the neutral gas, by collisions. When production of ionization is by precipitating electrically-charged particles, the electrons still carry most of the initial energy, so as to produce the same heat flow from electron gas to ion gas to neutral gas. Only when significant Joule heating occurs does this sign reverse, with heat flowing from a hot ion gas to a cooler electron gas. Here, heat goes directly into the ions because of very large electric fields driving collisions of very high-velocity ions, which collide with neutral particles at rest. For typical conditions then, the thermal balance of the electron gas is determined by thermal-balance relationships. These relationships define the rate of heat into the electron gas in a local volume, the rate of thermal heat conduction out of the volume, the rate of heat transfer from the electron to the ion gas within the volume. For altitudes well below 200km, there is an additional term for the rate of heat loss directly from the electron gas to the neutral gas. For practical ionospheric conditions, for a case where the heating rate of the electrons may be turned on or off quickly (seconds), illustrative profiles of ionospheric electron-gas time response can be found in Mantas and Carlson (1996), along with the cooling rate equations with coefficient values for representative thermospheric conditions. The electron-to-ion energy transfer rate is

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where atomic oxygen ions dominate. The rate of energy transfer to hydrogen or helium ions would be much greater per ion, because of their lower ion mass. However, they are minor species near F-layer peak ion densities, and all but absent at polar latitudes. The heat conduction electron-gas cooling rate is extremely electron-temperature depend ent, and can dominate whenever steep electron-density altitude gradients start to become large. The downward (or upward) rate of heat conduction is

where I is the magnetic field angle of inclination, and perpendicular to the magnetic field line

6.7.3

is the heat rate through a surface

Ion Gas Thermal Balance

The earlier discussion of Poynting flux energy deposition, dissipated as Joule heating, was formulated as a collective-interaction description with the ionosphere as a resistive load. It is often instructive to take both a collective view and a particle view of interaction processes. Thus, this section examines the fate of the Poynting flux energy deposition from the viewpoint of energy dissipation by particles. Auroral and transpolar electric fields drive plasma at an ion velocity at high latitudes. The plasma is electrically neutral, with An individual ion, upon collision with a neutral atmospheric particle, will exchange energy and be deflected in some new direction. Thus, the ion’s ordered motion in the plasma-drift direction becomes disordered motion, that is, heat. Prom this particle view, it is thus easy to see that the ion-gas temperature heats up rapidly over a few ion-neutral collision times. A useful number to recall for collision frequencies is that at about 140 km, the ion-neutral collision frequency roughly equals the ion gyrofrequency, about The decreases exponentially with decreasing altitude, in proportion to the neutral density. At F-region altitudes (~300km), the time for the ions to heat up by this process is only on the order of seconds. The ion temperature increases by this ion-neutral collision process at a rate (Banks and Kockarts 1973)

so that for a quasi-steady state, ion temperature

exceeds

by

where is the mean mass of the neutral atmospheric gas, and is Boltzmann’s constant, and is the ion gas or neutral gas velocity. Note that temperature enhancement varies as the square of the plasma velocity in the neutral rest frame, and is negligible for midlatitude velocities on the order of or less, but exceeds 1000K for auroral and polar cap velocities much greater than

6.7 Electron and Ion-Gas Thermal Balance

6.7.4

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Overall Energy Flow Within the Arc

In a previous section in this chapter, we discussed in some detail the flow of energy from Poynting flux input, driven by the solar wind, through ultimate energy loss to the thermo sphere. Many physical parameters were derived in the course of tracing the energy from its initial to final form. We do not repeat that information here; however, we need to note that several important elements of theory were applied to tracing the downward energy flow. These elements were summarized in Figures 5.26 to 5.28, which presented actual values derived for a representative strong arc. Figure 5.26 shows, from top to bottom, the derived parameters of, a) Pedersen conductivity, b) Hall conductivity, c) plasma velocity vectors, d) and e) electric field components in the X and Y direction, respectively, f) and g) horizontal currents in the X and Y direction, respectively, h) the Birkeland current, i) the ion-neutral gas temperature difference (Joule heating term), and j) the Poynting flux. Figure 5.27 for the same data set shows, a) height-integrated Pedersen conductivity, b) energy deposition from precipitating electrons, c) at 250km altitude, d) Poynting flux, e) at 110 km altitude, f) Joule heating rate, and g) energy transferred from ions to neutrals (for two assumed ion compositions, one of which shows excellent agreement, the other of which is low by the factor of two lower-collision cross-section for that ion). The physical relationships for deriving all these parameters from the four directly-measured parameters are found in Chapter 5, section 5.5. The narrative below describes the rationale for the calculations. Within the center of the arc, where is enhanced below 200km, must be enhanced by the incoming flux of impact-ionizing electrons. These electrons must also carry an incoming particle-energy flux, and an outgoing Birkeland current, to one side, while there must be a return (incoming) current on the other side. The Pedersen currents must decrease from dawn to dusk, as the antisunward plasma drifts must decrease from dawn to dusk. The plasma drifts may simply decrease, or reverse as in Figure 5.13. The ion temperature will be enhanced where the difference between the plasma and neutral atmospheric velocities is sufficiently great. This enhancement depends on the thermospheric winds across the polar cap, and the extent to which plasma drifts may have strong sunward flow on the duskside of the arc. will generally be enhanced on the dawnside, and occasionally on the duskside. Likewise, the Poynting flux typically peaks on the dawnside, but peaks on the duskside if there are strong sunward flows on the duskside. The magnitude of the Poynting flux is many for relatively intense sun-aligned arcs, exceeding the particle energy. Both are smaller by a factor to be statistically defined for more common weaker arcs. The cross-sectional area maps of the plasma density and thermal character across stable sun-aligned arcs show the electron-gas temperature is enhanced over the are, as must be the case, since the arc is produced by a sheet of incoming energetic electrons. Significantly, along the high-velocity dawnside of the incoming energetic electron sheet producing the arc, there is a channel of enhanced ion temperature, within which the ion temperature exceeds that of the electron gas. This sun-aligned channel, within which heat flows from the ion gas to the electron gas, coincides with the high antisunward velocity channel along the dawn edge of the sun-aligned region of enhanced electron density. In addition, if there is also return sunward flow at sufficiently high speed, ion heating is also found on the duskside of the arc. The ion gas heating can be explained solely on the basis of ion frictional drag of high plasma velocity in the thermosphere rest frame. Persistent strong Joule heating driven by Poynting flux down into a sun-aligned arc has been calculated and found to be at a rate well exceeding the particle energy flux. This rate is estimated by three independent calculations based on a horizontal current differential, a velocity differential, and a temperature differential. Furthermore, the magnitude of this newly-measured heat flux is several and as such is of significance to the polar thermospheric energy budget, and contributes to the solution of the missing polar thermospheric heat source.

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6.8

Theory of Polar-Cap Sun-Aligned Arcs

Thermospheric Heating and Momentum Transfer

As noted above, the thermosphere is largely the ultimate sink for the energy conveyed to near eartli space from the solar wind, via transmission through and storage in the magnetosphere, and is initially deposited in significant measure into the ionosphere. We have noted that the polar processes have transpolar scale effects on the plasma, thermal, optical, and electrical properties of near-earth space. This ultimate energy deposition into earth’s upper atmosphere, in turn, can have global impact on the upper atmosphere. That, in turn, has global impact on earth’s ionosphere, since the solar UV ionization effects depend on the upper-atmospheric densities, neutral-particle compositions, and global neutral-wind patterns. We shall see that all of these can be modified by polar processes.

6.8.1

Thermospheric Heating

In sharp contrast to the ions, the neutral-particle gas takes much longer to respond and, when it does, responds dramatically by a momentum change, rather than by a thermal change. Heat into ions exposed to an electric field and an (which physically may be thought of as resistive Joule heating in a conductor, randomization of ordered ion-particle motion, or frictional-drag heating of the ion gas dragged through the neutral-particle gas) is ultimately lost to the neutral gas. However, the thermospheric temperature increases much less dramatically than the ionospheric temperature previously discussed, because of its greater particle and mass densities, which lead to far greater heat capacity. The globally-averaged energy deposited in sunlit regions by solar UV is above roughly 110 km. The number is a valuable reference quantity to use as a measure of the significance of upper-atmospheric energy input. It should be compared with the integral (over altitude) of the rate of Joule heating expressed in the rest frame of the neutral atmosphere. For limited times and locations, this heating rate may be many times in polar cap arcs, due to Poynting flux and consequent Joule heating. Particle-flux heating is a fraction of this value, but still significant with respect to UV heating. Thus particle flux may also contribute to the “missing heat source” sought for polar heating by modelers, who empirically match global ab initio thermospheric-circulation model calculations against global data to derive empirical parameters for optimum fit. Earlier in this chapter, we discussed that not all electron-impact excitation of atomic oxygen states, capable of emitting a 630.0 nm photon, actually led to realization of such emission. Particularly for the more intense arcs, with electrons penetrating to lower altitudes, a major fraction of O excited states were quenched by collisional deactivation. Most of this energy goes into vibrational excitation of molecular nitrogen. This energy also needs to be included in addressing the thermal-balance issues in the upper atmosphere. Vibrational heating of molecular nitrogen takes away most of the energy that would otherwise be stored in the 2 to 4 eV energy range by atomic oxygen for altitudes much below ~ 225 km. The energy goes into the local molecular gas (thermospheric heating) vs. escaping the atmosphere as visible red light. The thermosphere cools by release of IR emissions, and by downward heat conduction.

6.8.2

Thermospheric Winds

For a steady electric field (E) moving the ions along an ordered trajectory, we noted that ion-neutral collisions cause the ordered motion of individual ions to become randomized in direction, resulting in heating. However, for an upper atmosphere at rest, the momentum transfer from the ions to the neutral particles is systematic, in that it always has a component downstream in the direction of plasma flow. Whereas an average F-region ion collides with a

6.9 Composite Essential Arc Features, Properties, and Processes

[ 265 ]

neutral particle on the order of once per second, an average neutral particle collides with an ion much less often. Thus, the thermosphere responds much more slowly, because, at F-region neutral particles altitudes, there are more than 1000 times as many neutrals as ions vs. typically ions and electrons). Thus, the average ion has to experience ion-neutral collisions, which occur with afrequency before the average neutral particle For large ion densities, experiences one neutral-ion collision, occurring with a frequency the thermosphere may eventually be brought up to the speed of the ions. Quantitatively, the ion-drag force on the neutrals acts over a time scale given by the neutral-gas momentum equation (neglecting gradient terms),

where we assume that the F-region neutral wind varies negligibly with altitude, and where is the gas density (number density times mass) for the ions and neutrals, as indicated by the i and n subscripts. The time for the thermosphere to respond is then given by Using typical val ues for and for atomic oxygen ions, we find that the response time for typical F-region altitudes and thermospheric temperatures/densities (250 -450 km, 750-1 500 K) is Thus, for a typical daytime ionospheric density of the ther mosphere is dragged up to the ionosphere plasma drift velocity (typically ~ 1 km/s) over the polar cap for a southward IMF) in about 30 minutes (Figure 6.13), whereas for ionospheric ion and electron densities of it takes 6 hours. However, in 6 hours, the earth has rotated 90°, and the thermosphere is exposed to a very different part of the ion convection pattern, which is fixed in the earth-sun reference frame for a fixed IMF condition. Thus, for ion densities near and above the upper-atmospheric winds are tightly coupled to the plasma drift within roughly 30 minutes. For densities near and below this wind coupling is almost negligible. Winds are also often seen to blow in all directions from the polar cap toward mid-latitudes, with strength and duration exceeding present ad initio model values. These excess equatorward winds may also be due to extra polar thermospheric heating associated with polar cap arc and patch energy deposition. This is another problem not yet fully treated.

6.9 Composite Essential Arc Features, Properties, and Processes Let us now summarize what we can conclude about the nature of sun-aligned arc phenomena in the ionosphere, in near-earth space, and in more distant space.

6.9.1

Signatures of Sun-Aligned Arcs

Polar cap arcs are the optical emission signatures of steep electric field gradients in the dawndusk direction within the polar cap. Given their simple arc electrodynamics (Carlson 1990), a straightforward first-principles approach to analyzing a sun-aligned arc reveals many properties it must have, with consequent signatures it must have (Burke et al. 1982). Briefly, solar-wind mechanical energy is transformed to electrical energy at the magnetopause interface. This energy is conveyed down magnetic field lines as Poynting flux to the ionosphere, where it drags high-speed curtains of plasma antisunward, dissipating the electrical energy as Joule heating of the ions at the feet of the field lines (ion-neutral collisions for particle view). The

[ 266 ]

Theory of Polar-Cap Sun-Aligned Arcs

resulting dawn/dusk plasma-velocity gradient has high velocity on the dawn side and low antisunward or sunward velocity around the dusk side of the velocity gradient (converging electric field gradient). The sense of the velocity difference across the boundary determines whether it tends to drive a horizontal convergence or divergence, and thus whether a fieldaligned upward or downward current is required to maintain a divergence-free state. Here the horizontal current gradient requires a field-aligned current up and out of the arc, carried by (soft and/or hard) precipitating suprathermal electrons (electrons being far more mobile than ions). The sheet of precipitating electrons produces secondary electrons, ambient electron-gas heating, and ionization, which thereby modifies the conductivity across the arc, and by electron impact, the optical signature of the arc. A self-consistent 3D current system is set up, adjusted to the modified conductivity, and with a return current sheet adjacent to the arc and carried by thermal electrons. As an arc grows stronger, the precipitating electrons grow harder (faster electrons meaning more current per unit particle flux), and multiple arcs form. Thus signatures of these arcs must include (see Figure 6.1),

6.9 Composite Essential Arc Features, Properties, and Processes

[ 267 ]

A curtain (sun-aligned) of rapid antisunward moving plasma on the dawn edge of the arc. The slowing of plasma velocity toward dusk, and often reversal toward sunward around the dusk edge. An upward current sheet within this converging electric field, carried by a precipitating sheet of soft and/or hard electrons (producing arc optical signature at their feet). A core of enhanced E-region (hard electrons) and lower F-region (soft electrons) ioniza tion under upward current sheet, and separately, modified ratio of molecular-to-atomic ions, as per modified production and recombination rates. An adjacent sheet of downward current carried by escaping thermal electrons. A core of high ion temperature in the E-region on the dawn edge of the arc (Joule heating under Poynting flux). A sheet of hot electrons over the arc, heated by precipitating suprathermal electron flux. For thermal balance, both heating by precipitating electrons and cooling to enhanced ionization vary across the arc. Furthermore, these signatures must also be present in any velocity-shear Ohm’s-law arc. For example, they are observed in the quiet dawn auroral oval, in flow channels in the south ward IMF cusp, at the poleward edge of the dusk auroral oval, in post-midnight “hook” arcs, as well as in other arcs. For non earth-sun aligned features, e.g., hook arcs, the dawn/dusk side of the arc instead becomes the fast antisunward plasma flow side (vice dawn), and slow or sunward flow side (vice dusk). Not all of these signatures have been necessarily appreciated in cusp velocity shear (reconnection) events.

6.9.2 Near-Earth Physics The presence of this sun-aligned sheet of incoming electrons causes the temperature of the electron gas to be higher above an arc (double or more) than elsewhere in the surrounding ionosphere. The actual increase depends on the incident flux of suprathermal electrons from the magnetosphere, and on how well the lower ionosphere is thermally coupled to its ther mospheric heat sink. For low electron densities, and thus low electron-gas cooling rates, the electron temperature increase may exceed 3000 K, leading the heated ambient electrons to enhance the 630-nm emissions. As the cross-arc gradient of antisunward plasma flow velocity (cross-arc gradient of E) increases, the upward current out of the arc must increase to carry off the enhanced hori zontal current converging on the arc. Field-aligned-current densities can grow by increasing the number and/or the speed of the current carriers. The latter effect causes precipitatingelectron fluxes to become harder, as is observed. The fluxes of precipitating hard electrons increase E-region ionization. It then follows that intense arcs will have underlying E-region ionization, whereas weak arcs will not. It also follows that the particle energy flowing into intense, stable sun-aligned arcs can be estimated from measurements of either molecular op tical emission (which can exceed kilorayleighs) or E-region electron densities (which can ex ceed Recall that the global mean thermospheric EUV heating rate is about Electron-particle energy deposition within an intense arc exceeds this refer ence solar-radiation rate. Plasma on the dawn edge of the arc is found to flow antisunward at about 1 km/s, and to decrease duskward across the arc. If thermospheric winds are light, the velocity difference

[ 268 ]

Theory of Polar-Cap Sun-Aligned Arcs

between the ions and the neutrals can be used to calculate the Joule heating rate. It follows from the square-law dependence of the velocity difference that the heating will be concentrated near the high-plasma-velocity (dawn) edge of the arc. Measured ion and neutral velocities across an intense arc imply Joule heating rates of several times concentrated near the dawn edge, with net energy into the arc exceeding that from the incident-electron source. This Joule heating rate has been confirmed, based on calculating the rate of ion heating required to maintain the approximately 1000 K by which ion temperatures have been observed to exceed those of the neutral gas. This energy that appears in the ionosphere in the form of heat is conveyed into the ionosphere in the form of electromagnetic energy (Poynting flux). Measurement of the Poynting flux simultaneously with Joule heating rates has demonstrated that the foregoing calculation is consistent with observation (Valladares and Carlson 1991). These physical arguments are not confined to sun-aligned arcs. Polar-cap convection can take different forms, depending on whether the solar-wind mag netic field is nearly parallel or nearly antiparallel to the subsolar field of the earth. When these fields are parallel (i.e., the IMF is northward), sun-aligned arcs are seen to occur. Near the central polar cap, the sun-aligned boundaries of the arc drift toward dawn (dusk) for negative (positive) IMF in the northern hemisphere. The connection of the arcs and the relation of their drifts to the coupling of the magnetosphcre and the solar wind are challenging topics, but beyond the scope of this treatment.

6.9.3

Far-Earth Physics

We have made it abundantly clear that the controlling processes and the properties of polar cap sun-aligned arcs are determined by solar wind driving forces, with dominant IMF de pendencies. Just as for the case of cusp and dayside aurora discussed elsewhere in this text, our interpretation of the behavior and character of polar arcs is based on models utilizing our present understanding of these IMF dependencies. Our understanding has evolved to our present state over time, but key work leading to our present understanding includes work by Reiff and Burch (1985), Heelis (1984), Heppner and Maynard (1987), Lyons (1980), Cowley and Lockwood (1992), (Crooker 1979), and others. Our present view of sun-aligned arcs is best explained in terms of the models developed and applied by Cowley and Lockwood. We present here our interpretation of sun-aligned arc properties and behavior within the context of such present models. As we discussed the behavior, we will note examples where the sun-aligned arc observations have themselves extended and refined the models now in use for such purposes. Such observations will continue to advance our understanding of such modeling, and of magnetospheric topology. The number of convective cells, and the plasma circulation within them during north ward conditions, has been presented by several authors. Reiff and Burch (1985) applied the antiparallel merging theory of Crooker (1979) to investigate the consequences of the merging of northward IMF field lines with closed field lines at the dayside polar cap boundary, and subsequent reconnection on the nightside. They postulated the existence of a region of closed field lines flowing sunward at the center of the polar cap. Convection patterns for northward IMF, inferred from experimental data, also show the existence of a region of the polar cap containing sunward flow (Heppner and Maynard 1987). Burch, Reiff, Heelis, Spiro and Fields (1980) have used DE-1 images and in situ measurements to further develop a suggested con ceptual model for the quiet-time polar cap, IMF dependencies included. The polar cap auroras they dealt with, and refer to as sun-aligned arcs, theta aurora, and horse-collar aurora, are of necessity sufficiently intense to be seen by the DE-1 imager. The database we deal with here, using ground-based all-sky image-intensified photometers (ASIPs), have over 10 times the sensitivity, and see sun-aligned polar cap aurora more like half the time, rather than the

6.10 Further Physical Insights From Statistical Studies

[ 269 ]

few percent of the time that they can be seen by satellite-based observations.

6.10

Further Physical Insights From Statistical Studies

Further physical insight into the underlying processes controlling sun-aligned arcs has come only from noting the consistencies of behavior of very large numbers of arcs. We list here the key findings from such statistical studies (Vallaclares et al. 1994). In agreement with findings reported by other researchers, more polar cap arcs were seen, above the sensitivity threshold of the images (tens of R), on the dawnside (40% probabil ity) than on the duskside (10%). This large difference in the observed occurrence of polar cap arcs may follow from the convergent/divergent nature of the electric fields that are usually encountered in the polar cap during values of the IMF. The dawn cell has been generally described as a region of convergent electric fields containing upward Birkeland currents. This fact facilitates the flow of more intense electron fluxes, thereby generating brighter auroral emissions. By contrast, the dusk cell has been classified as a region of downward currents, which are not conducive to intense precipitation. The overall probability of observing an arc in the polar cap was found to be at least 40%, for a threshold of tens of R in 630 nm. As expected, the probability of detecting a polar cap arc was found to depend strongly This probability becomes large when is positive, and increases on the value of with increasing It sharply diminishes during negative values of Unexpectedly, we found that about 20% of the total number of the polar cap arcs observed during the winter of 1986-1987 occurred under conditions. However, inspection showed that these arcs developed after prolonged periods of northward and followed steady displays of generated sun-aligned polar cap aurora. This delay between the reversal and the disappearance of the polar cap arcs, is interpreted as the time needed for the magnetosphere to complete reconfiguration into a topology, that is, the time for the IMF change to propagate from the dayside magnetopause to the nearmidnight region. All arcs visible to the ASIPs showed negligible dependence of the location of the arcs within the polar cap, on either the By or the components of the IMF. However, there is a significant By dependence of the dawn-dusk direction of motion of the arc. Therefore it must follow that if By maintains a fixed sign for an extended period of time (order an hour), the arc must shift to a favored hemisphere (e.g., duskside for By > 0). It is also in long-lived cases (hours) that the arcs become sufficiently intense (> kR) that they are visible from space or by conventional all-sky cameras. Because satellites only see the few brightest arcs, they would get the impression of a stronger By dependence. The polar cap arcs observed in the 0600 –1200 and 1200 – 1800 CGLT quadrants present a small offset angle with respect to the sun-earth line. The offset angles are such that the arcs in both quadrants tilt toward the cusp. In addition, there is a small, but detectable variation of the arc’s orientation with By. This latter modest rotation is interpreted as a displacement of the cusp region in magnetic local time coordinates. Polar-cap arcs observed in the 0000 – 0600 and 1800 –2400 CGLT were sun-aligned, with no detectable dependence on By. Polar-cap arcs located at the dawnside cell were observed to move duskward, and the arcs situated at the duskside cell moved toward dawn. However, the size of the cells was

[ 270 ]

Theory of Polar-Cap Sun-Aligned Arcs

found to depend on the magnitude and sign of Temporal variations in the X or Y components of the IMF do not seem to affect the arc motion. We suggest here that the arc motion is the result of the entrance of open flux tubes into the polar cap ionosphere-magnetosphere, producing a general displacement of the cells, and consequently of the arcs embedded within the convection cells. Following the line of reasoning of Reiff and Burch (1985), but guided by the additional database of ASIP continuous polar photometric images, it is suggested that polar plasma circulation for northward IMF leads to a persistent forcing of open and closed field lines poleward from both the dawn and dusk regions (Figure 6.14). For field lines open into a dusk cell, while field lines close near midnight, forcing a dawnward displacement of shear lines in a residual dawn cell. For the mirror-image situation will occur. This simple model can provide the behavior of the 150 sun-aligned stable polar cap arcs used as the basis of the Valladares et al. (1994) statistical study, as well as theta aurora (Frank et al. 1986), and the teardrop-shaped aurora (Meng 1981, Murphree and Cogger 1982), which have also been called horse-collar aurora (Hones Jr., Craven, Frank, Evans and Newell 1989).

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and AE (12) using

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