COSPAR COLLOQUIA SERIES VOLUME 9
MAGNETOSPHERIC RESEARCH WITH ADVANCED TECHNIQUES
PERGAMON
Sponsored by THE CHINESE N A T I O N A L C O M M I T T E E FOR COSPAR THE C O M M I T T E E ON SPACE R E S E A R C H (COSPAR) and
THE N A T I O N A L N A T U R A L SCIENCE F O U N D A T I O N OF CHINA Chairman Yixun Yah (China) G. Haerendel (Germany) Program Committee Ronglan Xu (China, Main Scientific Organizer) G. Rostoker (Canada, Deputy Organizer) A. T. Y. Lui (USA, Deputy Organizer) G.K. Parks (USA) B. Wilken (Germany) Wenyao Xu (China) Shui Wang (China) D. N. Baker (USA) R. Lundin (Sweden) Heng Du (China) U. Inan (USA) E. T. Sarris (Greece) V. N. Oraevsky (Russia) H. Matsumoto (Japan) Zhenxin Liu (China) L. Zelenyi (Russia) Zuyin Pu (China) C. Z. Cheng (USA) Guocheng Zhou (China) Associate Editor Lei Li (China)
MAGNETOSPHERIC RESEARCH WITH ADVANCED TECHNIQUES Proceedings of the 9th COSPAR Colloquium held in Beijing, China, 15-19 April, 1996 Edited by
R. L. Xu Centerfor Space Science and Applied Research, Chinese Academy of Sciences, P. O. Box 8701, Beijing 100080, China and
A. T. Y. Lui Applied Physics Laboratory, The Johns Hopkins University, Johns Hopkins Road, Laurel, Maryland 20723-6099, USA
PERGAMON
U.K
Elsevier Science Ltd. The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, U.K
U.S.A
Elsevier Science Inc., 655 Avenue of the Americas, New York, NY 10010, USA
JAPAN
Elsevier Science Japan, 9-15 Higashi-Azabu 1-chome, Minato-ku, Tokyo, 106 Japan
Copyright 9 1998 COSPAR All rights reserved No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means; electronic, electrostatic, magnetic tape, mechanical, photocopying, recording or otherwise, without permission in writing from the publishers.
First edition 1998 Library of Congress Cataloguing in Publication Data A catalogue record for this book is available from the Library of
Congress British Library Cataloguing in Publication Data A Catalogue record for this book is available from the British Library. ISBN 0 08 0433 308 In order to make this volume available as economically and as rapidly as possible the authors' typescripts have been reproduced in their original form. This method unfortunately has its typographical limitations but it is hoped that they in no way distract the reader. Whilst every effort is made by the publishers and editorial board to see that no inaccurate or misleading data, opinion or statement appears in this publication, they wish to make it clear that the data and opinions appearing in the articles and advertisements herein are the sole responsibility of the contributor or advertiser concerned Accordingly, the publishers, the editorial board and editors and their respective employees, officers and agents accept no responsibility or liability whatsoever for the consequences of any such inaccurate or misleading data, opinion or statement.
CONTENTS Preface R. L. Xu, and A. T. Y. Lui Opening Address of the COSPAR President to the COSPAR Colloquium on Magnetospheric Research with Advanced Techniques G. Haerendel Section I. Magnetospheric Observation and Measurement Techniques
Investigation of a Substorm Following an Extended Interval of Northward Interplanetary Magnetic Field A. T. Y. Lui, D. J. Williams, R. W. McEntire, S. Ohtani, L. J. Zanetti, W. A. Bristow, R. A. Greenwald, P. T. Newell, S. P. Christon, T. Mukai, K. Tsuruda, T. Yamamoto , S. Kokubun, H. Matsumoto, H. Kojima, T. Murata, D. H. Fairfield, R. P. Lepping, J. C. Samson, G. Rostoker and G. D. Reeves Structure and Evolution of the Current Sheet by Multi-Spacecraft Observations X. Y. Zhou, C. T. Russell and J. Gosling
17
Energetic Oxygen Ion Bursts in the Distant Magnetotail Q. G. Zong and B. Wilken
23
Present and Future Research Program in Solar-Terrestrial Physics at Zhongshan, Antarctica R. Y. Liu Evidence for Resonant Absorption of VLF Waves Obtained at Zhongshan Station, Antarctica K. Y. Tang, F. L. Peng, Z. L. Ning, W. Z. Cao, Q. F. Meng, Y. H. Yang and C. M. Jiao
33
37
Global-Scale Imaging: New Approaches in Magnetospheric Research J. L. Green, S. F. Fung, D. L. Gallagher, M.-C. Fok, G. R. Wilson, G. R. Gladstone, J. D. Perez, P. H. Reiff, J. L. Burch and T. E. Moore
41
UV Auroral Imaging J. S. Murphree
51
vi
Contents
High Altitude Electrostatic Fields Driving Subauroral Ion Drifts J. F. Lemaire, M. Roth and J. De Keyser
61
Search for Lunar Pickup Ions E. Kirsch, B. Wilken, G. Gloeckler, A. B. Galvin, J. Geiss and D. Hovestadt
65
The Tether System Used in the Magnetospheric Studies S. I. Klimov, F. L. Doudkin, V. E. Korepanov, A. A. Petrukovich, M. L. Pivovarov and A. V. Prudkoglyad
71
Simulations of a Spherical Section Electrostatic Analyzer J. H. Vilppola, P. J. Tanskanen and B. L. Barraclough
75
A Numerical, Double-Layer Solution for Plasma Contactors Collecting Electrons in Electrodynamic Tether Applications A. J. Li and D. W. You
85
Section H. Active Experiments
Modification Natural And Manmade EM Environment due to Ionospheric Plasma Barrier Transparency for Groundbased Transmitter Emission V. N. Oraevsky, Yu. Ya. Ruzhin, V. S. Dokukin, Kh. D. Kanonidy, B. P. Singh and G. S. Lakhina Intmins Project. The Three Levels Active Experiment in the Magnetosphere S. I. Klimov, O. V. Lapshinova, Yu. Lissakov, S. A. Romanov, W. W. L. Taylor, N. N. Antropov, F. L. Doudkin, V. E. Korepanov, M. N. Nozdrachev, W. E. Pine and M. P. Gough
91
97
Asymmetric Structure of the Luminous Neutral Barium Cloud in Chemical Release Experiments R. L. Xu and L. Li
107
Acoustic Gravity Wave (AGW) Generation during the Barium Injection Experiments Yu. Ya. Ruzhin, A. Kh Depueva and L. Palasio
111
Minimizing the Adverse Effect of the Photosheath around a Spacecraft H. Zhao, K. Torkar, R. Schmidt, C. P. Escoubet and W. Riedler Ti-C Reaction in Laboratory as a Heating Technique in Space Chemical Release Experiments F. Wu, R. L. Xu, L. Li, Z. G. Zhang and Y. B. Liu
115
119
Contents
vii
Section III. Numerical Simulation and Theoretical Modeling
Ion Beam Velocity Distributions in Plasma Sheet Boundary Layer L. M. Zelenyi, A. L. Taktakishvili, E. M. Dubinin, E. Yu. Budnik and L Sandahl Generation Mechanism of the Field-Aligned Current System Deduced from a 3-D MHD Simulation of the Solar Wind-Magnetosphere-Ionosphere Coupling T. Tanaka Configuration Instability of the Near-Earth Magnetotail in the Presence of an Earthward Plasma Flow and Substorm Onset M. H. Hong, Z. Y. Pu, X. M. Wang, Z. X. Chen, Z. X. Liu, A. Korth and R. H. W. Friedel
125
133
143
Generation of Electrostatic Waves by Nongyrotropic Protons P. D. Convery, M. Ashour-Abdalla and D. Schriver
153
Model Study of Polar Cap Arcs L. Zhu, R. W. Schunk, J. J. Sojka and D. J. Crain
159
Precipitation of Hot Protons from a Stretched Near-Earth Current Sheet W. W. Liu, G. Rostoker and J. C. Samson
165
Computer Simulation of the Three-Dimensional Decay of Thin Collisionless Current Sheets J. Bfichner and J.-P. Kuska
177
Magnetic Reconnection and Plasmoid Events in the Magnetopause Boundary Layer Z. X. Liu, H. Zhang, T. Chen, Z. Y. Pu and S. Y. Fu
187
A Simulation Study of Magnetic Reconnection in a Multiple Current Sheet System H. Zhang and Z. X.Liu
193
Theoretical Study of Vortex Induced Reconnection Processes C. Shen and Z. X. Liu
197
A Modulated Whistler Wave Model on Envelope Soliton in CRIT II Observation D. Y. Wang, N. Brenning, M. Raadu and O. Bolin
201
Author Index
205
List of Participants
207
List of Unpublished Papers
209
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Prof. R. L. Xu and A. T. Y. Lui, editors of the COSPAR Colloquia Series #9 on Magnetospheric
Research with Advanced Techniques, in the COSPAR Colloquium on Magenetospheric Research with Advanced Techniques, 15-19 April, 1996, Beijing, China
Prof. G. Haerendel, President of COSPAR, giving opening address in the opening ceremony of the COSPAR Colloquium on Magnetospheric Research with Advanced Techniques
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PREFACE
The COSPAR Colloquium on Magnetosphere Research with Advanced Techniques was held in one of the biggest garden hotels in Asia, Beijing Friendship Hotel, on April 15-19, 1996. The Colloquium was sponsored by the Chinese National Committee for COSPAR, COSPAR, and the National Natural Science Foundation of China. The theme of the meeting focused on four areas of modern magnetospheric studies, namely: (1) multi-point observation, (2) innovative measurement techniques, (3) active experiments in space, and (4) numerical simulation and theoretical modeling. This meeting was held after the launch of several major magnetospheric satellites which set the stage for most intensive investigations ever of the Earth's plasma environment in the framework of the International SolarTerrestrial Physics Program. These exciting new results and the ongoing discussions of innovative approaches to scientific instrumentation and spacecraft technology also provided the opportunity for the scientists, especially the Chinese Scientists to join the international space community and develop various international collaborative programs. More than seventy scientists from all over the world participated, from countries of Austria, Belgium, Canada, China, Finland, Germany, Japan, Korea, Russia, Sweden and the United States. A total of 93 papers were presented to provide a broad spectrum of exciting new multi-platform observations from the International Solar-Terrestrial Physics Program, state-of-the-art instrumentation design, and simulation and modeling on supercomputers. It was a busy time for meetings around April 1996 when this COSPAR Colloquium was held. Many colleagues who expressed an interest to attend this meeting felt unfortunate that they could not come due to conflicting commitments. Thanks to the hard work of the Scientific Organizers, Session Coordinators, and the Local Committee, the meeting preparation went smoothly without a hitch. We had received many letters and messages highly praising the organizational arrangement and scientific significance of this meeting. Fruition of this meeting includes plans of joint studies and protocols on some joint programs signed by both sides. We organized the manuscripts submitted to this Monograph into: (1) Magnetospheric Observation and Measurement Techniques, (2) Active Experiments, and (3) Numerical Simulation and Theoretical Modeling. Papers presented in the meeting but not submitted to this Monograph are listed by title as unpublished papers at the end of this Monograph. We would like to thank the Chairman and Co-Chairman of the Colloquium and the members of the Scientific Program Committee listed in the first page of this Monograph, the Chairman of the Local Organization Committee Houying Zhang and the committee members: Changchun Ai, Yidong Gu, Jicheng Kang, Kangwen Chen, Hai Lin, Quan Lin, Houren Pan, Faren Qi, Daheng Wang, and Yongren Zhao, the Executive Secretary: Fangyu Liao, Deputy Executive Secretary: Lei Li and Wei Li for an excellent job done in less than ideal circumstances. During the meeting the local Committee arranged a special program to visit the Great Wall and other historical sites around
2
Preface
Beijing, allowingthe participants to gain an impression on the long historical tradition and recent progress of China. We are grateful to W.-H. Ip for his help during the beginning of this Colloquium, and to S. Grzedzielski for his encouragement during preparing this Colloquium Monograph. We deeply appreciate the effort ofD. N. Baker, P. A. Bernhardt, J. F. Carbary, C.-L. Chang, J. Chen, S. P. Christon, R. B. Decker, R. E. Erlandson, W. R. Hu, Y. Q. Hu, E. P. Keath, K. Liou, A. T. Y. Lui, Z. Y. Pu, G. Rostoker, J. Wanliss, D. G. Wei, R. H. Wei, P. H. Yoon, L. J. Zanetti, H. Zhao, G. C. Zhou and anonymous referees for their help in improving the quality of the manuscripts published in this Monograph. In particular, the Associate Editor of this Monograph Lei Li, deserves special mention for her major contribution in bringing together the materials for this Colloquium Monograph for publication by Elsevier Science in early 1998.
Ronglan XU A. T. Y. LUI
OPENING ADDRESS OF THE COSPAR PRESIDENT TO THE COSPAR COLLOQUIUM ON MAGNETOSPHERIC RESEARCH WITH ADVANCED TECHNIQUES
Vice-President Yan, Chairman Xu, Ladies and Gentlemen, This is the Ninth COSPAR Colloquium and the second one I am attending since I became president of COSPAR. I am most pleased to be here and to speak to you, in particular, since the subject of this conference is in my field of research and since I am here also in a second function, namely as CoChairman of this event together with Vice-President Professor Yan. In this second capacity, I would like to welcome you and wish you an interesting and stimulating scientific, social and also touristic event. COSPAR was conceived at the beginning of the space era in order to promote space research and related applications with the emphasis on the exchange of results, information and opinions. By concentrating on data, methods and scientific insights COSPAR has always managed to keep clear of political interference, on the contrary, to build bridges across political barriers. Lately, after the fall of the iron curtain the bridging function of COSPAR in the area of space research may appear to have lost importance, but some barriers continue to exist and new challenges are emerging which need the platform offered by the Committee in order to bring together colleagues form East and West, from South and North, from poor and rich, from well-established and newly entering space-faring nations. In this context, COSPAR is watching with particular attention the developments in Eastern and SouthEastern Asia. Japan and China have both been highly active and successful in space for several decades, have developed powerful launchers, associated infrastructure and space industry, and have established a still growing space research activity. But other nations are just entering, like Thailand, Indonesia and Korea. China took the initiative to call in 1992 in Beijing an Asia-Pacific Workshop on Multilateral Cooperation in Space Technology and Applications. A follow-up conference was held in Islamabad last year and a third one will be held in Seoul at the end of May. This conference is meant as a forum for concerned space agencies, other institutions and administration with the aim to establish closer cooperation and to learn from each other in science and applications. COSPAR applauds such efforts and tries to raise the awareness that science is one of the best technology drivers. Therefore, even when applications like telecommunications, weather services and Earth's observations are the immediate goals of a young space nation, it should allocate a sensible fraction of the resources to basic research, because besides the technology impact there is nothing better to motivate the young generation and future technology leaders of a nation than an early exposure to the great questions and goals of basic research. But of o~ourse, to be able to pursue one's career and expand one's knowledge in research, largely free of commercial and political interests, is a great privilege. Let me now turn to this COSPAR Colloquium on Magnetospheric Research and Advanced Techniques with a few reflections. First on Magnetospheric Research. This field of research popped up with the very
4
G. Haerendel
first space vehicles. For almost four decades, it has produced an impressive list of discoveries of objects as well as of processes. It has profoundly affected our notion of the nature of the space between stars. At this point in time the field has entered its programmatic culmination with the Inter-Agency SolarTerrestrial Physics Program, with the successful launches of Geotail, Wind, Interball Tail Probe, SOHO, Polar and with CLUSTER, Interball Auroral Probe, FAST and Equator-S to follow soon. It is natural that in this phase, when thinking about he future of our field, we feel some anxiety. What will be the challenges after reconnection, flux-transfer events, boundary layer formation, substorm onset, plasmoids, coronal mass ejections, auroral acceleration and kilometric radiation have been largely understood? Of course, this understanding will not be achieved immediately. But did we not promise, when proposing all these missions, that this was what we were after, and that thesr missions, once successfully executed, would bring us close to the desired goals? What will be our new questions, the new frontiers? When I say "'our", I mean the younger ones among us, our students and successors. It is my belief that the more exciting tasks of the future will lead space plasma physics farther away from the Earth, to the planets, the outer heliosphere and above all to the Sun. We have already now an impressive program of ongoing missions in the solar system and very ambitious ones like Mars 96, Cassini and Rosetta in preparation. Solar Probe, Pluto Express, Mercury Orbiter, Intermarsnet, etc. are prospective candidates for the more distant future. All of them offer, at least in principle, opportunities for studying new aspects of plasmas and fields. But with the continuation of flight opportunities near Earth, also here new goals can be set and pursued, like the study of magnetic field-aligned processes as proposed in the IBIZA/IMPACT mission. A field of research is as good as its tools. In magnetospheric research the development of the key instrumentation has gone through several, if not many iterations. The progressive advancement of techniques, experimental, computational and theoretical, is the subject of this conference and rightly so. In addition, I regard it as highly significant that it is held in Eastern Asia, in the region of new space markets. With the expansion and increasing resolution of the accessible parameter spaces comes, almost necessarily, new knowledge and advancement of understanding. But in view of the growing distances from r arth of our future activities, the frequency of flight opportunities is bound to decrease, even with "'faster, better and cheaper" approaches. So, we are all challenged to work economically, proceed in the direction of miniaturizing, of data compression, of low-cost developments of systems and subsystems. Small satellites, use of flight opportunities mainly designed for other purposes, complementary groundbased work, these are some ways to go. I am confident that space plasma physics will not have to complain about a lack of tasks. For a while, however, the emphasis will be on harvesting and not on seeding. Let us enjoy this phase and exploit it fully! Finally, a word about COSPAR. Unified by the applications in, on and from space, COSPAR covers a wide variety of disciplines. Organized in seven Commissions, various Sub-Commissions, Panels and Task Groups, the authority for the definition of the scientific program lies entirely with the scientific community. The topics that need to be highlighted at the bi-annual Scientific Assemblies or the thematically more confined Colloquia and Workshops, they are chosen by the about 4000 Associates, who elect their Commission chairs and organize themselves in the business meetings during the Assemblies and bi correspondence in between. The Council, COSPAR's highest authority, assisted by the Bureau and Executive have the task to establish the right balance between disciplines. They, of course, also provide the necessary infrastructure and raise and distribute financial support for conference
Opening Address of the COSPAR President to the COSPAR Colloquium
5
attendees. The latter is one of the greatest concerns of the COSPAR Bureau, since the spirit of donation is fading rapidly in the present climate of economic crises. Nevertheless, we try hard to mitigate the hardships introduced by recent economic-political developments. I do not want to miss this chance to remind you all to our forthcoming Scientific Assembly in Birmingham (14 - 21 July) as well as of the preceding 10th COSPAR Colloquium on Asteroids, Comets and Meteorites at Versailles. The following Scientific Assembly, 1998, will be held in Japan, in Nagoya. This is my first visit to Mainland China. Although brief, I look forward with great expectations to this first glimpse of a great country with great people and, of course, to an exciting conference for all of us. G. Haerendel President of COSPAR
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Session I Magnetospheric Observation and Measurement Techniques
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INVESTIGATION O F A S U B S T O R M F O L L O W I N G A N E X T E N D E D INTERVAL OF NORTHWARD INTERPLANETARY MAGNETIC RELD A. T. Y. Lui 1, D. J. Williams 1, R. W. McEntirel, S. Ohtani 1, L. J. Zanetti 1, W. A. Bristow 1, R. A. Greenwald 1, P. T. Newell 1, S. P. Christon 2, T. Mukai 3, K. Tsuruda 3, T. Yamamoto 3, S. Kokubun 4, H. Matsumoto 5, H. Kojima 5, T. Murata 5, D. H. Fairfield 6, R. P. Lepping 6, J. C. Samson7, G. Rostoker7, G. D. Reeves 8
1The Johns Hopkins UniversityApplied Physics Laboratory, Laurel, MD 20723, USA 2Department of Physics, University of Maryland, College Park, MD 20742 USA 3The Institute of Space and Astronautical Science Kanagawa 229, Japan 4Solar-Terrestrial Environment Laboratory, Nagoya University, Toyokawa 442, Japan 5RASC, Kyoto University, Uji, Kyoto 611, Japan 6NASA/GSFC, Greenbelt, MD 20770, USA 7Department of Physics, University of Alberta, Edmonton, Canada T6G 297 8Los Alamos National Laboratory, Los Alamos, NM 87545, USA ABSTRACT Strong northward interplanetary magnetic field was observed for an extended period by the Wind spacecraft at an upstream distance of--200 RE from February 8-10, 1995. Within this period was a brief break of southward IMF on February 9 which led to a substorm of moderate inten,::.y " 500 nT) with its expansion onset at -0431 UT. In this paper, this substorm is examined with data flora eleven spacecraft in space and two networks of ground stations coveting both the northern and southern hemispheres. Detailed analysis of this event shows (1) an unusually long duration of the magnetospheric reconfiguration prior to expansion onset for this isolated substorm (2) new e,,:aence far mllltiple particle acceleration sites during substorm expansion, and (3) indications for sunward plasma flow in the plasma sheet during the late expansion phase of a substorm not related to a single acceleration site (e.g., an X-line) moving from the near-Earth tail to the more distant tail. INTRODUCTION One of the main objectives of the ISTP (International Solar Terrestrial Physics) program is to investigate the flow of energy, momentum, and mass from the Sun through the magnetosphere to the ionosphere and the atmosphere. Achieving this objective in the ISTP era has the distinct advantage over previous attempts because of the unprecedented multi-point measurements available and planned for ISTP activities. In this paper, we address this ISTP task by studying an isolated substorm with eleven spacecraft (Wind, IMP-8, Geotail, six geosynchronous satellites, one DMSP satellite, and Freja) and two networks of ground stations (Canopus and SuperDARN) from the ISTP data base. Studying an isolated substorm is particularly appropriate to address this ISTP task because a large amount of energy, momentum, and mass from the solar wind pass through the magnetosphere during a substorm episode. Furthermore, studying an isolated substorm instead of a substorm embedded within a sequence of substorm disturbances allows one to eliminate the possible interference from preceding substorm activity. Consequently, a clearer identification of features genuinely associated with the different phases of a substorm can be made without the ambiguity introduced by the remnants of previous activities. Before the onset of this substorm under study, a magnetic cloud passed over the Earth, engulfing the Earth's magnetosphere with a prolonged period of steady northward interplanetary magnetic field (IMF). This sets up an ideal situation for our study since the magnetosphere was then
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Lui
et al.
brought to a state with very little, if not the minimum, activity present. The extensive coverage of substorm activity during this event provides us with results which range from verifying some of our conventional expectations about substorm phenomena as well as revealing some surprises which are different from the anticipation of the traditional model for the substorm expansion and recovery development. OBSERVATIONS The suite of data available for this event may be divided into four categories according to the region where measurements were taken. These are solar-wind/magnetosheath, ground-based/auroral-region, geosynchronous altitude, and magnetotail. Figure 1 shows the locations of the high-altitude spacecraft Wind, IMP-8, Geotail, and six geosynchronous satellites during this event. Wind was in the far upstream region at -200 RE in front of the magnetosphere. IMP-8 was near the dusk meridian, initially in the solar wind but later entered the magnetosheath before the substorm onset. Geotail was near the midnight meridian at X -- -32 RE. In the geosynchronous orbit were four LANL satellites (1984-129, 1987-097, 1989-046, and 1990-095), GOES-7, and GOES-8 distributed at various local times. Not shown in the sketch are two low-altitude satellites DSMP F11 and Freja. This suite of satellites makes a total of eleven spacecraft in space gathering data for this event. Supplementing these data are observations from two networks of ground stations, Canopus and SuperDARN, which cover both the northem and southem hemispheres. These constitute a rich data set, truly unprecedented in terms of coordinated observations. Spacecraft Locations at Substorm Expansion Onset February 9, 1995 at 0431 UT
1995 Feb 8-9 (39-40) Solar Wind Observations WIND
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Fig. 2 The solar wind parameters as monitored by WIND and IMP-8.
Solar Wind/Magnetosheath Observations The IMF and the solar wind plasma parameters from Wind are given in Figure 2 for February 8-9, 1995. It can be seen that the IMF tumed northward at --14 UT on February 8 and remained strongly northward
Investigation of a Substorm Following an Extended Interval of Northward IMF (Bz in the range of 2-10 nT) for an extended period with a brief--2-hr break at --02-04 UT on February 9 when Bz was slightly negative. The IMF B Y component was . . quite strong, in the neighborhood of --412 nT throughout most of this period. The solar wind velocity was quite steady at the nominal speed of --400 km/s. The solar wind dynamic pressure varied somewhat from --1 to 4 nPa, mostly due to the variation in the solar wind number density. This is part of an interplanetary magnetic cloud studied earlier by Lepping et al. (1996). IMP-8 observations showed similar characteristics of the solar wind as that seen by the Wind spacecraft in spite of the fact that IMP-8 was separated from Wind by --65 RE in GSM Y-coordinate. The only major difference is on the duration and magnitude of the southward IMF seen by the two satellites during this time interval. While Wind observed the brief southward excursion of IMF for less than 2 hrs, IMP-8 detected the southward IMF for a duration of--4 hr, as shown in the bottom panel of Figure 2, which is about twice as long as that seen by Wind. This difference may be related to the bow shock crossings of IMP-8 as indicated by the abrupt changes in the Bz component and in the other two magnetic field components as well. Ground-Based/Auroral-Region Observations The brief southward turning of IMF led to an isolated substorm, as indicated in Figure 3. The IMF Bz component from Wind is reproduced in the top panel. The solar wind energy transfer function, Akasofu's e parameter (Akasofu, 1981), and the auroral kilometric radiation (AKR) index based on the observed kilometric radiation from Geotail, are also shown. The epsilon parameter, which measures the solar wind input energy to the magnetosphere, was quite small (below 103 MW) before the southward turning of the IMF. Afterwards, the epsilon parameter reached to large values (near 105 MW) where substorm activity is anticipated. Indeed, the AKR index at 64s resolution indicates an intensification of AKR emission starting at-0432 UT, followed by another one at--0436 UT. The magnetic stations from Canopus, which were in the pre-midnight sector (-23.5 MLT at the substorm onset time), registered this isolated substorm activity. Shown in the middle of the figure are the magnetograms from Eskimos, Fort Churchill, Back, and Gillam, illustrating a negative bay o f - 5 0 0 nT started near the time of the increase in the AKR. There were two Pi2 micropulsation onsets detected at Gillam, the first one commencing at -0431 UT and the next one at -0436 UT, similar to the onset times indicated by the AKR index. 1995 FEB 9 (40) OBSERVATIC..,.-.,
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12
A.T.Y. Lui et al.
Observations from Meridian Scanning Photometer (MSP) chain from Gillam and Rankine in the Canopus network (not shown) indicate that at 0200 UT the auroral luminosity was very low but still noticeable at-660-75 ~ (Pace latitude: Baker and Wing, 1989). A t - 0 3 0 0 UT, the auroral emission became enhanced at -690-72 ~ and moved equatorward. This equatorward motion is consistent with the expected expansion of the auroral oval in response to southward IMF. Comparison between the 5577 and 6300 A emissions indicate the polar cap boundary to be at least 72 ~ implying that the substorm expansion onset took place well within the closed field line region. The backscattered signals from Goose Bay radar of SuperDarn indicate that the auroral electrojet was at -68.5 ~ Pace latitude at -0200 UT and began equatorward movement a t - 0 3 0 0 UT, like the auroral luminosity seen by the Canopus MSP. By the end of the substorm growth phase, the auroral electrojet had moved -3 ~ equatorward, in agreement with the equatorward displacement of the auroral emissions from the Canopus stations. The radar observations from Halley station (conjugate to Goose Bay) show equatorward movement of the auroral electrojet from about-68 ~ at 0300 UT to a b o u t - 6 5 ~ (also 3 ~ equatorward displacement) at 0430 UT just before the substorm onset. This shows that the auroral electrojet in both hemispheres responded to southward turning of the IMF in a very similar manner. DMSP F11 crossed the auroral oval in the dusk-midnight sector. Using the identification scheme of Newell et al. (1996), particle precipitation from this satellite indicate that the plasma sheet boundary (defined as the latitude where the electron average energy is neither increasing or decreasing with latitude) expanded from --73.8 ~ Pace latitude (22.6 MLT) at 0008 UT to --67.7 ~ (23.4 MLT) at 0334 UT. This lowering in latitude of the plasma sheet boundary is quite consistent with the equatorward movement of the auroral electrojets in both hemispheres. There are two relevant passes of Freja over the auroral region. The first one at 0224-0254 UT crossed the midnight meridian at 71 o Pace latitude and showed no large-scale field-aligned current (FAC). The second pass at 0414-0444 UT crossed the auroral oval at -03 MLT between 0433:55 and 0435:00 UT, just after substorm onset on the ground. An east-west magnetic field perturbation o f - 1 2 0 nT at -67-69 ~ Pace latitude was observed. This low value implies the large-scale FACs to be still rather weak in the early morning local time a few minutes after the substorm onset. Geosynchronous Al:ltude Observations In the geosynchronous altitude at the time of substorm onset, there were six satellites distributed in local times of 3.4 hr (1990-095), 5.0 hr (1984-129), 11.4 hr (1987-097), 17.5 hr (1989-046), 19.5 hr (GOES7), and 23.5 hr (GOEs-8). As shown in Figure 4, dispersive energetic electron injections were seen by three of the four LANL satellites, namely, 1984-129 (LT=UT+0.5), 1987-097 (LT-UT+6.9), 1989-046 (LT=UT-11.0). For the fourth LANL satellite (1990-095), there was a data gap during this time. The onset time of electron injection based on the dispersive feature (i.e., extrapolating injection time to infinite energy particles) is found to be -0444 UT, which is -12-13 min delay with respect to the first Pi2 onset or AKR increase, and -8 rain delay with respect to the second Pi2 onset and AKR increase. The magnetic field measurements from GOES-8 (LT=UT-5.0) show a gradual depression starting a t - 0 3 UT, coinciding with the equatorward expansion of the auroral oval. The magnetic field dipolarized at -0440 UT, which is -9 min delay from the first Pi2 onset and --4 min delay from the second one. GOES-7 (LT=UT-9.0) which was 4 hr in LT to the west of GOES-8 also detected a substantial decrease in the Hp component just like GOES-8 during the growth phase. The H component is positive northward parallel to the Earth's spin axis, He is positive radially inward, and ~ is positive eastward. Magnetotail Observations For this event, Geotail was located slightly in the pre-midnight mid-tail region where magnetic reconnection is anticipated to occur at substorm expansion onset. The plasma, magnetic field, and electric field measurements from Geotail are displayed in Figure 5. Geotail was in the plasma sheet at the start of the interval and crossed the neutral sheet at -0408 UT. A noticeable sudden decrease in plasma density occurred at-0415 UT, accompanied by a temperature increase and small disturbances in
Investigation o f a S u b s t o r m F o l l o w i n g an E x t e n d e d Interval o f N o r t h w a r d I M F
13
the electric and magnetic fields. These are probably the signatures of Geotail's entry into the plasma sheet boundary layer. At -0425 UT, Geotail entered a low density region representative of the tail lobe. 1995 FEB 9 (40) GEOTAIL OBSERVATIONS
1 9 9 5 F e b 9 (40) Geosynchronous Observations ~o ~ ,.;
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?.~ --0435 UT, -4 min after the substorm onset, Geotail re-entered the plasma sheet boundary layer where a brief interval of weak (-100 km/s) sunward flow was observed. Moderate (-300-500 km/s) tailward flow occurred immediately afterwards but it did not last for the entire period of Geotail's residence in the plasma sheet boundary layer (0435-0445 UT). Therefore, the tailward flow burst was extremely transient and/or localized. Plasma flow reversal occurred not just in the x-component but also in the y-component with a smaller magnitude and a shorter duration. The local magnetic field turned southward a t - 0 4 3 5 UT before the start of the weak sunward flow. The southward field was accompanied by significant changes in the By component, indicating that the field change was due to a three-dimensional field structure rather than a two-dimensional field geometry as envisioned for an Xline. Southward field was associated with enhancements of the dawn-dusk electric field to an average of several mV/m (up to 10 mV/m) in the duskward direction. The computed total magnetic and plasma pressure indicates a relatively steady increase from --0.12 nPa at 0400 UT to -0.21 nPa at 0438 UT. This steady increase in total pressure is consistent with the expected increase in the tail lobe field strength during the growth phase. Since expansion onset occurred at 0431 UT, the pressure increase in the mid-tail region as monitored by Geotail continued for -7 min after the first Pi2 (substorm expansion) onset and for --2 min after the second Pi2 onset in this event. Sunward-dawnward flow occurred at the -0435 UT re-entry of the plasma sheet boundary. A careful examination of the ion and electron energy spectrograms indicates that the strong tailward flow seen after re-entry was mainly from the high energy (> --4 keV) portion of the ion population while the lower energy ion population was rather isotropic, much like what was observed prior to the plasma dropout at -0425 UT. This indicates that in spite of the large southward field ( - - 1 0 nT) seen at this time, the plasma in the plasma sheet boundary layer remained rather undisturbed, just like before the dropout at
14
A.T.Y. Lui et
al.
-0425 UT. The tailward flow was mainly due to a high-energy tailward flowing population streaming past the satellite (this is verified by an examination of the ion velocity distribution which shows distinctly the presence of two ion populations; the transient passing of this tailward flowing population is further indicated by its disappearance well before Geotail exited the plasma sheet boundary layer and while the magnetic field was still strongly southward). The measurements of energetic ions and electrons from the Energetic Particles and Ion Composition (EPIC) instrument on Geotail shown in Figure 6 substantiate the above interpretation of transient energetic particle population passing through an ambient plasma. The anisotropy spectrograms indicate that tailward streaming of energetic ions (-67 keV) and electrons (> 38 keV) were both quite intermittent. At-0522 UT, Geotail re-entered the plasma sheet once again and detected rather isotropic energetic electrons. However, tailward streaming of energetic ions was still quite prominent and lasted till -0541 UT. This implies that the process leading to acceleration of energetic particles and their tailward streaming during substorm was still active and lingered on in the region earthward of the Geotail location. In addition, intermittent sunward streaming of energetic particles were simultaneously seen to mingle with the tailward streaming activity. The co-existence of intermittent tailward and sunward streaming of energetic particles indicates that there were more than one particle source responsible for the streaming activity seen. The plasma at this time had characteristics quite similar to the pre-substorm activity at-0400 UT with no significant plasma flow.
95 Feb 9 (040) GEOTAIL/EPIC
Fig. 6 The anisotropy spectrogram of 67 keV ions and energetic electrons > 38 keV measured by EPIC instrument on Geotail. The left labels refer to the particle streaming direction and the line overlays denote the projected direction of the magnetic field. At-0535 UT, strong (>1000 km/s) sunward-dawnward flow began. There was no noticeable tailward flow preceding the start of the strong sunward-dawnward flow even though Geotail was well within the plasma sheet during this time (plasma beta being in the range of 1-100). Furthermore, the strong sunward flow arose at the time when tailward streaming of energetic particles was still detectable. This observation forms a powerful evidence that the process which produces tailward streaming of energetic particles did not move down the tail past the Geotail location to give rise to the observed strong sunward flow. SUMMARY AND DISCUSSION An isolated substorm is investigated with ISTP data from eleven spacecraft and two networks of ground stations covering both the northern and the southern hemispheres. These observations are unprecedented in providing global monitors of the substorm event, extending to all regions in the near-Earth space, namely, the solar wind, the magnetosheath, the magnetotail, the inner magnetosphere, the auroral altitude, and the ground. Analysis of this comprehensive set of measurements leads to confirmation of
Investigation of a Substorm Following an Extended Interval of Northward IMF
15
some expected substorm phenomena, to awareness of some unusual characteristics for this isolated substorm, and revelation of some surprising features which are difficult to reconcile with the traditional substorm model. The growth phase phenomena resulting from southward IMF (McPherron et al., 1973) are well substantiated in this isolated substorm. These phenomena include (a) the equatorward expansion of the polar cap, (b) the equatorward shift of the auroral oval/electrojet in both hemispheres, (c) the enhancement of the large-scale magnetic-field-aligned current system, (d) the increase in the lobe magnetic field strength (and thus the cross-tail current intensity) as monitored in the mid-tail region, and (e) the decrease in the poloidal component of the magnetic field at the geosynchronous orbit. The features (a) and (b) imply indeed energy storage of the magnetosphere during this interaction. Features (c), (d) and (e) indicate that the energy storage phase is associated with intensification of the large-scale magnetospheric current system which encompasses not just the cross-tail current but also regions 1 and 2 field-aligned currents. Feature (e) further indicates the earthward approach of the cross-tail current in this storage phase. The unusual aspect in the growth phase of this substorm is its long duration o f - 9 0 min instead of the typical -40 min (Foster et al., 1971). This long duration is quite comparable with that for another isolated substorm on March 9, 1995 studied by Rodger et al. (1997) in which the growth phase lasted for --80 rain. Therefore, long duration growth phase may be common among isolated substorms. Another unexpected feature found in this study is the difference in duration of southward IMF between Wind and IMP-8 observations. One possibility is that there were spatial structures of the IMF in the solar wind. Solar wind observations taken at locations far from the sun-Earth line should be viewed with caution. Another possibility is that the difference may arise as a result of the IMF being modified after passing through the bow shock. This is an important question because after all it is the magnetic field in the magnetosheath that interacts with the geomagnetic field. The second unusual aspect of this substorm is the time delay between ground indications of substorm onset and substorm injection onset inferred from observations at the geosynchronous orbit. Although some time delay (about a few min) is not unexpected between these onsets because of the intrinsic uncertainties. ~a timing onsets, the delay o f - 8 - 1 3 min is unusually long. It would be interesting to examine ,z- tv.e future whether this long delay may be common for isolated substorms just like the i2-:g duration of growth phase. Relevant to mention are the observations of an isolated substorm after a prolonged quiet period reported earlier (Lui et al., 1975). This earlier analysis suggests that particle injection iv a substorm following a prolonged quiet period may occur further down the tail (as indicat-_,~ by strong earthward flow detected at a further downstream distance o f - 1 1 RE) such that the injected particles do not reach the geosynchronous orbit. The most surprising observed features of this substorm were noted in the mid-tail region. About 4 min after the ground substorm onset, large southward magnetic field and tailward flow occurred in the midtail region. The traditional interpretation would be that these are signatures of an X-line. However, a detailed examination shows that tailward flow arises from a transient energetic ion population streaming down the tail while the ambient plasma appears to be undisturbed by the activity. Moreover, the ambient plasma remained undisturbed after the passage of the tailward streaming population. The lack of response from the ambient plasma is quite contrary to the expectation of magnetic reconnection occurring nearby as implied by the observed magnitude of southward magnetic field. The variation in the dawn-dusk component of the magnetic field simultaneous with the occurrence of southward magnetic field further suggests that the magnetic field structure at that time is complicated and threedimensional, unlike the relatively simple two-dimensional geometry for an X-line. This complex magnetic field structure may represent the diffusion region of the magnetic reconnection process. However, counter to this interpretation is the undisturbed (and unenergized) character of the ambient plasma after the passing of the tailward streaming population when the magnetic field was still strongly southward and complex. One possible reconciliation of the above conflicts is postulating the diffusion region (located at some distance from the Geotail) to be responsible for the intense particle acceleration in the energetic ion beam population while the slow mode shocks associated with magnetic reconnection provide rather insignificant particle acceleration. At the present time, this conjecture has not yet been tested by full particle simulation of magnetic reonnection.
16
A.T.Y. Lui et al.
Another surprising observation in the mid-tail region deals with the occurrence of strong sunward flow in the late expansion phase when Geotail was in the plasma sheet prior to its appearance. This is an extremely fortuitous situation because almost all previous studies of satellite re-entry to the plasma sheet during the late expansion phase showed an expanding plasma sheet with strong sunward flow. The traditional interpretation of these events in previous studies is that an X-line moved past the satellite location to further downstream, allowing the plasma sheet to expand and engulf the satellite and sunward flow arises from magnetic reconnection at the X-line. This interpretation can be verified or refuted if the satellite were within the plasma sheet before the commencement of strong sunward flow in the late expansion phase. The observations reported here provides one such case and they are inconsistent with such a traditional interpretation. Similar findings were reported by Mukai et al. (1996). These observations, however, are quite consistent with the substorm synthesis model in which multiple sites of current disruption occur during the substorm expansion which subsequently lead to magnetic reconnection further downstream at the late-expansion/early-recovery phase of a substorm (Lui, 1991). In this scenario, the process responsible for substorm expansion onset is different from the process which generates strong sunward flow in association with plasma sheet recovery during the late expansion or early recovery phase. Current disruption can generate earthward flow as well as tailward flow depending on the modification of the local Lorentz force. One distinction between current disruption and magnetic reconnection is that no slow mode shock needs to be formed in current disruption for fast conversion of magnetic field energy into particle energy. Finally, another important feature worthy of note from this study is that the observed strong sunward flow appears to be associated with a visible optical feature moving equatorward on the ground as shown by the Canopus MSP observation. This study represents an attempt to use the comprehensive ISTP data set to address several outstanding issues on substorms. It is important to conduct future studies for more isolated substorms to ascertain the general features exhibited by these substorms and to help resolving the various substorm theories currently under consideration. ACKNOWLEDGMENTS This work was supported in part by funding from the Space Physics Division of the National Aeronautics and Space Administration (under the Depa~,nent of Navy Task IAF; Contract N00039-9 lC-001) and in part by the Atmospheric Sciences Division of the National Science Foundation (Grant ATM-9622080) to the Johns Hopkins University Applied Physics Laboratory. REFERENCES Akasofu, S.-I., Energy coupling between the solar wind and the magnetosphere, Space Sci. Rev., 28, 121 (1981). Baker, K. B., and S. Wing, A new magnetic coordinate system for conjugate studies at high latitudes, J. Geophys. Res., 94, 9139 (1989). Foster, J. C., D. H. Fairfield, K. W. Ogilvie, and T. J. Rosenberg, Relationship of interplanetary parameters and occurrences of magnetospheric substorms, J. Geophys. Res., 76, 6971 (1971). Lepping, R. P., A. Szabo, K. W. Ogilvie, R. J. Fitzenreiter, A. J. Lazarus, and J. T. Steinberg, Magnetic cloud-bow shock interaction: WIND and IMP-8 observations, Geophys. Res. Lett., 23, 1195 (1996). Lui, A. T. Y., A synthesis of magnetospheric substorm models, J. Geophys. Res., 96, 1849 (1991). Lui, A. T. Y., S.-I. Akasofu, E. W. Hones, Jr., S. J. Bame and C. E. McIlwain, Observation of the plasma sheet during a contracted oval substorm in a prolonged quiet period, J. Geophys. Res., 81, 1415 (1976). McPherron, R. L., C. T. Russell, and M. P. Aubry, Satellite studies of magnetospheric substorms on August 15, 1968, 9, Phenomenlogical model for substorms, J. Geophys. Res., 78, 3131 (1973). Mukai, T., et al., GEOTAIL and INTERBALL-TAIL correlative observations of magnetotail dynamics and substorms, EOS Supplement, F611 (1996). Newell, P. T., Y. I. Feldstein, Y. I. Galperin, and C.-I. Meng, Morphology of nightside precipitation, J. Geophys. Res., 101, 10737 (1976). Rodger, A. S. et al., 9 March 1995 - a simple isolated substorm? Geophys. Res. Lett., submitted (1997).
STRUCTURE AND EVOLUTION OF THE CURRENT SHEET BY MULTI-SPACECRAFT OBSERVATIONS X.-Y. Zhou l, C. T. Russell 2, and J. Gosling 3
]Institute of Geophysics, The Chinese Academy of Sciences, Beijing 100101, P.R. China 2Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90095, USA 3Los Alamos National Laboratory, Los Alamos, NM 87545, USA
ABSTRACT On April 22, 1979, from 0840 to 1018 UT, ISEE 1, ISEE 2 and IMP 8 were all in or near the magnetotail current sheet at 17 Re, 16 Re and 35 Re respectively while ISEE 3 monitored the solar wind 206 Re upstream of the Earth. A global perspective of the four spacecraft observations and of the ground magnetic records is presented in this paper. The hyperbolic tangent current sheet model of Harris has been used to calculate the current sheet thickness and to analyze the plasma distribution in the vertical direction. It is fbuna that during this event the current sheet thickness varied from 2.5 Re to 1.5 Re for northward IMF but thinned abruptly to 0.5 Re when the IMF turned southward. INTRODUCTION The plasma and current sheets in the geomagnetic tail are the critical regions in both the coupling of the magnetosphere to the solar wind and the storage of energy for later release. The properties and evolution of these layers are intimately involved in the different substorm theories (Russell and McPherron, 1973; Lui, 1988; Kan and Akasofu, 1989). Under normal geomagnetic conditions the current sheet is relatively thick (Fairfield et aI., 1981), but under substorm conditions it can become very thin (McPherron et aI., 1987). This thickness might be controlled by either interplanetary conditions or by the substorm activity itself (Van Hoven et al., 1987; Wiegelmann and Schindler, 1995). But a quantitative determination of the temporal evolution of the current sheet thickness is difficult. In this paper we present a case study based on the observations of ISEE 3 in the solar wind, IMS ground magnetometers at the nightside and ISEE 1, ISEE 2 and IMP 8 in the magnetotail current sheet. We concisely introduce the conditions in the solar wind, in the near and mid-tail, and on the ground. Then we use this rare but ideal condition for calculation of the current sheet thickness and the analysis of the current sheet structure. GLOBAL PERSPECTIVE ..,Spacecraft Positions and,Magnetopause Configurations On April 22, 1979 from 0840 to 1018 UT, ISEE 1 and ISEE 2 stayed in the tail current sheet at about 17 Re, crossing the current sheet center several times while the initially northward I M Bz turned Southward under conditions of constant solar wind velocity and dynamic pressure. In this interval, ISEE-3 17
18
X. Y. Zhou et al.
monitored the solar wind 206 Re upstream and 82 Re to the dawn side of the Earth. IMP 8 was also located in the tail at about 25 Re and 6 Re above the expected position of the current sheet, and 16 Re to dawn side of the Earth. Figure 1 gives the calculated magnetopause and current sheet configurations corresponding to the northward IMF condition and dipole tilt angle of the Earth at 6.2 ~ toward the sun (Petrinec and Russell, 1995; Hammond et al., 1994). Over the event period ISEE 1 and 2 are expected to slowly cross the current sheet in the center of the tail from the north to south lobe and IMP 8 to stay firmly in the northern lobe. But we will see the latter spacecraft does not, implying a significant twist of the tail current sheet.
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Fig. 1. Location of ISEE 1 and 2 and IMP 8 during the observations reported herein. The top panel shows the projections of the positions in the GSM YZ plane. The tail boundary and the current-sheet location are those expected from statistical models under the observed solar wind conditions and tilt of the dipole.
Fig. 2. The interplanetary magnetic field in GSM coordinates as observed upstream by ISEE 3. The black shading denotes the period of strong southward IMF.
19
Structure and Evolution of the Current Sheet by Multi-Spacecraft Observations
A strong westward electrojet with an onset at about 0953 UT is detected by the ground magnetometers as shown by Figure 3. The vertical line drawn at 0953 UT in this figure is the onset time of the southward turning of the magnetic field seen by ISEE 1 and 2 at 17 Re in the tail. From College to Whiteshell the electrojet is very strong but at Ottawa and Saint John's it is weak, perhaps due to those stations being far from the center of the electrojet. I00 _ ~ / i
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Fig. 3. Ground-based measurements of the H component of the magnetic field (left) and Z component of the magnetic field (right) at selected stations at high latitudes near midnight.
Geomagnetic and Plasma Variations in the Tail The magnetic field and plasma observed by ISEE 1, 2 and IMP 8 are shown in Figures 4a and 4b respectively. An earthward propagating dipolarization of the magnetic field occurred at 0847 UT. Before 0920 UT the ISEE pair remained at the center of the current sheet, so that the total pressure is mainly due to the thermal pressure which is shown by the thin line in the bottom panel of Figure 4b. A rapid southward turning of the tail magnetic field occurred at 0953 UT corresponding to the expected arrival time of the IMF Bz southward turning seen at ISEE 3 at 0906 UT. This apparent reconnection event then appeared to move down tail, encountering first ISEE 2 and then ISEE 1. Another very significant phenomena occurred during the northward IMF conditions. IMP 8 entered the southern lobe at 0846 UT and remained there about half an hour. This implies that the tail current sheet is twisted significantly under these IMF Bz and By conditions. This twist is in the same direction as predicted by Cowley (1981). ISEE-1 - - - - - - - ISEE-2 20L,,
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Fig. 4a. The magnetic field at ISEE 1 and 2 from 0840 to 1018 UT on April 22, 1979. The GSM coordinate system is used.
20
X.Y. Zhou et al. ISEE-2 Plasma
400
Data
Fig. ilb. The plasma conditions at ISEE 2 from 0840 to 1018 UT on April 22, 1979. From top to bottom this figure shows the plasma velocity toward and away from the Earth; the number density; the ratio of the maximum to minimum ion temperature in one spin of the spacecraft, the average ion temperature and the plasma thermal and magnetic pressures, and their sum.
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THE HARRIS C U R R E N T SHEET S T R U C T U R E The Harris model is a simple analytical description of a one-dimensional current sheet model and is self-consistent using either MHD or kinetic theory. In a Harris current sheet, the magnetic field is given by Bx = Botanh(z~)
(1)
where B o is the lobe field, z is the distance between the observation point and the current sheet center, h is the current sheet half thickness. With observations from two vertically spaced spacecraft both in the current sheet, formula (1) can be rewritten
(2)
h = 2Az/ln[(Bo2+BoABx-Bx]Bx2)/(Bo2-BoABx-BxiBx2)]
where Az=zl-z2, zl and z2 are the positions of the two spacecraft in the Z direction of the GSM coordinates, ABx=Bxl-Bx2, B x l and Bx2 are the Bx components observed by the two spacecraft separately. The lobe field B o can be calculated under conditions of constant total pressure through the lobe. When we use a coefficient of 2.15 to recalibrate the plasma density to get a constant total pressure through the plasma sheet from one lobe to the other, the lobe field Bo=52.6 nT. The calculated current sheet center position z o in GSM and the half thickness are given in Figure 5. Up and down motion of the current sheet is very clear in the top panel before 0938 UT. Over the whole event the current sheet center was mainly above the equatorial plane, at an average height of about 1.1 Re. As shown in the bottom panel of Figure 5, before 0920 UT the average thickness is about 2.5 Re but it varied very much. ~o!~ , c
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Fig. 5. The magnetic field and inferred parameters from the Harris current sheet model. The top panel shows the magnetic field components in the GSM X direction. The middle panel shows the distance of the spacecraft from the center of the current sheet. The bottom panel shows the computed thickness of the current sheet.
21
Structure and Evolution of the Current Sheet by Multi-Spacecraft Observations
Between 0920 to 1000 UT the thickness was relatively smooth and the average value is about 1.5 Re, but oscillated near the time of the reconnection event. After 1000 UT the current sheet monotonically thinned to an average thickness of 0.5 Re. From the Harris current sheet model we also can analyze the structure of the plasma sheet along the vertical direction. We assume the current sheet magnetic structure is antisymmetric around the current sheet center and that the plasma properties as observed by ISEE 2 are symmetric about z o, so we reflect the plasma data about z o. Figure 6 shows the plasma distribution in the vertical direction. In Figure 6 the plasma density and average temperature are high at the center and low at the edges, which is very close to a Gaussian distribution. In Figure 6c the difference between the observed and the theoretical thermal pressure might be because the error in the observed Tperp and the limited detector range. Thus, while not a perfect match to the structure of the magnetotail current sheet the Harris model gives a good zeroth order approximation to it. 2
--- ,. --, .... , .... , .... 1.... I--'.-'-', .... i -~-
1.5 -
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,
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.II -
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0
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-
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,O~
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.
.
......".......... .
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g o ,O
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~IS ( 2
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,
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6
Fig. 6. The plasma parameters plotted as a function of the normalized distance from the current-sheet center using the Harris current-sheet model. The left-hand panel gives the plasma number density, the next panel the spin average ion temperature, the next panel the thermal pressure and the last panel the sum of the plasma thermal and magnetic pressures. REFERENCES Cowley, S. W. H., Magnetospheric asymmetries associated with the Y-component of the IMF, Planet. Space Sci., 29, 79-96, 1981. Fairfield, D. H., R. P. Lepping, E. W. Hones, Jr., S. J. Bame, and J. R. Asbridge, Simultaneous Measurements of Magnetotail Dynamics by IMP Spacecraft, J. Geophys. Res., 86, 1396(1981). Hammond, C. M., M. G. Kivelson, and R. J. Walker, Imaging the Effect of Dipole Tilt on Magnetotail Boundaries, J. Geophys. Res., 99, 6079 (1994). Kan, J., and S.-I. Akasofu, Electrodynamics of Solar Wind-Magnetosphere Ionosphere Interactions, IEEE Trans. Plasma Science, 17, 83(1989) Lui, A. T. Y., R. E. Lopez, S. M. Krimigis, R. W. McEntire, L. J. Zanetti, and T. A. Potemra, A Case Study of Magnetotail Current Sheet Disruption and Diversion, Geophys. Res. Lett., 15, 721(1988). McPherron, R. L., A. Nishida, and C. T. Russell, Is Near-Earth Current Sheet Thinning the Cause of Auroral Substorm Onset?, in Quantitative Modeling of Magnetosphere-Ionosphere Coupling Processes, pp. 252-257, Kyoto Sangyo Univ., Kyoto, Japan(1987). Petrinec, S. M., and C. T. Russell, Near-Earth Magnetotail Shape and Size as Determined from the Magnetopause Flaring Angle, J. Geophys. Res., 101,137(1995). Russell, C. T., and R. L. McPherron, The Magnetotail and Substorms, Space Sci. Rev., 15, 205(1973). Van Hoven, G., L. Sparks, and D. D. Schnack, Nonlinear Radiative Condensation in a Sheared Magnetic Field, Ap. J., 317, L91 (1987). Wiegelmann, T., and K. Schindler, Formation of Thin Current Sheets in a Quasistatic Magnetotail Model, Geophys. Res. Lett., 22,2057 (1995).
This Page Intentionally Left Blank
ENERGETIC OXYGEN MAGNETOTAIL
I O N B U R S T S IN T H E D I S T A N T
Q.-G. Zong and B. Wilken Max-Planck-Institut fiir Aeronomie, D-37191 Katlenburg-Lindau,
Germany
ABSTRACT This paper reports on the characteristics of O + beam sequences detected in GEOTAIL/HEP-LD observations in the distant tail (X=-60 and -90 Re). The relationship to concurrent plasma and magnetic field data is also discussed. The energetic O + burst usually last only 20 to 30 minutes, exhibit strong beam-like structures, appear after substorm onset, are located in the post plasmoid plasma sheet (PPPS), and are embedded in plasma flowing tailward at very high speeds (exceeding 1000Km/s). The bulk speed of the oxygen beam indicate that these ions were accelerated to the same velocity as the plasma. The energy dispersion in the oxygen bursts allows us to estimate the position of the acceleration source (presumably the acceleration region position) as lying between X=-15 and-20 Re. The distant tail is a unique location for monitoring substorm dynamic processes; details are smoothed out during the downtail propagation and only principal features are conserved. 1.INTRODUCTION Recent observations of the energetic oxygen abundance in flux ropes in the distant magnetotail showed no evidence for a flux increase above the ambient plasmajsheet abundance during normal geomagnetic activity(Lui et al., 1994). However, during an intense substorm,, energetic pxygen bursts with high speed tailward flow were found in both the CPS (Wilken et al., 1995; Zong et al., 1997b) arid post-plasmoid topologies (Zong et al., 1996, 1997a; Wilken et al., 1997). Cold ion beams (probably O + apparently of ionospheric origin) have indeed been observed in the lobes of the distant magnetotail (Mukai et al., 1994). Baker et al. (1996) showed that cold ions from the polar ionosphere can drift slowly toward the plasma sheet and reach locations relevant for the formation of acceleration regions (e.g. neutral lines) in the distant tail. Energetic atomic (0 +1 and N +1) and molecular (0 +1, N O +1 and N +1) ions of undoubtedly ionospheric origin were also observed in the deep tail (X ~ 146 Re) (Christon et al., 1994). In this paper, based on GEOTAIL HEP/LD measurements, we concentrate on the properties of energetic oxygen bursts in plasmoids and in the plasma sheet. 2. GEOTAIL/HEP-LD OBSERVATIONS 2.0 Instrument In this paper we use energetic ion measurements from the HEP-LD instrument on board the GEOTAIL spacecraft. A short account of the HEP-LD spectrometer was given e.g. by Zong et al. (1997a), whereas detailed information on the HEP-LD instrument was presented by Wilken et al. (1993) and Doke et al. (1994). For the easy reference we briefly describe the principal features of the instrument: HEP-LD is an advanced energetic particle spectrometer with time-of-flight(T) and energy(E) detection systems which determine the mass of incident nuclear particles. The novel design of the sensor head as a 'projection' camera allows the linear imaging of particle distributions in the polar angle (reference axis is the satellite spin vector pointing approximately towards ecliptic north): Twelve contiguous angular intervals over a range of 180 ~ Combined with the sectored spin plane of the spacecraft the unit sphere in phase space is covered with a total of 192 contiguous angular pixels (the satellite spin rate is 20 rpm). The energy ranges for hydrogen, helium and oxygen are approximately 40 - 3100 keV, 70 - 4000 keV, and 140 - 4000 keV, respectively. The instrument is not sensitive to the ionic charge state of nuclear particles; the mass resolution is marginal for separating carbon, nitrogen and oxygen ions. For the purpose of this paper the CNO group is referred to as 'oxygen' because this species is considered the major constituent. In addition to the energetic particle data , we use magnetometer measurements from the Magnetic Field Experiment (called MGF) on board GEOTAIL, which makes high resolution 23
24
Q.G. Zong and B. Wilken
vector field measurements. The MGF instrument is described in detail by Kokubun et aL (1994). 2.1 The Oxygen Burst in a Plasmoid
Fig. 1: Overview of the oxygen event on Jan.15, 1994, from 02:00 to 04:00 UT. From the top the panels show: Integral counting rates 'All Ions' (mass integrated) and 'CNO Ions' (the oxygen ion burst is marked in black); counting rates for hydrogen and helium; color coded azimuthal intensity distributions of the 'all ions' counting rate (look directions); GSE components and magnitude of the magnetic field (in nT).The circles in the third panel mark the magnetic field direction projected on the equatorial plane. Figure 1 gives an overview of the HEP-LD and MGF measurements for the time interval 02:00 to 04:00 UT on Jan. 15, 1994, when GEOTAIL was located approximately at (GSE: -96, 7,-5 Re) and a typical plasmoid event as described below was detected. This plasmoid event lasted from 0210 to 0330 UT. From 0210 to 0220 UT the spacecraft was in the northern lobe. In this location it detected , as a precursor of the plasmoid, a total magnetic field B, compression from 8 nT to 10 nT (the Bx and Bz components show an increase as well) prior to the arrival of the plasmoid at 0220 UT. This signature may have been caused by a compression of the lobe magnetic field in front of the moving plasmoid. The relative increase in the field magnitude is ~ 20%. In the plasmoid, the B u and Bz components show (+/-) bipolar waveforms with different times for the zero crossings (inflection point). The peak-to-peak variations are ABv=15 nT and ABe=9.6 nT. The zero crossing of the B u and Bz bipolar signatures occurred at 0243 UT and 022(3 UT, respectively. There is no indication of an enhanced core field at the time of the inflection points. This kind of plasmoid (bipolar signatures in the B v and Bz component), was first investigated by Moldwin and Hughes (1992). Inspection of Figure 1 shows that, during this event, the energetic particle peak count rates occurred when the instrument was looking sunward, i.e., there was a clear tailward flow in a downtail propagating magnetic structure. In particular, the magnetic field of this event is consistent with a 'closed loop' plasmoid with a symmetry axis roughly in the y-z plane forming an angle of approximately 450 with respect to the +z axis(Moldwin and Hughes, 1992). Because the zero crossing of the Bz and Bit bipolar signatures do not coincides, i.e., the spacecraft did not pass through the exact center of the structure (Slavin et al., 1993, 1994). Figure 1 shows the appearance of a strong energetic proton burst associated with this plasmoid event from 0220 UT to 0320 UT; embedded in this particle burst is a short intense increase in the oxygen flux from 0247 to 0310 UT. The
Energetic Oxygen Ion Bursts in the Distant Magnetotail
25
envelope of the energetic particle burst coincides with the interval of the disturbed magnetic field and as such appears to be part of the plasmoid morphology. The oxygen spike started 27 minutes after the onset of the particle burst and lasted for 23 minutes. The oxygen burst seems to be closely correlated with the strong negative B U component which developed after a strong positive excursion. The third panel of Figure 1 shows angular distributions for the particle bursts ('all ions' rate). The energetic particle burst (including the oxygen burst) display mainly tailward anisotropies. It should be mentioned that the (+/-) Bz bipolar signature passed over the spacecraft essentially during the interval for which the By component was positive. Between 0243 UT and the end of the event at 0320 UT the By component was essentially negative while Bz fluctuated with small amplitudes around zero. This indicates that GEOTAIL was traveling through a post-plasmoid-plasma sheet (PPPs). 2.2 Tailward Streaming Oxygen Event in the Plasma Sheet
Fig. 2: Overview of the oxygen from 17:45 to 19:15 UT on February 13, 1994. The format is the same as in Figure 1. Figure 2 gives an overview of the HEP-LD and MGF measurements on February 13, 1994, for the time interval 17:15 to 19:15 UT when GEOTAIL was located at (-63, 7.0,-3.8) Re in GSE coordinates, respectively. In figure 2, the Bx trace and the 'all ions' rate profile indicate that GEOTAIL was travelling in the PS and left the PS for the southern lobes at 19:15 UT. As shown in Figure 2 the angular distributions for hydrogen and helium (no oxygen) are quasi isotropic from 17:15 to 18:30 UT with a small net earthward flow. After 18:30 UT the distributions developed a strong tailward to duskward directed beam structure which ended abruptly at 19:10 UT when the spacecraft entered the southern lobes. The composition in the initial phase of the beam (18:30 to 18:47 UT) was primarily by hydrogen and helium. However, at 18:47 UT strong oxygen fluxes appeared and peaked at 19:02 UT. The oxygen peak coincided with an intensity reduction for hydrogen and helium as indicated in Figure 2. The decline of the oxygen peak after 19:02 UT began in the CPS shortly before the spacecraft passed through the PSBL towards the southern lobes. In antiphase with the decreasing oxygen abundance, the hydrogen intensity shows a transient returned to the pre-oxygen burst level as the spacecraft moved from the CPS to the lobes in the interval 19:05 to 19:10 UT. In the lobes the energetic particle flux dropped to background. In this event, the Bz component of the magnetic field was essentially positive with some fluctuations throughout the initial phase of the event and turned negative for the time of the tailward beam-like
26
Q. G. Zong and B. Wilken
distribution particular when the oxygen was present. The change of the B, polarity to principally negative values and the tailward streaming energetic particles can be explained by the widely accepted picture of a neutral line which has formed earthward of the spacecraft (Hones, 1984, e.g.). This neutral line is correlated with substorm onset at about 18:05 UT as obtained from ground-based and geosynchronous orbit observations (Zong et al., 1997b). 3. C H A R A C T E R I S T I C S O F THE OXYGEN B U R S T S
Color-coded representations of the angular distributions on the unit sphere are shown in Figure 3 for the time interval of the oxygen burst (02:15 to 03:30 UT, look directions). Four pictures show different views of the sphere: from the sun and from the tail (upper displays), from the north (lower left) and the south (lower right). Obviously, oxygen ions were flowing tailward in a highly collimated beam. Approximate values for the width in polar and azimuthal angle (determined at 10% of the maximum) for protons and oxygen are 37o x 125o and 90 o x 75~ respectively. Similar distributions are found for the event on February 13 as well. This difference in streaming direction may be caused by a mass-dependent process in the magnetic reconnection dynamics.
Fig. 3: Three-dimensional velocity distributions of the energetic oxygen burst from 02:15 to 03:30 UT on Jan 15, 1994. Particle Energy Spectra During the Oxygen Bursts Particle energy spectra for the Jan.15, 1994 plasmoid event are plotted in Figure 4. The integration time covers the entire plasmoid including the oxygen burst. As can be seen, the oxygen distribution has a well developed peak at 250 keV. Figure 4 shows no indication for the development of a peak in the proton and helium distribution functions. This can be explained by assuming that both proton and helium ions are flowing with the same bulk velocity as the oxygen. In this case the corresponding peak energies fall below the HEP/LD instrument energy window. Because of the strong collimation, the phase space density is computed only for the angular sector with the maximum flux. Number density ratios for the two oxygen events are calculated for the energy ranges covered by HEP-LD: 40 - 3100 keV for protons; 70 - 4000 keV for helium ions, and 140 to 4000 keV for the oxygen ions. Table 1 lists the results, value for the quiet plasma sheet and the normal solar wind (SW) (Bame et al., 1975) are added for comparison.
Energetic Oxygen Ion Bursts in the Distant Magnetotail
27
Fig. 4: Particle energy spectra for supra-thermal H +, He, and oxygen ions versus energy obtained for the plasmoid event on Jan. 15, 199~1. The integration time indicated on the upper right includes the oxygen burst. Even though the above number densities obtained from HEP-LD strictly speaking reflect only partial densities the following points with respect to the O / H ratios can be drawn: (1) ratios observed in the plasmoid and in the plasma sheet in the absence of enhanced oxygen abundance (burst) are less than 2.0%. (2) the solar wind ratio is approximately two orders of magnitude smaller than in the plasma sheet. (3) dynamic processes related with the generation of the observed oxygen beams can dramatically increase the oxygen/hydrogen density ratio. Lui et al. (1994) reported density ratios Nhr+ N~ obtained in flux ropes and/or plasmoids. In the energy range from ,-~ 10 keV to 230 keV, the ratio was between 0.07% to 0.75% and in the energy range from ~ 60 keV to 3 MeV (almost the same energy range covered by HEP-LD) the ratio was 1%. This is similar to the HEP/LD values for the quiet plasma sheet reference intervals. Oxygen ion bulk speeds The Geotail HEP-LD instrument measures the ion flux in 12 polar angles and 16 azimuthal sectors (total of 192 pixels); time-of-flight (T) and energy (E) are each analysed in 256 channels. The design allows the imaging of flux distributions over the complete unit sphere in phase space. This excellent angular coverage permits very good determination of
CASE 15 Jan. 1994
13 Feb. 1994
Solar Wind
Table 1: Ne s~~ Nn+ N2/+ 02:20-02:46 1.9% 13.3% 02:47-03:10 15.0% 4.8% Time [UT]
18:05-18:45 18:57-19:07
Comment Plasmoid Post-Plasmoid
0.96% 34.8%
15.5% 5.7%
Plasma sheet oxygen burst
052%
3.3%
Bame et al. (1975)
Q. G. Zong and B. Wilken
28
0247-0310 UT 15 Jan.94
Table 2: . First Order Anisotropies 0.7898
Oxygen Bulk Speed 1060 km/s
Source Location S _ - 1 6 . 7 Re
1857-1907 UT 13 Feb.94
0.890
1197 Km/s
S ~_ -20.9 Re
parameters relevant for particle distributions. The general form of a particle distribution function on the unit sphere for a given energy channel is I(O, r Applying the technique described by Sanderson and Page (1974); Sanderson and Hynds (1977); Sanderson et al. (1985) the expansion into a set of spherical surface harmonics (192 angular pixels) is represented by
I(O, r - E n c~--1AnOgr~O E :+= l e
(AnmYnem + BnmyOm)
(1)
Here, I(O, r is the particle intensity in the direction 0, r measured with respect to an arbitrary spherical coordinate system, Y~,~ are partially normalised surface harmonics, and An,~ and Bnm are the appropriate coefficients. Following (Daly et al., 1984, 1985), the oxygen anisotropy can be defined to first order in terms of the first harmonic A1 = (A10, All, Bll) which results from a spherical harmonic fit to the 192 directional ion flux measurements. The directional intensity
InI = lnAo + A1 9nl + h i g h e r - orderterms
(2)
is observed in the direction of the unit vector nl. With this representation the direction of the pole of the selected spherical coordinate system coincides with the direction of maximum intensity. Assuming that the ion distribution is isotropic in a frame moving with the (bulk) velocity V b relative to the spacecraft, then A1, the first-order anisotropy, can be related to the spectral index 7 and the bulk velocity Vb by writing
A1
-
--2(')' at-
1) -E, vb
where v is the mean ion speed in the given energy channel; the spectral index can be determined from the energy dependence of the anisotropy:
"[ -- lnAlcl-lnAlc2lnEcl"Z,~Ec2, here, subscripts cl and c2 denote different energy channels The oxygen bulk speed VD is calculated from A1 which is obtained from the spherical harmonic fit. Specifically, the bulk speed listed in Table 2 are derived from relatively broad energy channels for statistical reasons: Eel= 138.5397.5 keV, Ec2= 407.7-993.2 keV. As can be seen in Table 2 the bulk speeds of the oxygen beams exceed 1000 Km/s for the both two events (Jan. 15, 1994 and Feb. 13, 1994). Those are reasonable agreement with concurrent GEOTAIL CPI or LEP plasma bulk speed measurements. Geotail CPI measurements show the bulk speeds for those two events are -965 km/s and -1001km/s respectively. And in the same time interval, tailward proton beams at energies approximately 5 to 50 keV are also seen by GEOTAIL/CPI instrument (B. Paterson, Private communication). GEOTAIL/LEP Plasma measurements do not existing for the event on January 15, 1994; however, values for the plasma speed on February 13, 1994, vary between -1000 and-1600 km/s (Zong et al., 1997b). Those results indicate that the oxygen ions were accelerated to the nearly the same velocity as the plasma ions (proton and helium) in the course of these two events. Energy Dispersion of The Oxygen Ions The GEOTAIL satellite experienced numerous encounters with energetic oxygen bursts in the distant magnetotail. These bursts are usually related to substorm activity and exhibit strong beam-like angular distributions. Energy
29
Energetic Oxygen Ion Bursts in the Distant Magnetotail
dispersion effects are rarely seen in the time (T) or energy (E) spectrograms. However, the present two events display some dispersion effects as can be seen in the E versus T scatterplots in Figure 5 and 6.
15.01.1X=-96. 99402:55-03:11 DE,Ed_B,Ms/ode: E+T, Def=lkV Y=7. 2, Z=-4. 2, UT RE 6 1031i"~: 102~~""02:5:!i5i"~s~'~'2!02:56i":2:~..,.~.~:02:57 jj."[s~\.2~I02:58 Geotail HEP-LD
Ill
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l
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.................... ~................... ,................... ~......._.-:_. . . . . . . . . i"" _.. i"" . .... . "" -
~10311 ~~.,,I ~30:30 i ~ i : 03:0I4 ~ ~ 03:0ti5 ~ ~ 03:"0!6 ~,~102 7.:
102 101
2:
":2:
,
,,i
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~ ,
,
,,,,,,|
-.,.....: .
,
,
"
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.
.
,,,;,,i
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.
,
,
.
,,
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.
....
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.,
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,'"1
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'
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Time of flight [nsec]
'
'
.
- : ......
'''"'|
'
10 ~
'
'''"'!
m ~
101 '
......
_..
Fig. 5" A series of 16 scatter plots showing the mass and energy distributions during the oxygen burst in the post plasmoid on Jan. 15, 1994. Mass line in each frame indicate the loci of hydrogen, helium, CNO and SiFe (from the left). The frames show the oxygen ion with higher energy was detected previously than which with lower energy. With the assumption that all particles left the source region simultaneously the distance to the source region can be estimated (the method is also used by Williams (1981)). Consider two oxygen particles, A and B, with different energies and which leave the source at the same time (the spacecraft position is assumed to be stationary). The observations of the temporal evolution of the oxygen distribution via the time-of-flight effect can be wrote:
S ~r
=
~
S -
vlCos01
(3)
v2Cos02
where the 5T is the time difference between the different ions; S is the distance to the hypothetical source and 0 is the particle pitch angle observed by GEOTAIL. The subscripts 1 and 2 denote the different particles. From this we obtain for the distance S
S = vl v 2 C o s 0 1 C o s O 2 5 T
(4)
v2Co802 - I)1C0801
The results are listed in Table 2. The time intervals from 0259 to 0302 UT for Jan.15,1994 and from 1902 to 1906 UT for Feb.13,1994 and the energy channels (120 KeV and 686 KeV) are used in this back-trace calculation. It should be stressed, however, that the assumption of simultaneous emission of the particles is an essential pre-requisite for this estimate. Furthermore, the particle motion in the central plasma sheet may be nonadibatic or chaotic (Biichner and Zelenyi, 1989), and If the motion is chaotic, the particles will lose all "memory" of previous trajectory(Lyons and Speiser, 1982; Speiser, 1991). 4. DISCUSSION The discussed energetic oxygen bursts consisted of singly-charged O + ions as established by the EPIC instrument (private communication with S.Christon). This is strong evidence for an ultimate origin in the Earth's polar ionosphere. In the two oxygen bursts, the number density ratio No is much higher than that found in the normal plasma sheet. In addition, it is established that the oxygen bursts are closely related to substorms and can be understood as a direct
NH
30
Q. G. Zong and B. Wilken Geotail HEP-LD
13.02.1994 18:54-19:10UT
DE,Ed_B,Mode: s/E+T,Def=lkV :2:
X=-63.Y=7. 2, Z=-3. 0, RE 8
:
~--~102103 k ~5:.:
-4
9
~~_5_:_ ~~.~_
-
~_ 19:01
102 - 2:
-
!-=:
102103.~~6 ~..~::. 100
,,i
,
,
,,,,,,I
101 100 ,
,
,,,,,,I
,
--
!~:
: ~~.:: " ~.~.~~_119:09 ,
,
,,
101 100 101 100 Time of flight [nsec]
....
!
,
,
,,,,,,i
,
,
,,,,,,!
,
,
,,,,,,!
,
,
,,1,,,i
101 ,
",,
,,e_ ,,,,1
Fig. 6" A series of 16 scatter plots showing the mass and energy distributions during the oxygen burst in the central plasma sheet on Feb. 13, 1994. Mass line in each frame indicate the loci of hydrogen, helium, CNO and SiFe (from the left). The frames show the oxygen ion with higher energy was detected previously than which with lower energy.
product of the substorm process. By invoking the neutral line model we can explain the observed energetic oxygen bursts and the relation to substorm activity in this framework" First, the fast earthward flow with positive Bz was observed when GEOTAIL was located in the distant plasma sheet as seen in Figure 2. This is consistent with the existence of a 'constant' quiet time neutral line in the distant tail ( presumably at ~ 100 Re downtail) (Hones and McPherron, 1994; McPherron, 1991; Richardson et al., 1987; Richardson and Cowley, 1987). -
Second, groundbased magnetometer and geostationary particle observations show substorm expansion onsets at 02:10UT on Jan. 15, 1994(Zong et al., 1997a) and 18:05 UT on Feb. 13, 1994 (Zong et al., 1997b). At these times a second neutral line formed earthward of GEOTAIL as evidenced by the signatures of the magnetic field and the onset of tailward particle flow or the distant neutral line have moved Earthward of Geotail at 60 Re. The mean oxygen tailward flow speed is greater than 1000 km/s (Table 2).
-
-Third, magnetic field line reconnection at the new neutral line (closer to Earth) eventually results in the formation and rapid tailward motion of a plasmoid. Magnetic tension causes the plasmoid to move downtail at high speed (-400 to-600 km/s)(Richardson et al., 1987; Richardson and Cowley, 1987). Oxygen ions, extracted by the substorm process from the polar ionosphere, drift slowly (about 150 km/s) tailward in the lobe fields towards the active reconnection region which is located earthward of GEOTAIL and leads to appear after substorm onset about 20 to 30 minutes. In post-plasmoid-plasma- sheet (PPPs), plasma and oxygen ions are accelerated to velocities greater than 1000 km/s, which is far greater than the speed of plasmoids(Moldwin and Hughes, 1992). This process leads eventually to the observed bursts of energetic O + ions in the post-plasmoid plasma sheet (PPPS) characterised by a consistently negative Bz component (Zong et al., 1997a,b). 5.SUMMARY (1) Tailward flowing bursts of energetic oxygen (O +) were observed in the distant magnetotail (-X great than 65 Re) after the onset of a substorm. These particle bursts, embedded in the PPPS, showed bulk speeds of about V x = - l l 0 0 Km/s. The atomic charge state Q = I suggests a polar ionospheric origin. The oxygen bursts observed in the distant tail appear to be the products of global dynamical processes in substorms. The present particle observations are consistent with the widely accepted neutral line model of substorms. (2) The bulk speed of the oxygen burst, at least on February 13, 1994, is similar to the plasma speed; The energy dispersion of the oxygen ions allows us to estimate the position of the acceleration source (probably the neutral line position). Within the constraints of the assumptions, source locations between X=-15 to-20 Re were obtained.
Energetic Oxygen Ion Bursts in the Distant Magnetotail
31
(3) The number density ratio No observed during the bursts was much higher than values usually found in the distant NN+ plasma sheet and in the solar wind. This is interpreted as evidence for substorm associated extraction of oxygen ions from the polar ionosphere and subsequent acceleration in the magnetotail; by this process the oxygen abundance is temporarily drastically increased. (4) The angular distribution of the oxygen bursts is restricted to a rather small part of the unit sphere (about 7% to 10%). We suggest that a large amount of heavy ions from the ionosphere can be transferred to the distant tail and accelerated to high energies during substorm activity and these oxygen (O +) ions from the polar ionosphere can be considered as "Tracer Ions" in the substorm dynamical and magnetic reconnection process. REFERENCES Baker, D. N., T. I. Pulkkinen, P. Toivanen, M. Hesse, and R. L. McPherron, A possible interpretation of cold ion beams in the earth's tail lobe, J. Geomagn. Geoelectr., 48,699-710 (1996). Bame, S. J., J. R. Asbridge, W. C. Feldman, and M. D. Montgomery, Solar wind heavy ion abundances, Solar Physics, 43,463-473 (1975). Biichner, J., and L. M. Zelenyi, Regular and chaotic charged particle motion in magnetotaillike field reversals - 1. basic theory of trapped motion, J. Geophys. Res., 94, 11821-11842 (1989). Christon, S. P., G. Gloeckler, D. J. Williams, T. Mukai, R. W. McEntire, et al., Energetic atomic and molecular ions of ionospheric origin observed in the distant magnetotail flow-reversal events, 9rl, 21, 3023-3026 (1994). Daly, P., T. R. Sanderson, and K. P. Wenzel, Survey of energetic (e 35 kev) ion anisotropies in the deep geomagnetic tail, J. Geophys. Res., 89, 10733-10739 (1984). Daly, P., T. R. Sanderson, and K. P. Wenzel, A method to measure the bulk velocity of an energetic ion distribution in the presence of ion composition mixing, J. Geophys. Res., 90, 1499-1505 (1985). Doke, T., M. Fujii, M. Fujimoto, K. Fujiki, T. Fukui, et al., The energetic particle spectrometer HEP onboard the GEOTAIL spacecraft, J. Geomagn. Geoelectr., 46,713-733 (1994). Hones, E. W., Jr., Plasma sheet behavior during substorms, in Magnetic Reconnection in Space and Laboratory Plasmas, Geophys. Monogr. Set., vol. 30, edited by Hones, Jr., E. W., pp. 178-184, AGU, Washington, D. C. (1984). Hones, E. W., Jr., and R. L. McPherron, Evidence supporting the near-earth neutral line model of substorms: A reminder and update, in Proceedings of International Conference on Substorms 2, Fairbanks, U.S.A., March 7-11, edited by J. R. Kan, J. D. Craven, and S.-I. Akasofu, pp. 167-173, Univ. of Alaska, Fairbanks (1994). Kokubun, S., T. Yamamoto, M. Acuna, K.Hayashi, K. Shiokawa, et al., The GEOTAIL Magnetic Field Experiment, J. Geomagn. Geoelectr., 46, 7-21 (1994). Lui, A. T. Y., D. J. Williams, S. P. Christon, R. W. McEntire, V. Angelopoulos, et al., A preliminary assessment of energetic ion species in flux ropes/plasmoids in the distant tail, Geophys. Res. Lett., 21, 3019-3022 (1994). Lyons, L. R., and T. W. Speiser, Evidence of current sheet acceleration in the geomagnetic tail, J. Geophys. Res., 87, 2276 (1982). McPherron, R. L., Physical processes producing magnetospheric substorms and magnetic storms, Geomagnetism, 4, 593-739 (1991). Moldwin, M. B., and W. Hughes, On the formation and evolution of plasmoids: A survey of ISEE 3 geotail data, J. Geophys. Res., 97, 19259-19282 (1992). Mukai, T., M. Hirahara, S. Machida, Y. Saito, T. Terasawa, et al., Geotail observation of cold ion streams in the medium distance magnetotail lobe in the course of a substorm, Geophgs. Res. Lett., 21, 1023-1026 (1994). Richardson, I. G., and S. W. H. Cowley, Plasmoid-associated energetic ion bursts in the deep geomagnetic tail: Properties of the boundary layer, J. Geophgs. Res., 92, 9997-10013 (1987). Richardson, I. G., S. W. H. Cowley, J. E. W. Hones, and S. J. Bame, Plasmoid-associated energetic ion bursts in the deep geomagnetic tail: Properties of plasmoid and the postplasmoid plasma sheet, J. Geophys. Res., 92, 9997-10013 (1987).
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Q.G. Zong and B. Wilken
Sanderson, T. R., and R. J. Hynds, Multiple telescope measurements of particle anisotropies in space, Planet. Space Sci., 25,799-807 (1977). Sanderson, T. R., and D. E. Page, Spherical harmonic analysis of satellite anisotropy measurements, Nuclear Instruments and Methods, 110, 177-182 (1974). Sanderson, T. R., R. Reinhard, P. van Nes, and K. P. Wenzel, Observations of three-dimensional anisotropies of 35 to 1000-kev protons associated with interplanetary shocks, J. Geophys. Res., 90, 19-27 (1985). Slavin, J. A., M. F. Smith, E. L. Mazur, D. N. Baker, J. E. W. Hones, et al., Isee 3 observations of traveling compression regions in the earth's magnetotail, J. Geophys. Res., 98, 15425-15446 (1993). Slavin, J. A., C. J. Owen, and M. Hesse, Evolution of the plasmoid-lobe interaction with downtail distance, Geophys. Res. Lett., 21, 2765-2768 (1994). Speiser, T. W., Particle motion in the tail current sheet, Adv. Space Res., 9, 151 (1991). Wilken, B., W. Giittler, A. Korth, S. Livi, W. Weiss, et al., RAPID: The imaging energetic particle spectrometer on Cluster, in Cluster: Mission, Payload and Supporting Activities, pp. 185-217, Eur. Space Agency Spec. Publ., ESA SP-1159 (1993). Wilken, B., Q.-G. Zong, I. A. Daglis, T. Doke, S. Livi, et al., Tailward flowing energetic oxygen ion bursts associated with multiple flux ropes in the distant magnetotail: Geotail observations, Geophys. Res. Lett., 22, 3267-3270 (1995). Wilken, B., Q.-G. Zong, T. Doke, K. Maezawa, G. D. Reeves, et al., A isolating substorm occurred at first phase of cir: Geotail observations, J. Geophys. Res., P r e p a r i n g (1997). Williams, D. J., Ring current composition and sources: An update, Planet. Space Sci., 29, 1195-1203 (1981). Zong, Q.-G., B. Wilken, I. Daglis, S. Livi, J. Woch, et al., Geotail observation of energetic ion specics and magnetic field in plasmoid-like structures in the course of a substorm, in Proceedings of the third international conference on substorms (ICS-3), pp. 619-624, ESA SP - 389, ESA (1996). Zong, Q.-G., B. Wilken, G. Reeves, I. Daglis, T. Doke, et al., Geotail observation of energetic ion specics and magnetic field in plasmoid-like structures in the course of a substorm interval, J. Geophys. Res., 102, 11,409-11,428 (1997a). Zong, Q.-G., B. Wilken, T.Mukai, G. Reeves, T. Doke, et al., Energetic oxygen ion bursts in distant magnetotail as a product of intense substorms: Three case studies, J. Geophys. Res., 102, In Printing (1997b).
PRESENT AND FUTURE RESEARCH PROGRAM IN SOLARTERRESTRIAL PHYSICS AT ZHONGSHAN, ANTARCTICA R. Y. Liu
Polar Research Institute of China, 451 Jinqiao Road, Shanghai 200129, China
ABSTRACT Zhongshan Station has an ideal location for ground-based measurements in studying important problems related to solarterrestrial physics. The observations at Zhongshan Station include solar radiation, magnetic field variations, aurora data, ionospheric monitoring, middle and low atmospheric measurements and radio propagation. The present and future research include the program of global character research of solar-terrestrial system in the Antarctic, polar ionospheremagnetosphere coupling and auroral dynamics as well as two international collaborative studies. INTRODUCTION The Antarctic Zhongshan Station was established in February, 1989. It is located in Princess Elizabeth Land Larsemaan Hills of East Antarctica, having geographic coordinates of 69~ and 76~ In Fig.1 is shown the location of Zhongshan Station with respect to the Antarctic geospace network (AGONET) stations and the HF radar field of view (Dudeney, 1994). The annual mean temperature at Zhongshan Station is -9.5 ~ with maximum of 9.5 ~ in summer and minimum of-33.6~ in winter. The midnight sun lasts for 54 days, while the number of days with "Polar night" is approximately 58. The corrected geomagnetic latitude of Zhongshan is about 75 ~ and the equivalent L value is 14. Zhongshan Station is situated under the ionospheric projection of the magnetospheric cusp region at noon, and the polar cap region at midnight, twice passing through the auroral oval during a day. Zhongshan Station is also located in the area which is affected by ozone depletion. The weather is clear in most of the time. It is suitable for both optical and electromagnetic observations. Therefore Zhongshan Station is an ideal ground base for studying important problems related to solar-terrestrial physics. Another advantage of Zhongshan Station is that the distance between Zhongshan and the Australian Antarctic station, Davis, is about 100km. Both Zhongshan and Davis are located under the cusp region, having similar measurement instruments such as magnetometer, scanning photometer, all sky TV camera, riometer, ionosonde and so on. The Zhongshan-Davis pair will make a significant contribution to high latitude ionospheric and magnetospheric studies. It is also shown in Fig. 1 that Zhongshan Station has the same L value as South Pole Station with CGMT difference of about 5 hours. Coordinate observations at Zhongshan and South Pole would enlarge the observation time of high latitude phenomena. In Fig.2 the Antarctic continent is projected along the earth magnetic field lines to the Arctic (Dudeney, 1990). It can be seen that the conjugate point of Zhongshan is near Svalbard, a well equipped international ground base for studying solarterrestrial physics. This figure clearly illustrates the possibilities for carrying out magnetic conjugate observations between Zhongshan and the stations at Svalbard. THE PRESENT OBSERVATIONS AT ZHONGSHAN STATION A program named the global character research of solar-terrestrial system in the Antarctic was conducted over the past five years. The purpose of this program is to study the basic behavior of the coupling and interaction between various regions of solar-terrestrial system, and inquire into the response processes and the global effects of the system to the solar electromagnetic and particle radiation, using both ground based measurements and satellite in situ measurements. The main 33
R. Y. Liu
34
Fig. 1. Location of Zhongshan Station with respect to the AGONET stations and the HF radar field of view. (after Dudeney, 1994)
Fig. 2. Projection of Antarctica along the magnetic field lines on to the northern hemisphere. (after Dudeney, 1990)
research topics include: solar radiation measurement, cusp dynamic observations and modeling, polar ionospheric behavior and its effects on radio propagation, mechanisms of Antarctic ozone depletion hole and the effects of solar activity on the middle and low atmosphere, coordinate observations and analysis of solar bursts and responses of various regions of the system. This program was supported by the National Committee of Science and Technology and the National Committee on Antarctica, and 7 research institutes and universities took part. Now a composite ground based measurement system has been built which contains following observations: 1) Solar Radiation -- a telescope working at 10cm wavelength -- solar radiometer -- UV visible spectroscope -- solar UV spectroradiometry 2) Magnetic Field Variations -- standard magnetograms -- magnetic pulsations -- ELF/VLF emissions
3) Auroral Data -- all sky TV camera -- meridian scanning photometers (6300/?I,5577/~ ,4278/~ ) 4) Ionospheric Monitoring -- ionogram, using analog ionosonde TD-4 in early time, now using a digisonde DPS-4 -- drift measurement, digisonde DPS-4 -- absorption data, riometers (-30MHz and-~50MHz)
5) Radio Propagation -- HF field strength and time delay -- VLF field strength and time delay, navigation receiver.
Present and Future Research Program in Solar-Terrestrial Physics
35
6) Middle and Low Atmosphere -- column amounts of ozone, Brewer ozone spectrophotometer -- ozone profile, ozone-sonde -- stratospheric Lidar -- atmospheric electric field The measurement instruments at Zhongshan Station are listed in Table 1 with the responsible organizations and operating durations. Table 1.
Instrumentation on solar-terrestrial physics at Zhongshan Station
Item
Start Time
10 centimeter solar telescope Quartz photoelectric variometer Induction magnetometer Plasma wave measurement VLF receiver Ionosonde TD-4 Digisonde DPS-4 Imaging Riometer Riometer (-30MHz) Riometer (-50MHz) VLF navigation signal receiver HF field strength meter All sky TV camera Scanning photometer Surface ozone detector Ozone-sonde Brewer ozone spectrophotometer Solar UV spectroradiometry Solar radiometer UV visible spectroscope Stratospheric Lidar Atmospheric electric field mill
1993.4 1991 1992 1996.1 1991.4 1990.3 1995.1 1997.1 1995 1990 1990 1990 1995 1995 1995.1 1993.3 1993 1990.2 1990.2 1991.3 1993.3 1990.2
Responsible Organization BO IG IG PRIC IG CRIRP PRIC PRIC PRIC CSSAR CRIRP CRIRP PRIC PRIC PRIC lAP CAMS IAP IAP CSSAR lAP IAP
Remark
in coop. with UON
in coop. with NIPR
in coop. with NIPR in coop. with NIPR in coop. with NIPR
Abbreviation of Organizations: BO CAMS CRIRP CSSAR IAP IG NIPR PRIC UON
Beijing Observatory, Academia Sinica, Beijing 100080 Chinese Academy of Meteorological Sciences, Beijing 100081 China Research Institute of Radiowave Propagation, Xinxiang 453003 Center for Space Science and Applied Research, Academia Sinica, Beijing 100080 Institute of Atmospheric Physics, Academia Sinica, Beijing 100029 Institute of Geophysics, Academia Sinica, Beijing 100101 National Institute of Polar Research, Japan Polar Research Institute of China, Shanghai 200129 University of Newcastle, Australia
Most of the measurement instruments in Table 1 are used in regular observation and long-term monitoring. Some instruments, for example the solar telescope and solar UV spectroradiometry, will be .stopped during polar night. Based on the exiting measurement data from Zhongshan Station combined with other data from other places, we have done data analysis and theoretical and model studies and got some preliminary results which have shown various distinctive features of geospace in Antarctica (Liu, 1996).
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R.Y. Liu
FUTURE PLANS FOR SOLAR-TERRESTRIAL PHYSICS AT ZHONGSHAN STATION The program of global character research of solar-terrestrial system in the Antarctic will be continued. The research emphases in the future plans are on polar ionosphere-magnetosphere coupling and auroral dynamic processes, conjugate observations, monitoring and modeling of Antarctic Ozone, UV radiation and atmospheric composition. Here, only the polar ionosphere-magnetosphere coupling and auroral dynamic processes are mentioned in detail. High Latitude Ionosphere -- ionospheric properties by using digisonde DPS-4 ionograms. -- plasma convection by using digisonde DPS-4 drift measurement, response of dayside cusp and polar convection to the orientation and strength of the IMF. -- ionospheric absorption by using riometers (-30MHz and-50MHz) and an imaging riometer (8• elements) -- ionospheric signatures of cusp processes. Aurora and Particle Precipitation -- cooperative research on upper atmospheric physics between the National Institute of Polar Research, Japan, and the Polar Research Institute of China during the period from 1994 to 1999. Scientific Objectives: Auroral particles and auroral emissions in the cusp region, Ionospheric disturbances in the polar cap region associated with energetic particle precipitations, Correlative studies of high latitude ionosphere with HF radar observations from Syowa Station and ground based auroral and ionospheric observations at Zhongshan Station. The system consists of an all sky TV camera, a scanning photometer, a surface ozone detector, an imaging riometer, a monochromatic auroral TV canaera and an auroral spectrophotometer. In summer season, 1 or 2 Japanese scientists visit Zhongshan Station and establish observation systems in cooperation with Chinese scientists. In winter time, auroral and ionospheric observation instruments are operated by Chinese scientists. Space Plasma Wave Studies -- cooperative research between the University of Newcastle NSW Australia and the Polar Research Institute of China. The scientific aim of the cooperative project is to study the source and propagation characteristics of ultra-low frequency (ULF) hydromagnetic waves in the 0.001-1Hz band using identical induction magnetometers located at the Australian Antarctic station, Davis, and the People's Republic of China station, Zhongshan. Under the agreement the University of Newcastle, in cooperation with the Atmospheric and Space Physics Group of the Australian Antarctic Division, installed an induction magnetometer at Zhongshan Station in January, 1996, and the Polar Research Institute of China operate and maintain the instnament year round.
REFERENCES Dudeney, J. R., Antarctica, An International Platfom~ for Geospace Science, SCAR Atmospheric Science Working Group
Meeting, Invited Paper Review (1990). Dudeney, J. R., The Southern Hemisphere Auroral Radar Experiment, (SHARE), Short Note, Antarctic ,Science, 6(1), 123 (1994). Liu, R. Y., Advances in China in the study of Solar-Terrestrial Physics in Antarctica, in Geomagnetism, Atmosphere, Space Researches andApplJcations, edited by W. Y. Xu, pp. 125-136, Seismological Press (1996).
EVIDENCE FOR RESONANT ABSORPTION OF VLF WAVES OBTAINED AT ZHONGSHAN STATION, ANTARCTICA K.Y. Tang, F.L. Peng, Z.L. Ning, W.Z. Cao, Q.F. Meng, Y.H. Yang and C.M.Jiao
Institute of Geophysics, Chinese Academy of Sciences P.O.Box 9701, Beijing, 100101, P.R.China
ABSTRACT Evidence for resonant absorption of VLF waves was obtained at Zhongshan Station, Antarctica. The L value for Zhongshan Station is about 14. Due to such high geomagnetic latitude, no whistlers were received, but a lot of VLF emissions have been recorded since the station was founded in 1990. The VLF emission recorded in Zhongshan Station are usually wide-band emission, from a few hundred Hz to about 15kHz. In February and June of 1993, we recorded two sets of VLF emissions never seen before. The main feature for the emission is a blank curve around 5kHz cut from the wide-band emissions. We explain these observations in term of resonant absorption of VLF waves by ions. INTRODUCTION Many types of very low frequency plasma waves were received in Antarctica area. Whistler is one of the typical VLF waves that originates in lightning. The dynamic spectra of the whistler could help us to explore the plasma structure along the wave path inside the magnetosphere and ionosphere, and to understand the propagation processes and the excitation mechanism. Other VLF emissions, such as hiss, chorus, lion roars, periodic and quasi -- periodic emissions, originate within the plasma itself. All of those VLF waves could be easily received by a set of band pass VLF receiver with a tall enough antenna, especially in Antarctica area, due to the quiet electromagnetic environment. Besides using the VLF waves as a remote sensing tool to reveal the properties of plasma through which they travel, the high density and narrow bandwidth (a few hundred Hz to 25KHz, usually) also could indicate the presence of a new kind of wave particle interaction that converts the kinetic energy of charged particle into coherent electromagnetic radiation. This process is called coherent wave instability, or CWI. Thirdly, the energetic charged particles are precipitated into the ionosphere through resonance scattering by those VLF waves, causing enhanced thermal ionization, light and heat. Based on those reasons mentioned above, to better understand and use the CWI, a series of VLF simulation experiments were conducted by the Space, Telecommunications, and Radioscience (STAR) Laboratory of Stanford University. Controlled waves were injected from a 150-kW transmitter, through a 42 km antenna located at Siple Station, Antarctica into the magnetosphere and were received on the conjugate area in Quebec of Canada and by satellites. The key findings are 1) Coherent VLF signals often 37
38
K.Y. Tang et
al.
exhibit exponential temporal growth. 2) Temporal growth requires that the input signal exceed a threshold. 3) Narrowband triggered emissions can be entrained by Siple frequency ramps. 4)Simulated hiss shows coalescence of selected noise wavelets into longer and stronger chorus-like emissions(Helliwell, 1988). In 1994 and 1995, a further research on VLF simulation had been conducted by British Antarctic Survey, STAR Laboratory and University of San Paulo of Brazil, they used a chain of VLF receivers along the Antarctica Peninsula to record the lightning-induced electron precipitation (LED, i.e. trimpi events). The intersecting network of paths from the four northern hemisphere VLF transmitters NAA(Main, USA), NSS, NPM(Hawaii, USA) and NLK(Washington, USA) into four Antarctic receivers Faraday(UK), Ferraz(Brazil), Palmer(USA), Rothera(UK) had resulted in the recording of some interesting electron precipitation data excited by VLF wave(Smith et al., 1996). OBSERVATIONS In the year 1984-1985, the first Chinese Antarctic research expedition team started their work and founded the first Chinese Antarctic station -- the Great Wall station of China. Since then we have recorded many types of whistler waves, such as one and two-hop whistlers, multiple-component and multiple-source whistlers and echo trains at Great Wall Station. A new Antarctic geoscience station of China was founded in the winter of 1989. It has started to record geomagnetic and VLF emissions since the spring of 1990. The geographic latitude and longitude of Zhongshan Station are 69.37~ and 76.38~ the geomagnetic latitude and longitude are -77.06~ 122.12 ~ respectively, the L value for Zhongshan Station is about 14. Due to such high geomagnetic latitude, no whistlers have been received since the station was founded, but numerous VLF emissions have been recorded. The VLF emissions recorded at Zhongshan Station are usually wide-band emissions, from a few hundred Hz to about 15kHz (Tang, 1992). Fig. 1 shows a typical
Fig. 1 A typical spectrum of VLF wave received at Zhongshan Station. The horizontal axis is time in units of seconds, the vertical axis is the frequency in units of kHz, the darkness shows the spectrum density.
Evidence for Resonant Absorption of VLF Waves
39
dynamic spectrum o f VLF wave received at Zhongshan Station. In February and June o f 1993, we recorded two sets o f V L F emissions never seen before. The main feature for the new type o f emission is a blank curve cut from the wide-band emissions(Fig.2a and 2b).
Fig.2a On February 1, 1993, we recorded a set of VLF emissions never seen before, a clean blank curve around 5 kHz was cut form the wide-band emissions.
Fig.2b A consecutive part of Fig.2a, Feb. 1, 1993 From Fig.2a and Fig. 2b, we can see a clean blank curve around 5 kHz cut form the wide-band emissions.
40
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al.
Similar in Fig. 1, in Fig.2a and 2b, the horizontal axis are time in units of seconds, the vertical axis are the frequency in units of kHz, the darkness shows the spectrum density. S. Ohnami et al. [1993] have reported a type of triggered wave caused by a nonlinear wave-wave interactions in the subauroral ionosphere observed by ISIS-2 Satellite. On January 6 of 1988, VLF signals were ejected from Siple Station. Besides the 2.34kHz of ground-based VLF signals, an additional sideband structure around Siple signals were received by the ISIS-2 Satellite, they concluded that the sideband were caused by the nonlinear interaction between Siple signals and pre-existing ELF emissions(Ohnami et al., 1993) CONCLUSION AND DISCUSSION We have discovered a new interesting phenomena. It is different from Helliwell's finding, their controlled VLF signal triggered another VLF signal. It is also different from that of Ohnami et al., in their report, the interaction between two waves produced new waves. Our discovery shows that due to the interaction between natural VLF emission waves and ionospheric particles, a band of waves were absorbed, we infer that the phenomena were caused by resonance absorption, i.e. a type of Landau damping. Under a certain condition, particles with a certain velocity along a certain direction will interact with natural VLF emission waves, while the wave frequency, wave vector and particle velocity satisfy the following equation, CO - - n O , ) c i + K . V ,
n=0, +
1, _+2, _+3, ....
Eq.(1)
where co is the frequency of VLF emission wave, o3c~is the cyclotron frequency of some species of ion, K is the wave vector, and V is the particle velocity. The wave energy will be absorbed by the particle while the wave and particle satisfies equation 1. This could help us to better understand the particle wave interaction processes and mechanisms in the ionosphere and magnetosphere. ACKNOWLEDGMENT The authors would like to thank The Chinese Academy of Sciences and The National Antarctic Exploration Committee of China for supporting this project. We would like also to express our thanks to Mr. Gao Zonggang and Mr. Wang Zelin for their great work on designing and manufacturing the VLF receiver. REFERENCES Helliwell, R.A., VLF Wave Simulation Experiments in the Magnetosphere from Siple Station, Antarctica, Reviews of Geophysics, Vol.26, No.3, pages 551-578, August 1988. Ohnami, S., M.Hayakawa, T. F. Bell and T.Ondoh, Nonlinear Wave-wave Interactions in the Subauroral on the Basis of ISIS-2 Satellite Observations of Siple Station VLF Signals, Geophysical Research letter Vol.20, No.8, pages 739-742, April 23, 1993. Smith, A.J., M.A.Clilved, U.S.Inan, S.J.Lev-Tovand L.R.Piazza, Lightning-induced Electron Precipitation Studies with a Chain of VLF Receivers along the Antarctic Peninsula, Abstracts of 25th General Assembly of International Union of Radio Science, Lille, France, 1996. Tang, K.Y., Whistler Observations during 1990 magnetic Storms, Annual Report of Chinese Geophysical Union, Nov. 1992.
GLOBAL-SCALE IMAGING: NEW APPROACHES IN MAGNETOSPHERIC RESEARCH J. L. Green 1, S. F. Fung 1, D. L. Gallagher 2, M.-C. Fok 3, G. R. Wilson 4, G. R. Gladstone 5, J. D. Perez 6, P. H. Reiff 7, J. L. Burch 5, T. E. Moore 1
1NASA Goddard Space Flight Center, Greenbelt, MD, 20771, USA 2 NASA Marshall Space Flight Center, Huntsville, AL 35812, USA 3 USRA, NASA Marshall Space Flight Center, Huntsville, AL 35812, USA 4 NRC, NASA Marshall Space Flight Center, Huntsville, AL 35812, USA 5 Southwest Research Institute, San Antonio, TX 78228, USA 6Auburn University, Physics Department, Auburn, AL 36849, USA 7Rice University, Dept. of Space Physics and Astronomy, Houston, TX 77251, USA
ABSTRACT A completely different paradigm in magnetospheric research will begin with the launch of NASA's Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) in January 2000. All the instruments on IMAGE will provide global-scale measurements of various plasma regimes in the Earth's magnetosphere. The objective of the IMAGE mission is to determine the global response of the magnetosphere to changing solar wind conditions. IMAGE will address this broad objective in unique ways by using neutral atom imaging (NAI), far ultraviolet imaging (FUV), extreme ultraviolet imaging (EUV), and radio plasma imaging (RPI). The IMAGE theory and modeling team has accomplished an extensive number of simulations in order to model the response of the IMAGE instruments. INTRODUCTION The Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) is NASA's first medium-sized Explorer mission and is scheduled to be launched in January 2000. The overall science objective of IMAGE is to determine the global response of the magnetosphere to changing conditions in the solar wind. The science payload for IMAGE consists of instrumentation for obtaining images of plasma regions in the Earth's magnetosphere. The four types of imaging techniques used by IMAGE are: neutral atom imaging (NAI), far ultraviolet imaging (FUV), extreme ultraviolet imaging (EUV), and radio plasma imaging (RPI). These instruments are being designed to make concurrent global-scale images on a time scale of approximately four minutes, providing researchers with an opportunity to readily observe the structure and dynamics of the plasmasphere, ring current, aurora, geocorona, and the magnetopause within a substorm. In addition, simultaneous comparison of these data should also provide a global-scale understanding of how different important plasma regimes interact and their relationships with the changing conditions in the solar wind. The purpose of this paper is to provide background information on the IMAGE mission and what it is expected to observe. This will be accomplished by briefly describing the IMAGE mission and instruments and presenting a variety of computer simulations of the IMAGE instrument observations. ORBIT AND DATA SYSTEM Figure 1 shows a schematic of the IMAGE orbit. The IMAGE spacecraft will be spin-stabilized (--0.5 rpm) in an 90~ orbit with a perigee of 1000 km and an apogee of 7 Earth radii (RE) altitude. One 41
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J . L . Green et al.
IMAGE orbit is approximately 13 hours. The orbital evolution over the two-year nominal mission life is also shown. The precession of the orbit over the north pole of the Earth will provide the IMAGE instruments a global view of a number of important plasma regimes in the magnetosphere such as the plasmasphere, the cusp, and the ring current.
Fig. 1. The initial IMAGE orbit and the precession of its line of apsides. The IMAGE spacecraft will operate with about a 100% duty cycle with all instruments in their baseline operational modes and will generate nearly 200 gigabytes of Level-0 data per year. All IMAGE instrument and housekeeping data will be stored on-board and dumped to a Deep Space Network (NASA) ground station at a high rate approximately once per orbit. In order to support the National Space Weather program, IMAGE will has a 38 kb/s real-time link that will be monitered, on occasion, by the Space Environment Center of the National Oceanic and Atmospheric Administration in Boulder, Colorado. The IMAGE Level-0 data will be processed into Level-1 data (in the Common Data Format) from which browse images will be generated within 24 hours after their receipt in the Science and Mission Operations Control Center (SMOC) at the Goddard Space Hight Center. The SMOC will post only the latest IMAGE browse products on the World-Wide-Web (WWW) and transfer housekeeping, Level-0, and Level-1 data products to the National Space Science Data Center (NSSDC) for permanent archiving and long-term access. The NSSDC will ingest all IMAGE data into their NASA Data Archive and Distribution Service (NDADS) system for further distribution to the science community and the public. NDADS provides rapid (within minutes) access to a variety of space physics and astrophysics data and supports WWW and email requests for data. Higher level data products (ie: three dimensional images, movies, etc) will be generated by the IMAGE Science Team and archived at the NSSDC. The IMAGE Science Team will have the responsibility to generate and validate all the data products and claim no proprietary data rights. The IMAGE mission maintains a series of WWW pages which provides the latest information about all aspects of IMAGE, including the type and accessibility of IMAGE data (covered in the IMAGE Project Data Management Plan). These pages are accessible from the IMAGE homepage (see URL: http://image.gsfc.nasa.gov/). SCIENCE INSTRUMENTS There are three instruments to make NAI measurements: the Low Energy Neutral Atom (LENA) imager, the Medium Energy Neutral Atom (MENA) imager, and the High Energy Neutral Atom (HENA) imager. Each of these instruments covers a specific energy range and utilizes different instrument technologies. There are four instruments for photon imaging, the Extreme Ultraviolet (EUV) imager, the Spectrographic Imager (SI), Wideband Imaging Camera (WIC), and the Geocoronal (GEO) imager. The SI, WIC, and GEO instruments operate in the Far Ultraviolet (FUV) wavelength range. Finally, very long wavelength remote sounding will be accomplished with the Radio Plasma Imager (RPI). The performance requirements for the IMAGE instruments are listed in Table 1.
Global-Scale Imaging: New Approaches in Magnetospheric Research
43
The minimum time resolution for images from all instruments, except the RPI, is the spacecraft spin period of two minutes. The RPI will have modes which will allow density profile determination on shorter time scales. Images can also be constructed over time scales of multiple spin periods. Table 1. Instrumentation Required for the IMAGE Science Objectives Image Measurement NAI
EUV
Neutral atom composition and energy-resolved images over three energy ranges: 10-500 eV (LENA) 1-30 keV (MENA) 10-500 keV (HENA) 30.4 nm imaging of plasmasphere He + column densities.
FUV
Far ultraviolet imaging of the geocorona at 121.6 nm (GEO) and ',he aurora at 140-190 nm (WIC) and 121.6 and 135.6 nm (SI)
RPI
Remote sensing of electron densities and magnetospheric boundary locations using radio sounding.
Critical Measurement Requirements
FOV: 90~ 90~ (image ring current at apogee). AngularResolution: 8~ 8~(LENA),4~ 8~(MENA),4~ 4~(HENA), Energy Resolution (AF_./E): 0.8 Composition: distinguish H and O in magnetospheric and ionospheric sources,interstellar neutrals and solar wind. Image Time: 4 minutes (resolve substorm development). Sensitivity: effective area 1 cm2 for each sensor. FOV: 90~ 90~ (image plasmasphere from apogee). SpatialResolution: 0.1 Earth radius from apogee. Image Time: several minutes to hours (resolve plasmaspheric processes). FOV: 15~ 15~(SI) for aurora (image full Earth from apogee), l~ 360~for geocorona, and 22.4~ 30~OVlC) Spatial Resolution: 70 km (WlC),90 km (SI) Spectral Resolution: separate cold geocorona H from hot proton precipitation (Z,~0.2 nm near 121.6 nm); reject 130.4 nm and select 135.6 nm electron aurora emissions. Image Time: 2 minutes (resolve auroral activity). Density range: 0.1-105 cm-3 (determine electron density from inner plasmasphereto magnetopause). Spatial resolution: 500 km (resolve density structures at the magnetopauseand plasmapause). Image Time: 1 minute (resolve changes in boundary locations). .
Neutral Atom Imagers The NAI technique takes advantage of charge exchange between cold neutral geocorona hydrogen atoms and energetic magnetospheric and ionospheric ions. The hot neutral atoms created through the charge exchange process then travel in a straight line trajectory from the interaction region and can be measured at a remote location. The hot neutrals preserve the energy and direction of travel of the original ions. This allows the original ion distribution to be deconvolved from the observed energetic neutrals. Within the magnetosphere energetic neutral atoms have even been observed by instruments designed primarily for energetic charged particles on the IMP, ISEE, and POLAR spacecraft (see for example: Roelof et al., 1985; Spence et al., 1996). The science requirements driving the NAI instrumentation for IMAGE are to image the inner magnetosphere and to resolve the major species contributing to neutral atom fluxes. To meet these requirements a suite of three NAI instruments will provide angle, energy, and composition-resolved images at energies from 10 eV to 500 keV. The three NAI instruments are necessary because of the different techniques that apply to observing low (0.01 to 0.3 keV), medium (1 to 30 keV), and high (10 to 500 keV) energy neutral atoms. Angular information is obtained over 90 ~ fans with angular resolution between 4 ~ x 4 ~ and 8 ~ x 8 ~ depending on species and energy. Spacecraft spin is used to obtain angular information in the orthogonal (azimuthal) direction. All three instruments have collimators that consist of serrated, blackened surfaces to reduce internal scattering. The collimators contain deflection potentials of 10 kV that deflect and absorb charged particles below 100 keV/e. Small broom magnets remove electrons with energies <200 keV. LENA. The LENA instrument consists of a collimator, conversion region, dispersive electrostatic energy analyzer, and time-of-flight sensitive particle detection. Particles enter the instrument through a defined by the height of the collimator. Neutrals are converted to
unit, extraction lens and acceleration (TOF) mass analyzer with positioncollimator with elevation acceptance negative ions through near specular
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J . L . Green et al.
reflection from a low-work-function cesiated tungsten (Cs-W) conversion surface (Wurz et al., 1995). The surface is segmented to cover a 90 ~ azimuthal acceptance. The Cs-W is resurfaced approximately once or twice per week. LENA is also an ion mass spectrograph. Neutrals converted to negative ions at the conversion surface are accelerated and analyzed for energy, mass/charge and elevation angle by a spherical electrostatic analyzer. LENA Simulations. Simulations of the LENA observations are shown in Figure 2. The first and third panels of Figure 2 show the expected differential fluxes of neutral O and H produced from charge exchange of the upflowing ions in the auroral zone, polar cap, and cleft ion fountain. The images in the second and fourth panels (labeled instrument counts) were produced by integrating along the line of sight from a observing point just beyond perigee in the nominal IMAGE orbit. The instrument counts were constructed from O and H neutral atom fluxes by folding in the instrument response, which included the appropriate angular resolution and per pixel sensitivity, and adding random Poisson noise. The assumed geophysical conditions at the time of these simulated images were high solar activity (F10.7=200) and high magnetic activity (Kp=7).
Fig. 2. The first and third panels show line-of-sight integrated images of neutral atoms (23-55 eV) produced by modeled cleft ion fountain, polar cap and auroral zone ion upflows as well as backsplash oxygen neutral atoms produced by precipitating ring current O+. The second and fourth panels show the expected LENA results. The model used to produce these images assumes that the ionospheric ion outflow flux, from any location, is proportional to the local energy flux carded by precipitating electrons as given by the Hardy et al. [ 1987] empirical model, normalized so that the total outflow flux is that given by Yau et al. [ 1988]. The model finds the steady state ion flux as a function of energy, pitch angle, location and species by tracking particles in dipole magnetic and convection electric fields, launched from an altitude in the ionosphere where they are heated to a 10-30 eV initial energy. The ion fluxes are converted to neutral atom fluxes through charge exchange with thermospheric hydrogen, oxygen, helium and ionospheric O+. Results are dependent on Kp and F10.7 through the dependence of the model's inputs (thermospheric composition, ion outflow fluxes, auroral zone location, convection electric field) to these parameters. MENA. The MENA imager is a slit camera with straight-through optics, which samples and resolves velocities, masses, and polar angles within a 107 ~ fan. The MENA analyzer consists of a collimator, UV rejection grating, start foil, position-sensitive anode, TOF analyzer, and pulse-height analyzer. The collimator plates use electrostatic deflection to reduce charged particle background. The UV grating acts as a wave guide to reduce the Lyman a fluxes by a factor of 104 from the sun and geocorona. Velocity analysis, through TOF measurements made with the start/stop MCP, combined with pulse-height analysis, yields mass resolution sufficient to separate H + and O+. HENA. The HENA is a slit camera with a 90 ~ x 120 ~ field of view and a segmented focal plane incorporating an imaging solid-state detector (SSD) array in one portion and an MCP with position-sensitive anode in the other. Pulse height analysis of the SSD pulses provides total energy, which, combined with the TOF velocity determination, yields neutral atom mass.The MCP pulses are also pulse-height analyzed,
Global-Scale Imaging: New Approaches in Magnetospheric Research
45
separating of H and O. Each pixel is viewed both by the SSD array and the MCP as the scene is scanned. HENA acquires angular images by locating the start pulses on the entrance slit and the stop pulses in the image plane. The collimator serves to suppress charged particle entry by biasing adjacent collimating plates at+10 kV.
Fig. 3. Left hand panels: Simulated ring current flux of 1.5 keV H + and 31 keV H + at the equator during a geomagnetic storm on October 19, 1995 as predicted by the ring current model of Fok et al. (1995). Middle panels: The corresponding neutral atom fluxes projected along polar and spin azimuth angle. Right hand panels: instrument count of a 10 minute exposure as would be obtained by the MENA and HENA insmunents respectively from IMAGE near apogee. MENA and HENA Simulations. The neutral atom images that will be acquired by the IMAGE LENA, MENA, and HENA instruments are the result of line-of-sight integrations for which the number of counts in an individual pixel reveals how many particles came from that direction but not how far along that line-ofsight direction they originated. In order to extract the maximum amount of scientific information from these images, it is important to deconvolve the unknown source from the data. An arsenal of techniques are being developed to accomplish this task. All are based upon using the principles of Bayesian statistics to impose objective, clearly-defined criteria to select among the set of solutions that provide an acceptable fit to the data. To analyze simulated data (Moore, et al., 1995), such as that shown in Figure 3, it is assumed that the ions are trapped along magnetic dipole field lines while conserving both energy and the first adiabatic invariant. The equatorial pitch angle distribution of the ions is expanded in terms of bicubic splines for the two space dimensions and in terms of Legendre polynomials for the pitch angle dependence. Two conditions are imposed upon the solution, (1) fit the data, and (2) be as smooth as possible by minimizing the second derivative. The relative balance between these two criteria is determined using the technique of generalized cross validation which is based on the idea of repeating the solution with each of the pixels in turn removed from the computation and then choosing the smoothest solution. This technique yields a deconvolved 2dimensional pitch angle distribution from a single image (Perez, et al., 1996) or from a stereoscopic set of images of the same source that is consistent with the observations and that introduces no spurious structure. A method for analyzing a time-sequence of images has also be implemented. Similar methods that yield deconvolved 3-dimensional sources are being developed.
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al.
Photon Imagers There are four photon imagers on IMAGE. The EUV instrument images resonantly scattered solar emissions from cold plasmaspheric He + at 30.4 nm. Two FUV instruments, SI (Spectrographic Imager) and WIC (Wideband Imaging Camera) image the Earth's electron and proton auroral emissions, and a Geocorona Oxygen Cell (GEO) images the hydrogen geocorona at 121.6 nm. EUV. Effective imaging of plasmaspheric He + requires global "snapshots" in which the high apogee of the IMAGE mission and the wide FOV of the EUV imager provide, in a single exposure, a map of the entire plasmasphere from the outside with a sensitivity of 0.2 count/s-pixel-Rayleigh (R), a spatial resolution of 0.1 RE, and a time resolution of several minutes. The 30.4-nm feature is easy to measure because it is the brightest ion emission from the plasmasphere, it is spectraUy isolated, and the background is negligible. Measurements are easy to interpret because the plasmaspheric He + emission is optically thin, so its brightness is directly proportional to the He + column abundance. EUV consists of three identical sensor heads serviced by a common electronics module. It employs elements of new t~hnology, multi-layer mirrors. Because it is simple and lacks moving parts, the EUV is rugged and reliable. Each sensor head has a field of view of 30 ~ x 30*. The three sensors are tilted relative to one another to cover a fan-shaped instantaneous FOV of 90 ~ x 30 ~ As the satellite spins, the fan sweeps a 90 ~ x 360 ~ swath across the sky. Each EUV sensor achieves high throughput and a wide field of view by using a large entrance aperture and a single spherical mirror. A multi-layer reflective coating on the mirror selects a narrow 5-nm passband around the 30.4 nm line. To circumvent the red leak in the multi-layer mirror, a metal foil filter blocks H Lyman a, from the geocorona. The detector consists of two curved, tandem MCPs with an alkali halide front surface photocathode. The detector's spherical input surface minimizes the effects of spherical aberration. Readout from the detector is from a 128 x 128 wedge and strip anode. The sensitivity (accounting for the duty cycle inherent in a spinning spacecraft) is 0.2 count/(sec pixel) per R, where the pixel size is taken to be 0.1 RE. By summing pixels to make a spatial resolution element (or resel) of 0.5 RE, the count rate is 5 counts/(sec resel) per R.
Fig. 4. A simulated thermal plasma distribution during a storm time recovery is shown in the left hand panel. A modeled EUV image from near apogee of the IMAGE orbit is shown in the fight hand panel. The simulated image reflects what the EUV instrument would see for the distribution of plasma shown in the left panel (note: model oriented slightly differently from the EUV image). EUV Simulations. A simulation of convecting thermal plasmas in the magnetic equatorial plane is used together with a global empirical model of thermal plasma in the magnetosphere to predict the distribution of
Global-Scale Imaging: New Approaches in Magnetospheric Research
47
cold helium ions using (once again) the October 1995 magnetic cloud event. This simulation includes ionospheric filling, saturation, and solar wind driven convection. The left hand panel in Figure 4 shows the model plasma distribution during the recovery phase of the substorm that was observed during the event. The sun is to the left of this Figure. An important feature in the thermal plasma model is an extended plasmaspheric tail which results from the recapture and corotation of previously sunward convected plasmaspheric plasma. The right hand panel of Figure 4 is the corresponding simulated EUV instrument image that would be observed near apogee of the IMAGE orbit. The appearance of the Earth's shadow in the simulated EUV image provides an immediate orientation for the observer. The simulated image is a composite from all three EUV cameras. Poisson and dark counting noise has been added, along with instrument sensitivity and variation in the relative integration time across the conical instrument apertures. The relative sensitivity across the field of view has been deconvolved in this image. Enhanced noise at the left and right image edges and in two vertical bands results from that deconvolution. The extended plasmaspheric tail that has been swept up by corotation is near 10 hours MLT. FUV. Science requirements driving FUV imager designs are (1) to image the entire auroral oval from a spinning spacecraft at 7 RE apogee altitude, (2) to separate spectrally the hot proton precipitation from the statistical noise of the intense, cold geocorona, and (3) to separate spectrally the electron and proton auroras. The FUV consists of two imagers that combine high spectral discrimination, high spatial resolution, and the greatest possible sensitivity to meet these requirements. In the FUV range up to --160 nm, there are several bright auroral emission features that compete with the dayglow emissions. For the electron aurora, the brightest is 130.4 nm OI, which is multiply scattered in the atmosphere and thus is not optimal for auroral morphology studies. The next brightest is the 135.6 nm OI emission. Separation of the 130.4 and 135.6 nm lines necessitates the use of a spectrometer because even reflective narrow-band filter technology cannot satisfy the -3 nm wavelength resolution requirement. Above 135.6 nm, weak LBH lines can be detected using narrow-band filter technology. Separate imaging of the intense, cold geocorona (Lyman tx emissions at 121.6 nm) and the less intense, Doppler-shifted Lyman ct auroral emissions requires significantly higher spectral resolution (0.2 nm). Spectrographic Imager ($I). The relatively high wavelength resolution requirement is satisfied by the SI instrument. The 0.2-nm wavelength resolution drives the size of the instrument and consequently the number of mirrors in the optics system. Also the narrowness of the slits in the spectrometer limit the dwell time during which a pixel is in the field of view. The SI is a Wadsworth spectrometer, which uses a diffraction grating to produce separate images of 135.6 nm emissions from the electron aurora and 121.8 122.2 nm Doppler-shifted Lyman tx emissions from the proton aurora. The 130.4 nm oxygen airglow emission and the geocorona 121.6 nm Lyman tx emission are blocked out. The detectors use a KBr photocathode on a MgF2 window image tube with MCP intensification. The intensified image is detected by a crossed delay-line type detector with two 32 x 128 pixel active areas. The Geocorona Oxygen Cell (GEO) provides three 1~ narrow-band photometer channels of geocorona data at 121.6 nm. For some orientations of the spin axis, the Sun may enter the field of view of SI at some spin phases. For these times the control microprocessor will automatically reduce the high voltage to the MCPs to avoid excessive counting rates. The filters will prevent damage to the detector by focused visible sunlight. Excellent observations of the geocorona have been previously accomplished by Rairden et al., [1986] on the Dynamics Explorer spacecraft. Wideband imaging c~merit (WIC). The relatively high sensitivity requirement for auroral imaging is satisfied by the WIC. This imaging camera uses the basic design flown on the Freja and Viking (Anger et. al, 1987) satellites to measure the auroral LBH emissions in a relatively broad band from 140 nm to 190 nm. The large field of view permits a long integration period and increases its sensitivity. The WIC optics design is identical to that of the Freja camera. Incident photons pass through a filter that blocks Lyman tx emissions and protects the detector from direct, focused sunlight. The primary and secondary mirrors have a coating that is highly reflective (>60%) in the FUV but has minimum (<3%) reflectance out of band. MCPs are used to intensify the image, which is produced on a phosphor and fiber-optically coupled to a diode array. Operation of the WIC is essentially identical to that of SI. Readout occurs once per 0.1 ~ of rotation for a frame rate of 30 frames/s at 0.5 rpm. The camera data are digitized and co-added in memory and the addresses are selected according to the rotational phase of the spacecraft. This technique minimizes distortion.
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J . L . Green et al.
FUV Simulations. The magnetic cloud event of October 18-19, 1995 was used as a comparative period for simulating IMAGE data. In order to simulate an SI observation during this time period, Hardy statistical models (Hardy et aL 1987; 1991) were used to represent auroral activity. Using the model energy flux and mean energy of precipitating electrons and protons the brightness of HI Lyman a, OI 135.6 nm, and N2 LBH emissions (using the yield curves determined by Strickland et aL, 1993) can be estimated. Realistic levels of dayglow emissions were added and images simulated using the code of Gladstone (1994). Estimated brightnesses were converted to counts using the estimated SI sensitivity of 5 cps/100R/spin, and appropriate dark and counting noise were added to make the images more realistic. Auroral emissions stronger than the dayglow would allow aurora to be measured on the dayside with FUV. Figure 5 shows an example OI 135.6 nm image generated for the substorm on October 18, 1995. The IMAGE spacecraft is assumed to be near apogee. This vantage point provides an excellent view of the northern auroral oval when the magnetic cloud event is most intense, and demonstrates that the FUV imagers should obtain excellent data during the IMAGE mission.
Fig. 5. Simulated OI 135.6 nm image as seen from by the SI instrument on IMAGE. The SI selects photons from a 3-nm region centered on the OI 135.6 nm doublet, and has a field-of-view of 16~x 16 ~with 128 (0.125 ~ pixels on a side. Radio Imaging The Radio Plasma Imager (RPI) is a transmitter/receiver system that provides remote sensing measurements of plasma densities, structures and dynamics in the magnetosphere and plasmasphere. The instrument measures the time delay, angle-of-arrival, and Doppler shift of magnetospheric echoes over the frequency band from 3 kHz to 3 MHz. This frequency range makes possible remote sensing of plasma densities from 0.1 to 105 cm-3. Programmable operational modes will focus on specific magnetospheric and plasmaspheric features. RPI will have two crossed 500-m tip-to-tip thin wire dipole antennas in the spin plane, and a 20-m tip-to-tip dipole antenna along the spin axis. All three antennas will be used for reception to determine the angles of arrival of the echoes (Calvert et al., 1995; 1997). The large distances, low power, and short antennas (relative to the wavelength) require onboard signal processing. Pulse compression and coherent spectral integration techniques will be used to increase the signal-to-noise (S/N) ratio. The nominal range resolution is approximately 500 km. The number of sounding frequencies selected for a given measurement, together with the coherent integration time, determines the time resolution. RPI Simulations. The primary presentation of RPI data will be in the form of plasmagrams, which are the magnetospheric analogs of the ionograms. A plasmagram is plot of the echo power as a function of frequency and echo delay. Ray tracing calculations have been performed to simulate the return pulses from the RPI instrument on IMAGE located in a model magnetosphere. The model magnetosphere used in the ray
Global-Scale Imaging: New Approaches in Magnetospheric Research
49
tracing has been described by Green et al., 1997. Figure 6 shows simulated RPI echoes (delay times as a function of frequency) as would be observed by IMAGE near apogee during the October 1995 magnetic cloud event. Two RPI echoes are clearly seen, one from density regions toward the magnetopause and the other toward the Earth. The Ne profile that was calculated from the change of echo delay with frequency using the technique developed by Huang and Reinisch, [1982] is also shown. The calculated Ne profile is in excellent agreement with the model Ne profile used in the ray tracing calculations. Plasma
Frequency ., ,, s " - I Maonet
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Fig. 6. Simulated plasmagram showing the expected echo amplitudes and delay times as a function of delay and frequency. In addition, the corresponding radial Ne distribution is also shown.
Based on these simulations, RPI will determine the location of the magnetopause and plasmapause and their respective Ne values. It should be possible to deduce global-scale boundary structures using the directional and range measurements, as well as from a sequence of many plasmagrams along the IMAGE orbit. SUMMARY The Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) mission will produce forefront science by quantifying the response of the magnetosphere to the time variable solar wind. As the first dedicated imaging magnetospheric mission, IMAGE will acquire a variety of simultaneous threedimensional images of magnetospheric boundaries and plasma distributions extending from the magnetopause to the inner plasmasphere. The images will be produced on time scales needed to answer important questions about solar wind-magnetosphere interactions. Computer simulations presented in this paper illustrate the type of data that will be routinely obtained on IMAGE.
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J . L . Green et al.
REFERENCES Anger, C. D., S. K. Babey, A. Lyle Broadfoot, R. G. Grown, L. L. Cogger, R. Gattinger, J. W. Haslett, R. A. King, D. J. McEwen, J. S. Murphree, E. H. Richardson, B. R. Sandel, K. Smith, and A. V. Jones, An Ultraviolet Auroral Imager for the Viking Spacecraft, Geophys. Res. Letts., 14, 387-390 (1987). Calvert, W., R. F. Benson, D. L. Carpenter, S. F. Fung, D. Gallagher, J. L. Green, P. H. Reiff, B. W. Reinisch, M. Smith, and W. W. L. Taylor, The Feasibility of Radio Sounding of the Magnetosphere, Radio Sci., 30, 1577-1595 (1995). Calvert, W., R. F. Benson, D.L. Carpenter, S. F. Fung, D. L. Gallagher, J. L. Green, D. M. Haines, P. H. Reiff, B. W. Reinisch, M. F. Smith, and W. W. L. Taylor, Reply to: Comment on the Feasibility of Radio Sounding in the Magnetosphere, Radio Science, 32, 281-284 (1997). Fok, M.-C., T. E. Moore, J. U. Kozyra, G. C. Ho, and D. C. Hamilton,Three-Dimensional Ring Current Decay Model, J. Geophys. Res., 100, 9619-9632 (1995). Gladstone, G. R., Simulations of DE 1 UV Airglow Images, J. Geophys. Res., 99, 11441-11448 (1994). Green, J. L., W. W. L. Taylor, S. F. Fung, R. F. Benson, W. Calvert, B. Reinisch, D. L. Gallagher, and P. Reiff, Radio Remote Sensing of Magnetospheric Plasmas, Accepted for publication in the Chapman Conference on Measurement Techniquesfor Space Plasmas (1997). Hardy, D. A., M. S. Gussenhoven, R. Raistrick, and W. J. McNeil, Statistical and Functional Representations of the Pattern of Auroral Energy Flux, Number Flux, and Conductivity, J. Geophys. Res., 92, 12275-12294 (1987). Hardy, D. A., W. McNeil, M. S. Gussenhoven, and D. Brautigam, A Statistical Model of Auroral Ion Precipitation, 2, Functional Representation of the Average Patterns, J. Geophys. Res., 96, 5539-5547 (1991). Huang, X., and B. W. Reinisch, Automatic Calculation of Electron Density Profiles from Digital Ionograms. 2. True Height Inversion of Topside Ionograms with the Profile-Fitting Method, Radio Science, 17, 837-844 (1982). Moore, T. E., M.-C. Fok, J. D. Perez, and J. P. Keady, Microscale Effects from Global Hot Plasma Imagery, in Cross-Scale Coupling in Space Plasmas, J. L. Horwitz and N. Singh, eds., Geophys. Mono., 93, AGU, Washington, DC, 37-46 (1995). Perez, J. D., J. P. Keady, T. E. Moore, and M.-C. Fok, Microphysics from Global Images, in Physics of Space Plasmas, T. Chang and J. R. Jasperse, eds., MIT Center for Theoretical Geo/Cosmo Plasma Physics, Cambridge, Mass. (1996). Rairden, R. L., L. A. Frank, and J. D. Craven, Geocoronal Imaging with Dynamics Explorer, J. Geophys. Res., 91, 13613-13630 (1986). Roelof, E. C., D. G. Mitchell, and D. J. Williams, Energetic Neutral Atoms (E~50 keV) from the Ringcurrent: IMP 7/8 and ISEE 1, J. Geophys. Res., 90, 10991- (1985). Spence, H. E., R. B. Sheldon, T. A. Fritz, and J. Chen, First Energetic Neutral Atoms (ENAs) Measured by POLAR: ENA Source Regions at Both Low and High Altitudes, (abstract), EOS, 77, 565 (1996). Strickland, D. J., R. E. Daniell, Jr., J. R. Jasperse, and B. Basu, Transport-Theoretic Model for the Electron-Proton-Hydrogen Atom Aurora, 2. Model Results, J. Geophys. Res., 98, 21533-21548 (1993). Wurz, P., M. R. Aellig, P. Bochsler, A. G. Ghielmetti, E. G. Shelley, S. Fuselier, F. Herrero, M. F. Smith, T. Stephen, Neutral Atom Imaging Mass Spectrograph, Optical Engineering, 34, 2365 (1995). Yau, A. W., W. K. Peterson, and E. G. Shelley, Quantitative Parametrization of Energetic Ionospheric Ion Outflow, in Modeling Magnetospheric Plasma, T. E. Moore and J. H. Waite Jr., eds., Geophys. Mono., 44, AGU, Washington, DC, 211-217 (1988).
UV
AURORAL
IMAGING
J. S. Murphree
Department of Physics and Astronomy, University of Calgary
ABSTRACT Auroral images acquired by satelliteinstrumentation have proven to be a crucial component of the scientific enquiry into the physical processes of the Earth's magnetosphere. These processes often result in the emission of photons and the ability to remotely sense these processes becomes possible. Further, with suitable instrumentation the global character can be measured providing a means whereby the activity of vast regions of the magnetosphere is monitored. Perhaps the most powerful technique established thus far is the usage of global ultraviolet imaging because in this wavelength region sunlight scattering is low. While normally the spatial and temporal characteristicsof the aurora are used to infer the character of the plasma processes operating in the near-Earth environment-on a morphological basis, there is an trend towards using the imagery in more quantitative fashions. Investigation into the dynamics of substorm bulge expansions shows bulge extents reach 2 x 106 k m 2 by the time the bulge has reached its poleward limit. The expansion process often occurs in a series of intensificationswhich result in rapid poleward advances. The total time period of expansion is approximately 20 minutes although shorter and longer periods are observed. THE CASE FOR SATELLITE
AURORAL
IMAGING
As the trend toward more multipoint, in situ measurements continues, the need to establish the context within which those measurements are made gains significance. One outgrowth of this are the growing efforts at multi-experiment studies where observations are made in the same temporal frame, but different spatial domains. Depending on the questions asked however, it is often difficultto establish a sufficientlylarge-scale context unless some type of remote sensing is performed. Currently an excellent means of providing this context is to utilize satelliteplatforms to image as much of the auroral distribution as possible. Because the aurora provides a mapping of magnetospheric processes into the ionosphere (which in turn modifies the processes themselves) an extensive region of space can be monitored relatively simply. Initial steps toward providing this context were taken with the ISIS 2 spacecraft which contained two photometers (Anger et al., 1973; Shepherd et al., 1973) that provided for the firsttime views of the auroral distribution. This was followed with the D M S P series of spacecraft which used photographic techniques for its data system (Rogers et al., 1974). The success of these programs was followed by other missions (HILAT [Meng and Huffman, 1984]; Polar Bear [Schenkel and Ogorzalek, 1987]) all of which provided (through various means) one image (per channel) during an orbit of the satellite. While useful for morphology, the absence of dynamic information for time scales less than an orbital period (roughly 2 hours) limited the usefulness of the data for questions involving magnetospheric dynamics. This shortcoming was removed with the Dynamics Explorer (DE 1) (Frank et al.,1981) program wherein images could be acquired over roughly 12 minute intervals by combining scan lines into an image. This was followed by the major technical advance of providing simultaneous exposure in two-dimensions (hence construction of an image instantaneously). Initiated by the U V Imager on Viking (Anger et al., 1987) the success was rapidly followed by the Akebono program (Oguti et al., 1990), Freja (Murphree et al., 1994), Polar (Frank et al., 1995; Tort et al., 1995) and Interball(Cogger et al., 1995). ADVANTAGES
AND
DISADVANTAGES
OF UV IMAGING
Aside from the improvement in what one might call image formatting obtained over the last number of years, an equally significantadvance has been in the area of wavelength coverage. Originally (eg ISIS 2 and D M S P ) measurements were made using the visible part of the spectrum. Populated by atomic lines (557.7 51
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J.S. Murphree
Figure 1: UV images (Lyman-Birge-Hopfield emissions) obtained during the Viking mission. Two images (top and bottom) from three separate passes are shown. The auroral distribution appears in grey-level format with more intense emissions whiter. An example of a Poleward Intensification (PI) is shown in each pass.
nm, 630.0 nm) and well understood molecular band systems (eg N2 +), the interpretation of intensities were relatively straightforward and further, ratios of measurements led to information on some characteristics of the particles causing the emissions (Rees and Luckey, 1974). However, a major problem with using visible emissions is the contamination by scattered sunlight and hence the inability to make observations except when the ionosphere is in darkness. Thus early observations of dayside auroral activity were restricted with a few notable exceptions (eg Cogger et al., 1977; Dandekar and Pike, 1978). Beginning with the DE 1 mission, the significant advantage of UltraViolet (UV) imaging was established, and this approach has been a cornerstone of satellite imaging ever since. Because scattering in the UV is minimal, the auroral distribution can be extracted from dayside observations even during full sunlight. Figure I shows two images from each of three orbits illustrating this from the Viking program. The images on the left in particular, taken in the UV during September, show the northern hemisphere auroral distribution exhibits strong auroral activity on the dayside of the terminator (this boundary runs roughly from bottom left in the images to top right). The importance of this cannot be overstated. The possibility of including dayside measurements provides : 9 the potential to make estimates of the size (ie area) which the auroral distribution covers and hence implicitly the area of the polar cap 9 the opportunity to calculate two-dimensional ionospheric conductivities 9 observations of the site of expected direct interaction with solar wind/Interplanetary Magnetic Field (IMF) interactions can be observed during all seasons 9 the ability to study the auroral distribution in relation to global magnetospheric phenomena
UV Auroral Imaging
53
Partly as a result of the Viking UV imager measurements it has become apparent that the dayside aurora is a dynamic, highly structured part of the auroral distribution. Of particular interest have been the observations of a series of vortices which appear to reflect processes associated with shear flows in the magnetosphere (Lui et al., 1989; Murphree et al., 1989). However, usage of UV has brought with it certain difficulties which have actually led to considerable studies in their own right. The most common approach (eg DE 1, Viking) has been to concentrate on broad band UV measurements. Popular choices are to cover the molecular nitrogen Lyman-Birge-Hopfield (LBH) emissions which extend from 130.4 nm to 200.0 nm as various vibrational and rotational transitions, and the strong line at 130.4 nm due to atomic oxygen. These choices reflect both relative ease in obtaining wavelength discrimination and known auroral output. However, some difficulties associated with these two wavelength intervals should be noted : 1. Although materials are readily available with which passbands can be constructed, the relative proximity of Lyman c~ potentially overwhelms auroral observations due to its high intensity in the geocorona. This turns out to at least partly be due to filter cutoffs shifting downward toward that wavelength with decreasing temperature. 2. While LBH emissions are primarily optically thin, those at 130.4 nm are not with the result that considerable radiative transfer effects occur, principally broadening of narrow features. 3. UV broad band measurements do not directly provide integrated, precipitated energy measurements such as for example, the N2 + visible emissions. 4. LBH emission response to particle precipitation depends on energy (as does 130.4 nm but less strongly). Thus the ratio (1304/LBH) could be indicative of characteristic energy with higher ratios indicating softer fluxes (Rees et al.,1988). However, the dependence of this ratio seems highly variable (Steele et al., 1992), although characteristic energies, which generally agree with in situ~measurements, have been inferred from the ratio of an LBH band centered on about 140 nm and an LBH band centered on about 170 nm (Germany et al., 1997). 5. Specification of intensities (for line emissions given in Rayleighs) becomes suspect when the distribution within a broad wavelength band is unknown. Efforts to relate UV observations to equivalent visible intensities (eg Vallance Jones, 1987) have only been partially successful. As a result of these difficulties UV satellite images have been used almost entirely within a morphological context. That is, they are used to delineate spatial regions of emission and monitor the dynamics of these regions, irrespective of sunlight conditions. THE SUBSTORM PROBLEM Extensive advances in our understanding of the substorm have been made using satellite images of the auroral distribution. Some of these have been recently formulated in terms of a new substorm paradigm, wherein basic morphologies are presumed to represent specific classes of magnetospheric/ionospheric processes (Elphinstone et al., 1996). In fact much of the effort in substorm work using images has been of such a qualitative nature, because the significance of the context has been so important. Questions such as "Where in relation to existing ionospheric features (and hence to magnetospheric source regions) do onsets occur?" and "What role does the dayside play in the overall substorm process?" require fundamentally morphological data. Both of these essentially take the basic auroral distribution observations and ask where in the magnetosphere they come from. Important results have been obtained from such studies including : 9 Based on extensive observations of substorm onsets the statistical location has been established to have a median location of 66.7 degrees (Corrected Geomagnetic Latitude 1985.0) and 22.8 hours Magnetic
54
J. S. Murphree Local Time (MLT)(Henderson, 1993). Similar results have been provided by Craven and Frank (1991) and comparison of the results of these studies reveals likely dependencies on solar activity levels. The onset position has a broad range of probability in both latitude and MLT however. Mapping (using the Tsyganenko 1987.0 model) of specific substorm onset locations yields surprisingly close locations to the nightside of the Earth. Murphree et al. (1993a) showed that for 14 onsets the X location was -7.SRE (in the Y dimension they were shifted approximately 1RE toward dusk from midnight). Caution should of course be used in such static mapping during periods when the magnetosphere is changing (ie the growth phase). Attempts to account for such dynamics (Pulkkinen et al., 1995) however, suggest the same result. Further, detailed multipoint observations confirm these suggestions (Lopez et al., 1990). Observations have shown that optical substorm expansions can occur well equatorward of the poleward extent of emissions, both during quiet and a~tive periods (Murphree et al., 1991; Murphree et al., 1993a). There is no reason to suspect that this poleward region of emissions is not on closed field lines and that the onset location is therefore unrelated to the open/closed field line boundary, a result consistent with some (but not all) near-Earth mechanisms. The auroral distribution at MLT other than midnight can undergo significant changes prior to the expansion phase (Elphinstone et al., 1991). In particular intensifications originating in the dayside (both afternoon and morning) propagate nightward suggestive of wave activity. Normally when substorm activity occurs the region poleward of the auroral distribution becomes dark (ie absent of emission). However, satellite imager views have shown (eg Murphree et al., 199ab) that previously existing polar arcs can maintain their form (and sometimes intensity) during substorms. Further, in the dawn sector the region well poleward of the auroral distribution can commonly intensify during the latter stages of a substorm.
In most of these cases information available from the dynamics has not been utilized. However, such information has an important role to play in quantifying substorm evolution just as the morphology led to revisiting where in the magnetotail onset occurs. The manner in which a typical substorm bulge expands is shown in Figure 2. Here the onset was located at 23.2 MLT and Eccentric Dipole (1990.0) magnetic latitude 2600 Onset Date: April 1,1986 Onset Tune: 1850:00 UT - 1855:37 UT 0 1858:18 UT x 1901:01 UT 9 191 :
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Figure 2" Expansion of the substorm bulge poleward boundary 64.4 degrees. The figure shows data extracted from several of the one minute spaced sequence of images observed by the Viking UV imager. The poleward edge of the substorm bulge was extracted at all MLT and the distance in km from the onset location determined. For reference purposes the onset was assumed to occur at a point although this could not be confirmed. The times chosen were arbitrary except in the case of the final curve (1916:23 UT) which represents the maximum latitude to which the bulge expanded
UV Auroral Imaging
55
along the onset MLT. The latitude to which a substorm bulge extends in the poleward direction most likely represents the physical limit to which the substorm process can extend in the magnetotail. Normally, the poleward limit appears in satellite imagery to occur with respect to no identifiable morphology because in general the region poleward of the auroral distribution is essentially void of emission. However, at times prior activity (either quiet or active) leaves markers of emission which can be used as a reference. It is found in these cases (Murphree et al., 1993a) that the bulge expands to the poleward limit of these morphologies. These observations have been interpreted as showing that the bulge expands to the limit of closed field lines (at least in these cases), raising the question whether this is always so. In the case of observations presented here the maximum extent was determined to be when propagation in the poleward direction ceased for at least several minutes. In this particular example, the expansion distance turns out to be a minimum along the onset MLT (approximately 900 kin). Previous work on such motions has been reported by Craven and Frank (1987), Craven and Frank (1991) and Henderson (1993) with maxima consistent with this example. Using data from a selected set of substorms observed by the Viking UV imager, an average, maximum displacement along the onset MLT has been determined to be 700 km. The maximum distance observed (Viking observations took place during an interval of generally low period of solar activity) was just over 900 km. An interesting question is what this ionospheric projection corresponds to in the magnetotail. An immediately obvious feature of this series of observations is the larger displacement from the onset position in the longitudinal as opposed to the latitudinal direction. The propagation of auroral forms during a substorm expansion often has focussed on the westward motion, but this data show that while that occurs, the eastward expansion is larger. Compared to the poleward motion the westward and eastward expansions are 1.4 and 2.6 times as large respectively. This implies that the substorm process in the magnetotail is less constrained in the azimuthal direction and particularly effective in the postmidnight direction. In terms of projecting this extent into the ionosphere, the area covered by the substorm bulge has been found to be an average of 2 x 106 km 2 at ionospheric altitudes. The mapping of this projection into the three dimensional magnetosphere would establish some important topological constraints. Characteristics of bulge expansion rates have been reported by Craven and Frank (1987) for a few examples. In that study emphasis was placed on quite extended substorm examples, yielding expansion rates of 230 m/s in two cases and one reaching 1000 m/s. Figure 3 shows (for the same substorm as shown in Figure 2) the speed at which the poleward boundary has moved. This data shows that the rate of expansion along the onset MLT was initially above 1 km/s and decreased to an overall average of approximately .5 km/s. Further the rate exhibits some variability which is illustrated in Figure 4 for points along the onset MLT. There clearly is an initial period where the speed rapidly increases to a maximum (some two minutes after onset) and then a slower decrease in the expansion rate over the next 3 minutes or so till the expansion rate Onset Date: April I, 1986 Onset Tune: 1850:00 UT n 1855:37 UT O 1858:18 U T 1901:01 UT ~ 1916:23 UT \
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J. S. Murphree
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reaches a fairly constant value. Note that these rates are all determined by referencing the instantaneous position of the poleward boundary to the onset position so they represent the cumulative expansion rate for the specific time. Thus each speed calculation provides the average over the total time interval to that point as if only that one position measurement had been made. The average rate for this dataset is .6 km/s along the onset MLT to reach the poleward maximum. Referring again to Figure 3 the expansion rate in the longitudinal directions are consistently higher than in the poleward. Again the cumulative rate decreases as the expansion reaches its poleward limit. Further, motion eastward has a higher speed than westward, a significant result given the dominance of interest in the westward propagation of substorm auroral features. The effective rate of eastward propagation still exceeds 1 km/s at the time when the poleward limit along the onset MLT has been reached. An important issue which has not yet been dealt with adequately is the amount of time it takes for the bulge to reach its maximum latitude. During special cases when previously existing auroral forms turned out to be the demarcation line for expansion, Murphree et al. (1993a) found for one case 13 minutes was the time it took to evolve to this poleward limit. This number is quite low compared to the classical view of substorm expansion phases. For example, Frank and Craven (1988) point out expansions can last for more than an hour after onset. Using the definition of the poleward limit adopted here however, that length of time probably does not represent a reasonable average of expansion periods. In the dataset used in this paper the average time is approximately 20 minutes. There are several points which should be noted regarding this period : 1. it potentially represents the time it takes to propagate substorm expansion effects to the open/closed field line boundary 2. discriminating between periods when prior activity was quiet or disturbed may be important. In the cases where prior activity occurred the time period appears to be generally shorter. This is certainly the case when a high latitude arc system (remnants of a double oval morphology) remains, with the few such examples yielding times right around 13 minutes. These cases therefore tend to lower the average quoted above. 3. there may well be 'episodic' expansions (Craven and Frank, 1987) on intervals of 10-15 minutes which merge together in some cases to form overall expansion phases reaching an hour in length. It is therefore of interest to understand what might happen when an expansion bulge goes through such episodic activity. In Figure 1 three examples acquired by the Viking UV imager of substorm bulge activity have been shown. In each example the substorm process is well underway and the images are ordered top to bottom for each case.
57
UV Auroral Imaging DEVELOPMENT OF POLEWARD MOTION
PI
o PI
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20
30
Figure 5: Schematic of poleward evolution of bulge
The first case shows that at 1838:41 UT a well developed bulge exists in the midnight sector. The activity at its western edge (a westward traveling surge?) is most intense in the nightside substorm region. The next image shows that the poleward edge of the bulge has intensified (denoted by "Poleward Intensification" (PI)). This intensification occupies a little more than half of the MLT extent of the poleward edge of the bulge. In this example the surge has significantly intensified as well. The second example (861005) shows no evidence of a large scale surge. Here the poleward intensification manifests itself as a series of distortions along the poleward boundary and they cover essentially the entire MLT range of the bulge. It is significant that the intensification has occurred within the 1 minute time interval between the images. This rapid intensification is also apparent in the third example (861015). Although not apparent in the figure, the intensifications represent not just an increase in emission, but as well form part of the poleward expansion. Figure 5 illustrates in schematic form the process which appears to occur in these cases. After onset the bulge propagates poleward with speeds in the range of .5 km/s reaching some latitude where there is a sudden jump in position (at 10 minutes in this figure). Such Multiple 'jumps' can occur depending on the sustainability of the substorm process in the magnetotail. Are these intensifications new substorms? Certainly their optical character would not suggest so. Nor are they the optical signature of a refilling of the plasmasheet after neutral line retreat (Hones et al. 1987; Frank and Craven 1988). The particle characteristics of the arc at the poleward edge of the bulge suggest rather plasmasheet boundary layer activity. However, the episodic nature is of yet not understood. SUMMARY Satellite UV imaging provides an important context within which in situ measurements can be placed. For dynamic studies however, they further provide the most easily understood set of measurements of magnetospheric processes projected onto the ionosphere. Datasets now exist which can effectively exploit this capability and provide new constraints on, for example, theories or models of substorm activity. Results shown here provide several new insights which should be considered when looking at understanding the global auroral substorm : 9 the differential expansion in azimuthal and radial directions implies an asymmetric region of substorm instability. Overall projected areas on the order of 2 • 106 km 2 need to be accounted for. 9 expansion rates vary within a substorm and as well may depend on the prior state of the magnetosphere. Typical rates along the onset MLT are .6 km/s. 9 the basic expansion unit may repeat several times (ie is episodic) being accompanied by intensifications at the start of each. Periods somewhere between 10 and 20 minutes seem to be in the range. Coupled with the various modules suggested for substorm characterization (Elphinstone et al. 1996) these
58
J.S. Murphree
more detailed observations should help solve the continuing problem of what causes an auroral substorm. ACKNOWLEDGEMENTS G. Enno and M. Johnson provided invaluable help in analyzing certain data. This work was supported under a grant by the Natural Science and Engineering Research Council of Canada. REFERENCES Anger, C. D., S. K. Babey, A. L. Broadfoot, R. G. Brown, L. L. Cogger, R. Gattinger, J. W. Haslett, R. A. King, D. J. McEwen, J. S. Murphree, E. H. Richardson, B. R. Sandel, K. Smith and A. Vallance Jones, An Ultraviolet Auroral Imager for the Viking Spacecraft, Geophys. Res. Lett., 14, 387-390, 1987. Anger, C. D., T. Fancott, J. McNally, and H. S. Kerr, ISIS-II Scanning Auroral Photometer, Appl. Opt., 12, 1753-1766, 1973. Cogger, L. L., J. S. Murphree, S. Ismail and C. D. Anger, Characteristics of Dayside 5577 ti and 3914 .;i Aurora, Geophys. Res. Left., 4, 413-416, 1977. Cogger, L. L., D. J. Hearn, J. S. Murphree, R. A. King, E. P. King, Yu. I. Galperin, B. Gordon and J. Matsushita, Ultraviolet Auroral Imager (UVAI), in Interball Mission and Payload, edited by Yu. Galperin, T. Muliarchik and J.-P. Thouvenin, Russian Space Agency, Space Research Institute, and French Space Agency, 382-400, 1995 Craven, J. D. and L. A. Frank, Latitudinal Motions of the Aurora During Substorms, J. Geophys. Res., 92, 4565-4573, 1987 Craven, J. D. and L. A. Frank, Diagnosis of Auroral Dynamics Using Global Auroral Imaging with Emphasis on Large-Scale Evolutions, in Auroral Physics, edited by C.-I. Meng, M. J. Rycroft and L. A. Frank, Cambridge University Press, New York, 273-288, 1991 Dandekar, B. S. and C. P. Pike, The Midday Discrete Auroral Gap, J. Geophys. Res., 83, 4227, 1978. Elphinstone, R. D., J. S. Murphree, L.L. Cogger, D. Hearn, M. G. Henderson and R. Lundin, Observations of Changes to the Auroral Distribution Prior to Substorm Onset, in Magnetospheric Substorms, edited by J. R. Kan, T. A. Potemra, S. Kokubun and T. Iijima, Amer. Geophys. Un. Mono., 64, 257-275, 1991. Elphinstone, R. D., J. S. Murphree and L. L. Cogger, What is a Global Auroral Substorm?, Rev. Geophysics, 34, 169-232, 1996. Frank, L. A., J. D. Craven, K. L. Ackerson, M. R. English, R. H. Eather, and R. L. Carovillano, Global Auroral Imaging Instrumentation for the Dynamics Explorer Mission, Space Sci. Inst., 5,369-393, 1981. Frank, L. A. and J. D. Craven, Imaging Results from Dynamics Explorer 1, Rev. of Geophys., 26, 249-283, 1988. Frank, L. A., J. B. Sigwarth, J. D. Craven, J. P. Cravens, J. S. Dolan, M. R. Dvorsky, P. K. Hardebeck, J. D. Harvey and D. W. Muller, The Visible Imaging System (VIS) for the Polar Spacecraft, Space Sci. Rev., 71,297-328, 1995. Germany, G. A., G. K. Parks, M. Brittnacher, J. Cumnock, D. Lummerzheim, J. F. Spann, L. Chen, P. G. Richards and F. J. Rich, Remote Determination of Auroral Energy Characteristics During Substorm Activity, Geophys. Res. Lett., 24, 995-998, 1997. Henderson, M. G., Implications of Viking Imager Results, PhD. Thesis, Department of Physics and Astronomy, University of Calgary, 1993 Hones, E. W., Jr., C. D. Anger, J. Birn, J. S. Murphree and L. L. Cogger, A Study of a Magnetospheric Substorm Recorded by the Viking Auroral Imager, Geophys. Res. Lett., 14, 411, 1987. Lopez, R. E., H. Luhr, B. J. Anderson, P. T. Newell and R. W. McEntire, Multipoint Observations of a
UV Auroral Imaging
59
Small Substorm, J. Geophys. Res., 95, 18,897, 1990. Lui, A. T. Y., D. Venkatesan and J. S. Murphree, Auroral Bright Spots on the Dayside Oval, J. Geophys. Res., 94, 5515, 1989. Meng, C.-I., and R. E. Huffman, Ultraviolet Imaging from Space of the Aurora under Full Sunlight, Geophys. Res. Lett., 11,315-318, 1984. Murphree, J. S., L. L. Cogger and R. D. Elphinstone, Observations of Distortions of Optical Features in the UV Auroral Distribution, IEEE Trans. on Plasma Sei., 17, 109, 1989. Murphree, J. S., R. D. Elphinstone, L. L. Cogger and D. Hearn, Viking Optical Substorm Signatures, in Magnetospherie Substorms, edited by J. R. Kan, T. A. Potemra, S. Kokubun and T. fijima, Amer. Geophys. Un. Mono., 64, 241-255, 1991. Murphree, J. S., R. D. Elphinstone, M. G. Henderson, L. L. Cogger and D. J. Hearn, Interpretation of Optical Substorm Onset Observations, J. Atmos. Tevr. Phys., 55, 1159, 1993a. Murphree, J. S., J. B. Austin, D. J. Hearn, L. L. Cogger, R. D. Elphinstone and J. Woch, Satellite Observations of Polar Arcs, J. Atmos. Terr. Phys., 56, 265-284, 1993b. Murphree, J. S., R. A. King, T. Payne, K. Smith, D. Reid, J. Adema, B. Gordon and R. Wlochowicz, The Freja Ultraviolet Imager, Space Science Revs., 70, 421-446, 1994. Oguti, T., E. Kaneda, M. Ejiri, S. Sasaki, A. Kadokura, T. Yamamoto, K. Hayashi, R. Fujii and K. Makita, Studies of Auroral Dynamics by Aurora-TV on the Akebono (EXOS-D) Satellite, J. Geomag. Geoeleetr., 42, 555, 1990. Pulkkinen, T. I., D. N. Baker, R. J. Pellinen, J. S. Murphree and L. A. Frank, Mapping of the Auroral Oval and Individual Arcs During Substorms, J. Geophys. Res., 100, 21,987-21,994, 1995 Rees, M. H. and D. Luckey, Auroral Electron Energy Derived from Ratios of Spectroscopic Emissions, J. Geophys. Res., 79, 5181-5186, 1974. Rees, M.H., D. Lummerzheim, R. G. Roble, J. D. Winningham, J. D. Craven and L. A. Frank, Auroral Energy Deposition Rate, Characteristic Electron Energy, and Ionospheric Parameters Derived from Dynamics Explorer I Images, J. Geophys. Res., 93, 12,841, 1988. Rogers, E. tI., D. F. Nelson, and R. C. Savage, Auroral Photography from a Satellite, Science, 183,951-952, 1974. Schenkel, F. W. and B. S. Ogorzalek, Auroral Images from Space : Imagery, Spectroscopy, and Photometry, Johns Hopkins APL Digest, 8, 308-317, 1987 Shepherd, G. G., 2". Fancott, J. McNally and H. S. Kerr, ISIS-II Atomic Oxygen Red Line Photometer, App. Optics, 12, 1767, 1973. Steele, D. P., D. J. McEwen and J. S. Murphree, On the Possibility of Auroral Remote Sensing with the Viking Ultraviolet Imager, J. Geophys. Res., 97, 2845, 1992. Torr, M. R., D. G. Torr, M. Zukic, R. B. Johnson, J. Ajello, P. Banks, K. Clark, K. Cole, C. Keffer, G. Parks, B. Tsurutani and J. Spann, A Far Ultraviolet Imager for the International Solar Terrestrial Physics Mission, Space Sei. Rev., 71,329-383, 1995. Vallance Jones, A., R. L. Gattinger, F. C. Creutzberg, P. Prikryl, R. A. King, L. L. Cogger, D. J. McEwen, F. R. Harris, C. D. Anger, J. S. Murphree and R. A. Koehler, A Comparison of CANOPUS Ground Optical Data with Images from the Viking UV Camera, Geophys. Res. Lett., 14, 391-394, 1987.
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HIGH ALTITUDE ELECTROSTATIC SUBAURORAL ION DRIFTS
FIELDS DRIVING
J. F. Lemaire, M. Roth and J. De Keyser
Institut d'Agronomie Spatiale de Belgique, Avenue Circulaire 3, B-1180 Brussels, Belgium
ABSTRACT A subauroral ion drift (SAID) layer is characterised by a narrow peak of westward ion drift speed exceeding 1000 m/s. It is confined in less than 1 degree in latitude and located equatorward of the nightside auroral zone but poleward of the plasmapause. We propose a mechanism for the narrow peak electric field driving this phenomenon: an electrostatic potential is produced by thermo-electric charge separation across the front edge of a hot plasma cloud moving inward from the tail and penetrating into the colder background plasma in the plasmatrough and at the plasmapause. Quantitative calculations corroborate this scenario. INTRODUCTION Discovered by Galperin et al. (1973), subauroral ion drifts (SAIDs) have been observed by many satellites in electric field measurements in the ionosphere (Smiddy et aI., 1977: Maynard et al., 1978; Burch et aI.; 1976: Rich et al., 1980) and in the magnetosphere (Maynard et al., 1980), as well as by ion drift meters in the ionosphere (Spiro et al., 1979; Anderson et al., 1991). SAID events usually are observed more than half an hour after the onset of a magnetospheric substorm and have lifetimes less than 3 hours (Anderson et al.. 1991). Figure 1, taken from Anderson et aI. (1993), illustrates the distribution of ion temperature, ion concentration and horizontal and vertical ion drifts across a SAID at 60.2 ~ invariant latitude (ILAT), 20.7 h magnetic local time (MLT), and 385 km altitude. The top panel shows the spectrum of precipitating energetic electrons observed along the orbit of DE-2 across the auroral region. SAIDs are characterised by a band of westward ion drift exceeding 1000 m/s in a narrow latitudinal range, located equatorward of the nightside auroral zone and poleward of the plasmapause (Anderson et al., 1991). The half width of a SAID can occasionally be as narrow as 0.1 degree in latitude (10 km) but is usually wider (100-300 km). The ion temperature across SAIDs peaks at, 300-400 km altitude, coinciding with the ion drift speed peak. The ion concentration is depleted by a factor up to 10 below the ambient subauroral concentration. The detailed structure of SAIDs may vary considerably from event to event, but the drift velocity spike is nearly always westward. On the equatorward side the drift speed decreases with decreasing latitude often over more than 10 ~ Close to the spike the convection can deviate several hundred m/s from the corotation velocity, which is equal to 230 m/s at 60 ~ latitude in a magnetospheric frame of reference. QUALITATIVE MODEL Coordinated DE-1 & 2 observations led Anderson et al. (1993) to propose a qualitative scenario for the formation of SAIDs. "At substorm onset, precipitating ions and electrons of all energies occupy a common equatorward boundary." This common boundary may possibly be identified with McIlwain's substorm injection boundary (McIlwain, 1974). "After about 10 minutes, the equatorward edge of the region of precipitating electrons separates from that of the plasmasheet ions in the pre-midnight local time sector. The equatorward extent of the downward directed, region 2 field-aligned currents is coincident with the eqlla~orward extent of the region of ion precipitation. These currents close via Pedersen currents with the 61
62
J. F. Lemaire et al.
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Fig. 2: An EMF is generated at the surface of a plasma cloud drifting inward from the magnetotail. The equatorial electric field is projected in the terrestrial ionosphere along magnetic field lines and drives the SAID.
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Fig. 1: DE 2 observations of a SAID event at low altitude from Anderson et al. (1993). The top panel shows the spectrum of precipitating, energetic electrons, while the zonal component of the horizontal ion drift is plotted in the second panel. The SAID is seen a~ the equatorward edge of the electron precipitation region., just prior to 0058 UT. The third, fourth and fith panels show ion temperature, vertical ion drift, and total ion concentration,
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"% % % % % % % % %%%%~Z, Fig. 3: Theoretical profiles of: (a) Electric potential; (b) Electric field in the equatorial plane (Eeq) and at an ionospheric altitude of 400 km (El); (c) Subauroral ion drift ~elocitv,, (vi). The upper and bottom scales give the invariant latitude (ILAT) and the L values, respectively. The model assumes that the cloud is moving with a speed of 20 km/s with respect to the background plasmatrough, in the westward direction.
outward flowing region 1 currents at higher latitudes. The Pedersen currents flow in the region of low conductivity equatorward of the electron precipitation, and a broad region of relatively large, poleward directed electric fields is produced. These electric fields in turn produce relatively large westward ion drifts. The frictional heating of the ions caused by collisions with the corotating neutral atmosphere substantially increases the rate of reaction between O + and N2 (O + + N2 ~ NO'- + N). Subsequent fast recombination of NO + with electrons (NO + + e -~ N + O) further reduces the already low subauroral Pedersen conductivity thereby decreasing the current drain on the thinning of the Alfv6n layer. Subsequent frictional heating leads to thermal expansion, field-aligned plasma flow. and very large depletions in the F-peak concentration. additionally reducing the height-integrated Pedersen conductivity. The E-region conductivity is also lowered in the region of large electric field through poleward ion transport. Very large electric fields are eventually produced between the converging equatorial boundaries of the electron and ion precipitation, resulting in the latitudinally narrow spike signature of a SAID event. As the substorm recovers, the field-aligned
High Altitude Electrostatic Fields Driving Subauroral Ion Drifts
63
currents diminish, but the SAID is maintained by the loss of ionospheric conductivity until the Alfv6n layer disappears." The origin of the EMF producing the magnetospheric electric field and the field-aligned currents is not specified by Anderson et al. (1993); the purpose of this paper is to provide a model for the high altitude voltage generator driving the SAID. Figure 2 illustrates how a hot plasma cloud or plasmoid drifts inward from the magnetotail into the low density plasmatrough. The plasma injected in the plasmatrough consists of energetic electrons (2-30 keV) and 30-3000 keV protons. Due to the different thermal velocities and masses of electrons and ions their Larmor radii are different. Therefore. the ions at the surface of the plasmoid tend to penetrate further out into the cloud's environment than the electrons. This builds up a space charge layer whose thickness (AXeq) is a few ion gyroradii (p~) in the direction perpendicular to the magnetic field like in the transitions described, e.g., by Lemaire and Burlaga (1976), Roth (1978) and Roth et al. (1993). The gyroradius of 50 keV protons is of the order of 350 km at 6-7 RE in the equatorial plane where the magnetic field Beq is about 100 nT. The actual thickness (AZeq) of the layer is 5-10 proton gyroradii: AXeQ ~ 1500-3000 km. The corresponding width (Axi) at ionospheric altitude is 2 L 3 / 2 ( 1 - 1/L) 1/2 times smaller, i.e., Axi ~ 50-120 km for L = 6. This compares well with the characteristic width of SAIDs. The electric potential difference L.~Yeq across this layer is determined by the temperature difference of the plasmas on both sides of the interface, i.e., 1-10 kV. The quantitative model calculation presented in the next section indicates that the peak electric field is of the order of or smaller than AVeq/AXeq: Emax ~ 2-20 m V / m . Mapped down into the ionosphere along the field line L = 6. this corresponds to 50-500 m V / m and an electric drift velocity of 1000-10000 m / s at the altitude where SAIDs are observed. The half life time of the transition can be shown to be of the order of 104 s or more (following the reasoning by R oth et al., 1993). This is the time interval during which dissipative processes will not significantly alter the potential difference across the initially unloaded EMF. We therefore can model the source as approximately unloaded. In view of this. the interface can be regarded as a tangential discontinuity (TD): no energy conversion takes place in a TD as the currents are perpendicular to the electric field in such an interface. As illustrated in Figure 2 we can consider the SAID to be located between the magnetic field lines 1 and l ~, which are tangent to the interface layer at P and P~ in the TD approximation. We will now determine the electric potential between P and P~ by means of a simplified version of the TD model developed by Roth (for a comprehensive review of this model with various applications, see (Roth et al., 1996)). "r
QUANTITATIVE D E S C R I P T I O N We illustrate our model quantitatively with an example. The density in the cloud is taken to be .Nr+ cl = 0.25 cm -3, less than in the plasmatrough background (N~:~ = 2 cm-3). Plasmatrough ion and electron temperatures are typically TQ = 5 keV and Tb~ = 0.5 keV; a reasonable choice for the plasma cloud temperatures is Tc~- = 50 keV and Tc~ = 2.3 keV. Taking Bbg to be 60 nT (typical at an equatorial nightside distance of 7-8 Earth radii) pressure balance requires Bd = 86.62 nT. We consider two sources for the electric field across the plasma cloud's front surface. First. there is the thermo-electric electric field due to the different plasma temperatures. Second. since the plasma cloud is traveling sunward at an average velocity of the order of tens of km/s, a typical azimuthal velocity in the premidnight sector (where most SAID are observed) is U~.cl = 20 km/s. while the background is essentially at rest. The kinetic model (Roth et aI., 1996) uses a parameterized description of the electron and ion VDFs; the parameters are the so-called transition lengths, which are the length scales over which the anisotropy of the VDFs becomes important. In this example, the electrons are taken to have maxwellian distributions centered around their bulk velocity. The ion VDFs are anisotropic with transition lengths l~ and It;%. Selecting a reference length (in casu the 50 keV cloud proton gyroradius t5 = 373 km in the Bd field) the normalized transition length is defined as l* = lp/p. where p is the ~-roradius of the considered species in the magnetic field of its source region. ~ took l cl = 10 and lbg' = 5. Figures 3a and 3b show the electric potential (in kT) and electric field intensity (in V/m) profiles. The plasma cloud is located on the iefthand side while the plasmatrough is on the righthand side. The electric field peaks at .Eeq -" 3.5 IllV/1TI i dashed line in 3b). directed away from Earth. This electric' field maps along
64
J.F. Lemaire et al.
geomagnetic field lines into the ionosphere. The upper scale in the figure gives the invariant latitude, taking the magnetic field lines to be dipolar and Xeq ---- 0 at L = 8, i.e., at an invariant latitude A = 68.6 ~ If the field-aligned electric potential difference between the ionosphere and the equatorial plane does not change significantly over this latitude range, Figure 3a also gives the ionospheric electric potential. The intensity Ei of the ionospheric electric field (left vertical scale) versus L (bottom scale) is given by the solid line in Figure 3b. Figure 3c shows the subauroral ion drift velocity vi = Ei x Bi/B~ where Bi is the magnetic field at 400 km altitude. It is directed westward. The peak velocities determined from the model and those observed in the subauroral ionosphere are quite comparable. Their latitudinal widths are also comparable. CONCLUSIONS In this paper we propose a quantitative model for the EMF driving SAIDs. We determine the electric field which exists at the boundary of a hot plasmasheet cloud drifting earthward into the plasmatrough environment. When mapped down to the ionosphere, the electric field has profile and intensity observed in SAIDs. This result nicely complements the scenario outlined by Anderson et al. (1993). REFERENCES Anderson, P.C., W.B. Hanson, and R.A. Heelis, The ionospheric signatures of rapid subauroral ion drifts, J. Geophys. Res., 96, 5785 (1991). Anderson, P.C., W.B. Hanson, R.A. Heelis, J.D. Craven, D.N. Baker and L.A. Frank, A proposed production model of rapid subauroral ion drift and their relationship to substorm evolution, J. Geophys. Res., 98. 6069-6078 (1993). Burch, J.L., S.A. Fields, and R.A. Heelis, Substorm effects observed in the auroral plasma, in Physics of Solar Planetary Environments, edited by D.J. Williams, pp. 740-759, AGU, Washington D.C. (1976). Galperin, U.I.. Y.N. Ponomarov, and A.G. Zosinova, Direct measurements of ion drift velocity in the upper atmosphere during a magnetic storm, Kosm. Issled., 11,273 (1973). Lemaire, J. and L.F. Burlaga, Diamagnetic boundary layers: A kinetic theory. Astrophys. Space Sci., 45, 303 (1976). Maynard, N.C., On large poleward-directed elecric fields at sub-auroral latitudes, Geophys. Res. Lett., 5, 617 (1978). Maynard, N.C., T.L. Aggson, and J.P. Heppner, Magnetospheric observation of large sub-auroral electric fields, Geophys. Res. Lett., 7, 881 (1980). McIlwain, C.E., Substorm injection boundaries, in Magnetospheric Physics, edited by B.M. McCormac, pp. 143-154, D. Reidel. Norwell, Mass. (1974). Rich, F.J.. W.J. Burke, M.C. Kelley and M. Smiddy, Observations of field-aligned currents in association with strong convection electric fields at subauroral latitudes, J. Geophys. Res.. 85, 2335 (1980). Roth. M., Structure of tangential discontinuities at the magnetopause: the nose of the magnetopause, J. Atrnos. Terr. Physics, 40. 323 (1978). Roth, M., D.S. Evans, and Lemaire, J., Theoretical structure of a magnetospheric plasma boundary: application to the formation of discrete auroral arcs. J. Geophys. Res., 98, 11,411 (1993). Roth. M.. J. De Keyser and M.M. Kuznetsova. Vlasov theory of the equilibrium structure of tangential discontinuities in space plasmas. Space Sci. Rev., 76, 251-317 (1996). Smiddy. M.. M.C. Kelley. W. Burke, F. Rich, R. Sagalyn, B. Shuman, R. Hays. and S. Lai, Intense poleward directed electric fields near the ionospheric projection of the plasmapause, Geophys. Res. Lett., 4, 543 (1977). Spiro. R.W., R.H. Heelis. and W.B. Hanson, Rapid sub-auroral ion drifts observed by Atmospheric Explorer C.. Geophys. Res. Lett., 6. 657 (1979).
SEARCH FOR LUNAR
PICKUP
IONS
E. Kirsch 1, B. Wilken 1, G. Gloeckler 2, A.B. Galvin 2, J. Geiss a, D. Hovestadt 4
1Max_Planck_institut fb,'r Aeronomie, D-37189 Katlenbu, rg-Lindau, Germany 2 University of Maryland, Department of Physics, College Park, MD 20742, USA 3International Space Science Institute, CH-3012 Bern, Switzerland 4Max.Planck_institut fiir Extraterrestrische Physik, D-85748 Garching, Germany
ABSTRACT During lunar flybys in December 1994 and January 1996 pickup ions, generated by solar photons, solar wind and micrometeoroid impacts, were detected by the SMS experiment on the W I N D - S / C in the energy range E = 6 . 5 226 KeV/e. Near the moon the ions have not yet reached their flfll pickup energy, but they can be identified by the directional properties of the additional flux increases in sectors 4-8, the high mass and the charge state (singly charged ions) in contrast to solar wind ions which are multiply charged. Lunar pickup ions ill the mass range 12-42 amu/e could be identified but no singly charged iron ions were found. The pickup ions give a qualitative picture of the lunar surface composition. The results obtained earlier by Hilchenbach et al. (1991) near the Earth with a similar experiment on board the AMPTE satellite could generally be confirmed. INTRODUCTION The WIND-S/C, launched on November 1, 1994, performed first elliptical orbits around the Earth and was then deflected by the gravitational field of the moon so that the libration point. L1 (,~ 235 BE) between Earth and Sun could be reached (see Ogilvie a,nd Parks, 1996). The WIND mission together with GEOTAIL and POLAR, belongs to the Global Geospace Science Program of NASA. WIND measures the solar wind plasIna, solar particles, magnetic fields and plasina waves before they reach the Earth magnetosphere. Here particle lneasurelnents of the SMS experiment package (S_WlCS - Solar wind ion composition sensor, MASS - High resolution nlass spectrolneter, S_TICS - Suprathermal ion composition sensor) are presented which were especially obtained during lunar flybys of the WIND-S/C. The purpose of this study is to identify lunar lnatter near the lnoon which is released from the lunar surface by various erosion processes. The gas atoms become ionised by the solar UV light and then picked up by the solar wind. Such pickup ions of lunar origin were already detected by the AMPTE satellite outside of the Earth magnetosphere (Hilchenbach et al., 1991). Pickup ions are singly charged and can therefore be distinguished from the multiply charged solar wind ions. The detection of pickup ions can principally be used to study the surface and/or atmospheric composition of smaller celestial bodies such as the Earth lnoon, planet Mercury, Jupiter l n o o n Io, comets and asteroids, as well as planet Pluto and its moon Charon. However, the efficiency of the erosion processes, the ionisation potentials of the elelnents and the varying solar wind velocities must be considered in each case. ORBIT AND EXPERIMENT DESCRIPTION Between December 1994 and March 1996 WIND performed a total of 7 lunar flybys, the dates of which are listed in Table 1. Only the flybys 1) and 6) will be discussed in this study because they were characterised by a high solar wind velocity. Selected orbits for the W I N D - S / C were pulished by Acuna et al. (1995) (their Fig. 3). Some relevant lunar parameters are compiled ill Table 2. The SWICS and STICS sensors on WIND (a detailed description is given by Gloeckler et al., 1995) apply an electrostatic deflection potential for the energy/charge separation, time-of-flight and energy measurements to identify the particles mass. The mentioned technique allows the determination of the mass and lnass/charge ratios for energies E > 30 keV/e. The count rate as function of m / e can be measured for E = 6.5 - 226 keV/e by the STICS sensor. The orientation of the three sensors and the sectorisation can be seen ill Figure 1. The SWICS and STICS sensors use 16 azimuthal sectors in the spill plane of the spacecraft. The fan shaped field-of-view is 40 by 450 for SWIGS and 4.50 by 1560 for the STICS sensor. 65
66
E. Kirsch et al.
Table 1.
Lunar flybys of WIND
Date
1) 2) 3) 4) 5) 6) 7)
Dec. Jul. Aug. Sep. Nov. Jan. Mar.
27, 1994 30, 1995 23, 1995 20, 1995 25, 1995 16, 1996 24, 1996
Table 2.
Location in degree
Solar wind speed
r r r r r r r
~ 550 km/s ~ 300 km/s ~ 350 km/s ~ 380 km/s ~ 300 km/s ~ 550 km/s ~450km/s
~ 293 ~ 34 ~ 325 ~ 306 ~ 53 ,,~ 301 ~ 61
Lunar Parameters
Radius Escape velocity Escape energies for
R V H C O OH H20 K
= = = = = = -=
1738 km 2.38 km/s 0.03 eV 0.36 eV 0.48 eV 0.51 eV 0.54 eV 1.20 eV
r - azimuthal angle of WIND during lunar flybys in solar ecliptic coordinates Pickup ions move perpendicular to the local magnetic field in space. For an average magnetic field, inclined ~ 450 to the S/C-Sun line, it is expected that pickup ions appear mainly ill the sectors 4-8. They reach a maximum energy of Emax = 1/2 M (2 Vs sin v~)2 where M = mass of the pickup ions, Vs = solar wind velocity and f) = angle between the solar wind and the magnetic field direction. The composition of the lunar surface material was directly studied with samples brought back froln the moon by the Apollo missions 11, 15, 16. Elphic et al. (1991) irradiated such samples with beams of argon, helium and hydrogen ions, silnulatillg tile solar wind interact.ion with the surface of the Moon. The most important compounds in the lunar soil are: SiO2, TiO2, A1203, FeO, MnO, C,a O, Na20, I(20, Cr2Oa and P205. The most important ions generated by irradiation with solar wind type particles are: O, Si, A1, Ca., Fe, Mg, Na, K (Morgan and Shemansky, 1991 ). The transport of lunar pickup ions toward the Earth was studied by Cladis et al. (1994). OBSERVATIONS Lunar pickup ions are shown here for tile flyby events 1) and 6) (see Table 1) which are characterised by high solar wind speeds. The SWICS instrument measured 500-550 km/s in both cases. The STICS sectored measurelnents (> 10 amu/e, Fig. 2) indicate an additional flux ill the sectors 4-8 (dashed lines) which should result fi'om lunar pickul) ions. Such ions are not detectable during later high speed strealns (full lines) when WIND had no fly by with the moon. The count rate as a function of ln/e (Figure 3) indicates enhanced heavy ion fluxes in the lnass range m/e 10-42 amu/e averaged over the days Dec. 25, 12:00 UT to Dec. 29, 24:00 UT, 1994. Peaks appear around m / e 12, 14, 16-18, 28-34, 40. Such flux enhancements cannot be seen at later times (not shown a.s figure). The second example (Figure 4a) from the flyby on Jan 16, 1996 shows also enhanced heavy ion fluxes which again should result from lunar pickup ions. The count rates shown as a function of m / e indicate an additional flux in the range m/e 16-18 anm/e which should result, froln O and OH ions. Also Na, Mg, A1, Si and some heavier ions such as K, Ca contribute to the lunar pickup ions. Singly charged iron ions were not detected by the STIC, S sensor. Ill Figure 4b are shown the triple coincidence count rates (E/e >_ 30 keV/e) of the lunar fly by Jan. 1,5-20, 1996 (above) and of a later high speed stream (below) without a lunar fly by. C, O, A1, Si and S-ions were detected during the moon fly by. Only few O ions could be detected during the high speed stream without moon fly by (below).
67
Search for Lunar Pickup Ions MASS
SWICS
STICS
y
Table 3. Ions
x
9 Noon
-
-
~
t
~
-
H20 Na A1 Si S K Ti Mn Fe
Midnight
Dusk
Rigidities of 100 keV Ions Mass m m n] m ill in in m nl
= = = = = = = = --
18 23 27 28 32 40 48 55 56
Gauss H H H H II H H H H
9
cln
R = 1.93 R = 2.18 R = 2.37 R = 2.41 R=2.58 R = 2.88 R=3.16 R = 3.38 It. = 3.41
105 105 105 105 105 105 10 .5 105 105
H = magnetic field strength ill Gauss R = gyroradius ill cm nl
--
nlass
ill
anlu
Fig. 1. Sectorisation (0-15) of the SWICS and STICS lneasurements and the field-of-view pointing directions of the three sensors MASS, SWICS, STICS. The S/C is spilming clockwise. DISCUSSION From December 1994 until March 1996 seven WIND lunar flybys took place. Pickup ion measurenlents were shown for two selected examples which were characterised by high solar wind velocities. Flux increases in the azinmthal sectors 4-8 were detected for in > 10 alnu/e particles which we illterprete as hmar pickup ions ill the mass range 10-42 alnu/e. However, singly charged Fe-pickup ions were not found. In Table 3 are compiled the rigidities for some typical lunar ions (E = 100 keV). For an interplanetary magnetic field lnagnitude of ,5 nT follows that the gyroradii are then of the order of 20-40 lunar radii for in = 18-56 a mu particles. Thus it ca.n be expected that a part of the pickup ions have energies well below 100 keV near the moon and do not exceed the detection lilnit of the STICS sensor. Generally three processes contribute to the release of gaseous material from the lunar surface: 1) photon sputtering, 2) solar wind sputtering and 3) impact of lnicrometeoroids. A review of model calculations was recently published by Smyth and Marconi (1995). It is worth noting that the results of the various authors diifer by few orders of magnitude. Ground based observations of the photon sputtering effect especially for the sodiuln content of the lunar atmosphere were made by Mendillo and Baumgardner (1995) during a lunar eclipse period during which the impact of the solar wind was shielded by the tail of the earth magnetosphere. The authors found spectroscopically that the sodium atmosphere extends up to ~ 10 luuar radii. As mentioned earlier Elphic et al. (1991) lnade laboratory measurements with lunar soil salnples which can be considered representative for the surface composition of the moon. Irradiation of that material with 5 keV Argon ions, 4 keV Heliuln ions and 1.5 keV protons showed that solar wind ions are able to release ions from the lunar surface which contribute then to the pickup ion flux. For example 1 keV protons generate about 10 -5 secondary particles. Thus a solar wind ion flux of typical 3 x l0 s ions/cm2s generates about 103 secondary ions/cm2s. The generation of pickup ions by photon sputtering, solar wind and inicrometeoroid ilnpacts was also studied by Morgan and Shemansky (1991) and Ip (1991). The authors Morgan and Shemansky applied an equation proposed by Zel'dovich and Raizier (1967) and found a maximum ejection velocity of 5.1 km/s. Thus at least part of the released atoms is able to leave the gravitational field of the Moon which has v= 2.38 km/s escape velovity. Ip (1991) calculated the extension of the lunar exosphere under the influence of the solar wind for various release velocities and concluded that meteoroid impact and solar wind sputtering are the major sources for the lunar exosphere. Froln the presented study it is generally concluded that during the lunar flybys in December 1994 and January 1996 pickup ions, generated by solar photons, solar wind ions and microlneteoroid impacts were detected by the SMS eperiment onboard the W I N D - S / C , although a contribution of singly charged lnagnetospheric ions calmot be excluded. ACKNOWLEDGMENTS We thank J. Cain, R. Lundgren, S. Lasely and Ed. Turns fi'oln the University of Maryland for their work during tile development of the SMS experiment and S. Chotoo for the analysis of tile STICS calibration data,. The Gerlnan contribution to the SMS-Experiment was funded by DARA-Contract FKZ 50 OC 89149.
58
E. Kirsch et al. WIND/STICS . . ~ I O ~ ,
Iu'''
....
T-r-r~q
ec 25,1200 don
30
-
7 ' -
Feb
>
m/q
Dec 29,
'
10
I ' ' '
omu/e I
'
'
r-~-w-,--q
i ' '
'~
1994
WlND/STICS 10000 ] i
.5, 1 9 9 5
I~ 2
.~
Q) 1
1
94/,De,c/,25-,94,,(Dec/29
tdoy 3597363) '
"~
. . . . . 2
o
I 4
,
,
,
I 6
,
,
I
, I , , , I , , , I , , ,
,
8
10
12
14
sun sect. 1000
100
16
Sector ,
~Io
2
i
i
i
i
i
i
i
i
i
i
i
i
i
i
i
. . . . ~:~:;_~:~. . . . . . . _:
i .....
i ,
i
,
,
i
i
,
,
i
i
i
i
h
I
2
3
4
5
6
7
8 9
mass/charge
IO-I-~
i
i
I 2
L
,
,I
L,
4
,
L~
6
ii
I 8
,
I,
l t, 10
, I L l ,
:;
I, 14
12
16 Sector
Fig. 2. Sector measurements (> 10 ainu/e)of the 2 WIND-moon fly bys (dashed lines) and of later high speed streams measured far away from the moon (full lines). The enhancements in sectors 4-8 (dashed lines) are interpreted as lunar pick-up ions. Such ions do not appear in the later measured high speed streams (full lines). Sectors 9-10 measure solar wind or singly charged lunar ions, whereas sectors 0-3 detect most likely magnetospheric ions (dashed l!nes).
I0
20
30
40 50 60
[ ainu/e]
Fig. 3. Tile additiol}al ion fluxes (Dec 25-29, 1994) measured by STICS (E/e = 6.5- 226 keV/e) in the mass range 10-42 amu/e should be caused by lunar pick-up ions and for < 10 amu/e by solar wind and interstellar He + ions. Only the sunward directed sectors 6-13 are used here (see Fig. 1).
, 96/Jan/15-96fJan/20
WIND/STICS
tdoy 015T020) ~ sun sect.
o 100.0
WIND/STIC, S 10000~
96~Jan/,15-,96,/Jan(/20
10.0
tdoy 015702j0) sun sect. ~ o
~ ,ooo! 0
1.o
100
1
10.0
2
WIND/STICS ~
L
3
,
t
4 5 6 7 8 910 mass/charge [am./~] 96/(Feb/10-96/Feb/15 t
~
~
I
i
~ J
20
30
40 50 60
tdoy 041704,6I ) sun sect
i 1
2
3
4
5
6
mass/charge
7
8 9 10
,
1416
,
i
20
30
40 50 60
[ amu/e]
Fig. 4a. The additional ion fluxes (Jan 15-20, 1996) measured in the mass range 10-42 amu/e (E/e = 6.5226 keV/e)are caused most likely by lunar pick-up ions (further details see Fig. 3).
1.0
0.1
1
2
3
4 5 6 7 8910 mass/charge [amu/e]
I
I
20
30
I
I
40 50 60
Fig. 4b. Triple coincidence count rate of the lunar fly by Jan 15-20, 1996 (above) and of a later high speed stream (below). C, O, A1, Si and S-ions of E/e = 30 - 226 keV/e were detectable during the moon fly by (above) but only few O-ions during the high speed stream (below) without moon fly by.
Search for Lunar Pickup Ions
69
REFERENCES
Acuna, M.H., K.W. Ogilvie, D.N. Baker, S.A. Curtis, D.H. Fairfield, and W.It. Mish, The Global Geospace Science Program and its Investigations, Space Sci. Rev., 71, 5 (1995). Cladis, J.B., W.E. Francis, and R.R. Vondrak, Transport toward Earth of Ions Sputtered from the Moon's Surface by tile Solar Wind, J. Geophys. Res., 99, 53 (1994). Elphic, R.C., H.O. Funsten, B.L. Barraclough, D.J. McComas, M.T. Paffett, D.T. Va.ninlan, and G. Heiken, Lunar Surface Composition and Solar Wind Induced Secondary Ion Mass Spect.rolnegry, Geophys. Res. Left., 18, 2165 (1991). Gloeckler, G., H. Balsiger, A. Biirgi, P. Bochsler, L.A. Fisk, A.B. Galvin, J. Geiss, F. Glieln, D.C. Halnilton, T.E. Holzer, D. Hovesgadt, F.M. Ipavich, E. Kitsch, R.A. Lundgren, K.W. Ogilvie, R.B. Sheldon, and B. Wilken, The Solar Wind and Supratherlnal Ion Composition Investigation on the WIND Spacecraft, Space Sci. Rev., 71, 79 (1995). Hilchenba.ch, M., D. Hovestadt, B. Klecker, and E. M6bius, Detection of Singly Ionised Energetic Lunar Pickup Ions Upstrealn of Earth's Bowshock, in Solar Wind Seven, ed. E. Marsch and R. Schwenn, COSPAR Coll. 3, pp. 349-355 (1991). Ip, W.-H., The Atomic Sodium Exosphere/CoIna of the Moon, Geophys. Res. Lett., 18, 2093 (1991). Mendillo, M., and J. Baumgardner, Constraints oil the Origin of the Moon's Atmosphere from Observations During a Lunar Eclipse, Nature, 377, 404 (1995). Morgan, T.H., and D.E. Shelnansky, Lilnits of tile Lunar Atmosphere, J. Geophys. Res., 96, 1351 (1991). Ogilvie, K.W., and G.K. Parks, First Results from WIND Spacecraft: An Introduction, Geophys. Res. Lett., 23, 1179 (1996). Smyth, W.H., and M.L. Marconi, Theoretical Overview and Modelling of the Sodiuln and Potassiuln Atmospheres of the Moon, Astrophys. J., 443,371 (1995). Zel'dovich, Ia.B., and Yu.P. Raizer, Physics of Shock Waves and High Temperature Gases, Academic, San Diego, Calif. (1967).
This Page Intentionally Left Blank
THE TETHER SYSTEM USED IN THE MAGNETOSPHERIC STUDIES
S. I. Klimov l, F.L.Doudkin 2, V.E.Korepanov 2, A.A.Petrukovich l, M.L.Pivovarov l, and A.V.Prudkoglyad 1
1Space Research Institute (IKI), Profsoyuznaja 84/32, 117810 Moscow, Russia 2Lviv Center of Institute of Space Research, Naukova 5-a, 290601 Lviv, Ukraine
ABSTRACT The Electromagnetic remote sounding of the low density plasma boundaries is discussed. Using a tether system, with transmitting and receiving antennas in the frequency range 9-200 kHz, the bow shock sounding is possible. INTRODUCTION Tethered systems carried by spacecraft provide the unique opportunity to perform the remote sensing of the specific boundaries in space plasma (shock wave, bow shock, magnetopause etc.). The signal reflected from the iboundary has frequency fpe.,./<< fR << fPe., , where fpe., is the electron plasma frequency of i-layer; fR the frequency of reflected signal. As an example, the remote sensing of the Earth's bow shock is considered. It is known, that the plasma electron density of the solar wind (SW) ne~ and of bow shock (BS) neb are given by the following relation: neb = artes where a-- 3-5 and nes ~ 1-100 cm "3. It is obvious also, that 1 < fR / fpes <-~ ,
(1) (2)
where fpes~ 9-90 kHz - electron plasma frequency of SW. From the last equation it is seen that efficient sounding of BS is possible only in LF and ULF bands, requiring long antennas for the electromagnetic(EM) field sources(FS) and field receivers(FR). THE TETHER SYSTEM The best solution to the antenna length problem is provided by tether systems created by the system "satellitesubsatellites" (Klimov et al., 1995a, 1995b, 1996), connected by long ropes in order to have stable geometry (Figure.I). Let us analyze whether the development of such a system is realistic from the point of view of its dimensions and power consumed. In the plasma EM transparency band, ifEq.(2) is valid one can assume, that 71
72
S.I. Klimov et al.
IkLbl > >
1,
Ikrl > > 1
(3)
where k is the wave number, Lb ,r - characteristic BS dimension and the distance to BS correspondingly. This permits the use of the far zone and geometric diffraction approximations. The best suitable FS for such a case is a linear electric antenna when following relation is fulfilled: kolt_
kolrS~
(4)
where ko is the free space wave number, It and lr- length of FS and FR antennas. In this case, the electric field of the reflected signal in the reception point is ER ~ 0.25j ,uol lt.t R F r -1 exp(-2jkr)
(5)
where I is the current in the antenna, lt.t- effective antenna length (King and Smith, 1981), R - EM wave reflection factor (Krall and Trivelpiece, 1973) R = (k~-kb)( ks +kb) l,
(6)
F - foc:~ing factor (Green et al., 1993): (7)
F = (l+r/p) ~,
where 9 is the equivalent spherical curvature radius of the reflecting surface, + o r - is for convex or concave surface correspondingly.
Fig. 1. The tether system 'satellite-subsatellites'.
Taking into account the expression for plasma permeability (King and Smith, 1981), from Eq.(5) it follows that for the plasma opaque band: (2rt I1- all/2)1 <_8 (2rtko)-l < 1 al/2< f / f oe < 1- bl
Here 8 is the skin depth, bl,~ 1.25•
(8)
2. For BS thickness hb~ 100 km and referring to Eq. (2) and Eq.(8), we have: 8b << hb;
8s << r
(9)
This provides practically the full reflection of the signal with frequency given by Eq.(2) from the BS and full cutoff when f/fpes <1. Figure 2 shows the dependence of relative value of IERIn on the relative frequency f/fpe~ for the non-matched FS antenna when the transmitter output voltage Ut=const. Numbers 1 and 2 correspond to values of
a = 3 and 5 (see Eq.(1)).
Tether System Used in the Magnetospheric Studies
lento
73
The points where f/fpes =
1 and a
are the
characteristic points of the reflected signal curves, which give the concentrations of electrons in the SW and the BS. The distance from spacecraft to the BS can be determined by measuring the pulse lag, with the resolution Ar equal to (Green et al., 1993,
0,~-
Calwert et al., 1995): Ar ~ 0.5 c At,
(10)
where At- time resolution of the FR, c - light speed.
~4 ..............
The estimation of the FS power and antenna length
-~
2
gives the following expressions. The maximal necessary current in the FS antenna is Zt.max
Fig. 2. Normalized value of reflected electric field.
~
(15 ~r l ,
2~IRI F ) "
(SN")E,r
max
(11) where l , ~ 2l ,.1; 2 - EM wave length in free space, field
value reduced to the field
SN
"1 -
signal to noise ratio at the FR input, E, - noise electric
in the antenna vicinities, and rmax- maximal distance to the BS. Eq. (4) gives the
limitation on the FS antenna length: /tmax = ~ ' ( 2 ~ a ) -1
(12)
l , = 2(2red) ~
(13)
Then, taking Eq. (2) into account, where d ~ 1~ ~ ,
and possible antenna length is within the limits It max= 2.4 kin;
l t m,<_ 2 4 0 m.
The active FS power can be estimated as follows: Pat=It 2(Rrt+Rst+Ru)
(14)
where R,.t is the radiation resistance of the antenna, Rst - active part of surface impedance of the antenna, Rtt output transmitter resistance. The full transmitter power is P , = It 2 Ig,,+g;,I
(15)
where Z,t, Z , - antenna and transmitter impedance. The voltage at the output of the transmitter:
Ut = I, IZat+Z,,l
(I 6)
The numeric estimation of these parameters for rmo~ ~ 30RE, the non-matched antenna, using the non-magnetic steel rope with diameter D = 4 mm,
gives the following results: Pat ~ I . I W ;
It~0.47A;
Ut~1400V
CONCLUSION So, the theoretical estimation of the possibility of EM sounding of the boundaries in space plasma gives hope that it couk, be realized. Certainly, it is difficult to achieve the necessary dimensions of the tether system, but exciting rewards offered by such a new instrument for plasma boundaries monitoring may accelerate the corresponding tethered antenna development.
74
S.I. Klimov et al.
REFERENCES Calwert, W., R.F.Benson, D.L.Carpenter et al., The feasibility of radio sounding in the magnetosphere, Radio Science, 30, 1577-1595 (1995).
Green, J.L., R.F.Benson, W.Calwert, S.F.Fung, P.H.Reiff, B.W.Reinish, and W.W.Taylor, Radio plasma imaging, NASA Report, Feb. (1993).
King, R.W.P., and G.S.Smith, Antennas in matter, The MIT Press, Mass. (1981) Klimov,S.I., A.A.Petrukovich, M.L.Pivovarov, A.V.Prudkoglyad, V.G.Rodin, A.A.Skalsky, and V.E.Korepanov, Tethered systems in the Magnetospheric studies, Proceedings of the Fourth International Conference on Tethers in Space, Smithsonian Institution, Washington, D.C., 10-14 April, pp.1259-1268 (1995a). Klimov, S. I., Yu.N.Agafonov, A.A.Skalsky, and V.G.Rodin, The electromagnetic clean subsatellite SPELIS for studies on plasma-wave phenomena caused by operations of the electrodynamical tethered systems (EDTS) in space plasmas, Proceedings of the Fourth International Conference on Tethers in Space, Smithsonian Institution, Washington, D.C., 10-14 April, pp.1643-1652(1995b). Klimov, S. I., M.L.Pivovarov, E.N.Alekseeva, A.V.Prudkoglyad, and V.G.Rodin, Use of tethered systems for fundamental studies of the magnetosphere. Dynamics of tethered system deployment and operation. Kosmicheskie Issledovanij'a, 34, pp. 106-109 (1996). Krall, N.A., and A.W.Yrivelpiece, Principles ofplasma physics, McGraw - Hill Book Co., N.Y. (1973).
SIMULATIONS OF A SPHERICAL SECTION ELECTROSTATIC ANALYZER J. H. Vilppola, 1 P. J. Tanskanen 1 and B. L. Barraclough 2
1Department of Physics, University of Oulu, Oulu, FIN-90570, Finland. 2Space and Atmospheric Sciences Group, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA.
ABSTRACT The response function of a hemispherical electrostatic analyzer equipped with a curved aperture plate (to collimate the stray electric field at the entrance apertures) has been simulated. A cylindrical threedimensional simultaneous overrelaxation algorithm has been introduced to solve for the stray field. The influence of the non-symmetric deformed hemisphere on the transmission properties of the hemispherical electrostatic analyzer was considered in order to explain the unexpected calibration results. It has been shown that using a specially selected deformation of the shape of the inner hemisphere yields a particle distribution which closely resembles the results of the calibrations. Thus, it can be concluded that the results of the simulations and the results of the calibrations are in good agreement with each other. ION BEAM SPECTROMETER FOR THE CASSINI MISSION The ESA/NASA Cassini mission to Saturn has a planned launch in October 1997, with an arrival at Saturn in 2004. A hemispherical electrostatic analyzer will be used as an Ion Beam Spectrometer (IBS) as a part of the Cassini Plasma Spectrometer (CAPS instrument; NASA proposal and Bame et al. (1986)). The other scientific instruments in CAPS are the Electron Spectrometer (ELS) and the Ion Mass Spectrometer IMS. The CAPS also consists of a Data Processing Unit (DPU), High Voltage Units (HVU 1&2) and an Actuator (ACT). The Actuator provides the mechanical interface to the Cassini spacecraft. The Actuator also provides the rotation capabilities of the CAPS instrument together with the spacecraft spinning. The outlined picture of the CAPS instrument is shown in Figure 1. The IBS is a high energy/angular resolution, nearly 180 ~ spherical section electrostatic analyzer that utilizes a unique crossed-fan viewing geometry to analyze ion beams expected to be encountered during the course of the mission. The IBS uses three custom channel electron multipliers (CEMs) as detectors (McComas and Bame (1984) and McComas et al. (1987)). INTRODUCTION TO IBS SIMULATIONS The simulation routines used in this paper are introduced more detailed in two articles published in Rev. Sci Instrum. (see Vilppola et al. (1993 and 1996)). In our first article about the IBS simulations (see Vilppola et al. (1993) hereafter called Paper I) we considered the IBS as a hemispherical electrostatic analyzer where the stray electric field effects were considered only as they effected ion energies. The 75
76
J.H. Vilppola et al.
Fig. 1. The cross section view of the Cassini Plasma Spectrometer (CAPS). The CAPS contains an Ion Mass Spectrometer (IMS), an Electron Spectrometer (ELS) and an Ion Beam Spectrometer (IBS). It also contains a Data Processing Unit (DPU), an Actuator (ACT) and High Voltage Units (HVU 1&2).
effects of these fields on different entrance angle trajectories were entirely ignored. This was reasonable when considering that the aim of that study was to investigate the effects of manufacturing tolerances of the hemispherical analyzer plates. The geometrical properties of the hemispheres were estimated by calculating the trajectories of the ions after their entrance into the instrument. In our second article (see Vilppola et al. (1996) hereafter called Paper II) we reported on improvements in our simulation routines, which took stray electric field effects into account, as well as the effects of the front aperture plate geometry and the shape of the apertures. Paper I presents a calculation method that describes the performance of the IBS instrument and the estimation of the influence of the geometrical manufacturing and assembly errors on the measurements. In particular, the misalignment of the hemispheres were studied. Also the influences of indentations on the surfaces of the hemispheres were considered. As a result, the 25 lam specification was given to all geometrical manufacture errors although only the cases of the misalignments were studied using simulations directly. It was found that the indentations up to a depth of 300 ~tm and a quite large area did not affect significantly to the transmission properties of the IBS instrument. However, indentations or elevations, which can be expected in manufacturing, are typically much smaller than those used in calculations presented in Paper I. In practice the 300 Jam value for the depth of the indentation should be considered quite large since even an indentation depth of 100 ~tm could easily be mechanically detected. Therefore, a nominal upper limit of 100 ~tm for a depth of an indentation was chosen, although the calculations clearly indicated that the hemisphere does not have to be rejected as long as the indentation depth is less than 300 jam. The hemispheres of the IBS flight model have been manufactured and the measured shape tolerances were Within the range _+25 Jam. The assembly tolerance of the hemispheres has
Simulations of a Spherical Section Electrostatic Analyzer
77
been confirmed to be of the same order of magnitude. The hemispheres were measured mechanically by using a touch sensor which yields a 5~m accuracy. In Paper II the stray electric field was calculated using a cylindrical (3D) simultaneous overrelaxation (SOR) algorithm (see also Press et al. (1986)). The hemispherical electrostatic analyzer equipped with an aperture plate to improve the collimation of the stray electric field at the entrance apertures was considered. The influence of a curved entrance aperture had also been added to the simulation model, and its effects had been studied in detail. In practice this gave an opportunity to study the butterfly effect considered by Gosling et al. (1978) and (1984). Ion trajectories inside the IBS instrument were calculated using the method described in Paper I. Basically the aim was to simulate the function of the IBS instrument as realistically as possible in distinction to Paper I where the only interest was the properties of the hemispheres. As a result of Paper II the energy resolution of the IBS instrument improved from AE/E=(1.6_+0.2)% given in Paper I to &E/E=(1.3_-+O.2)%. The improvement occurred as a result of the front aperture plate cutting off some ion trajectories with very low or very high relative energies and also the collimating effect of the stray electric field on the ion beam. Because of the butterfly effect the azimuthal angle resolution changed from [3=(1.4_+0.1)~ given in Paper I to 13=(2.3_+0.1)~ In Paper II it was also proved that an accuracy of 251am of the manufacture of the IBS instrument given in Paper I was still sufficient. After the simulations were published in Paper I and II, the IBS flight model has been calibrated at Los Alamos National Laboratory (LANL) in June 1996. From the calibration data unexpected results were found in the energy versus alpha plot. In order to understand the results of the calibrations new simulations were implemented. In the new simulations the case of asymmetric hemispheres is studied. In this paper we introduce the theory of simulations which differs from the theory given in Papers I and II. Those results of the simulations and calibrations are presented which are necessary to understand the unexpected results of the calibration. Finally at the end of this article discussions and conclusions about the results of the simulations and the calibrations are given. THEORY AND ALGORITHMS FOR SIMULATIONS In the simulations presented in this paper we use similar simulation methods as were introduced in Paper I and II. All three IBS apertures shown in Figure 2 are identical. In this paper we can consider the instrument as a single aperture, spherical section electrostatic analyzer. The simulation result given here is identical for all three apertures since the geometry used in the simulations is rotation invariant for rotation around the z-axis. In the case of the calibration data the results given here cannot be generalized to the other apertures since the data are from the middle aperture measurements and the real geometry of the hemispheres of the IBS is still unknown. All three coordinate systems, shown in Figure 2, are chosen so that the xy-plane at z=0 lies at the edges of the hemispheres, the z-axis points directly towards the top of the hemispheres, and the x-axis passes directly through the center of the aperture. Ion Trajectories Inside the IBS In an ideal case, IBS is simply a hemispherical condenser with a voltage applied between the hemispheres. Solving Gauss' law leads to an equation where the electric force F on an ion with charge Q is given as a function of radius r (see for example Spiegel (1959), Grant and Phillips (1976) and Arfken (1985)), F -
r
2
R 2-R l
,
where ,4 U is the voltage between the hemispheres. Parameters R! and R 2 are indicated in Figure 2.
(1)
78
J. H. V i l p p o l a et al.
Z
RI'
/-///1"~ ,
//"/~.
!
iI
/ Fig. 2. Coordinate systems (x,y,z), (r,~,O) and (p,9,z), angular variables a and [3 used for the initial velocity vector and the parameters of the IBS instrument as appearing in the simulations. The inner radius R 1 is 0.09875m and the outer radius R 2 is 0.10125m. The figure has been published earlier in Paper II.
The equations of motions can easily be solved analytically and the solutions are Kepler's elliptical orbits. In our case, these orbits cannot be used since spherical symmetry is broken when asymmetric hemispheres are considered. In that case the force given in Eq. 1. depends on the distance between the hemispheres (AR=R2-R 1 -> RI=R2-zkR). The geometry used in the simulations presented here can be defined using equation l~used "- l~idea I -- (~l~as sin 2(20),
(2) where ARused is the distance between the hemispheres used in the simulations, ARidea I is the distance between the hemispheres for an ideal instrument, 8Z~as is a maximum deviation from the ideal distance and 0 is an angle defined in Figure 2. In the simulations presented here a value of 50~m for the parameter ~Z~as is used. The correction term 8Agassin2(20) has its maximum when 0 =45 ~ and it has its minimum 0 when 0 =0 ~ or 0 =90 ~ A view of the geometry used in the simulations is presented in Figure 3.
Z
Ideal"
Asym."
Z
z ~ -
/
/ /
]
-.
I
L
,y
'X Fig. 3. The exaggerated view of the asymmetric inner hemisphere used in the simulations.
\
Y
Simulations of a Spherical Section Electrostatic Analyzer
79
Eq. 1. and 2. yield ion orbits which have to be solved numerically. For our purposes the fourth order Runge-Kutta method was used to obtain the solutions (see Press et al. (1986)). For more details about solving ion trajectories inside the IBS instrument see Paper I. Ion Trajectories Outside the IBS A stray electric field accelerates incoming ions and thus it also changes the ion trajectories. The geometry of the front aperture plate has dramatic influence to the stray electric field. With the front aperture plate installed, about 90% of the stray electric field can be confined to the volume between the front aperture plate and the hemispheres. Removal of the ions which collide with the front aperture plate, but otherwise could traverse the sensor, improves the energy resoiution as well as the angular resolution of the instrument. The geometry of the front aperture plate of the IBS instrument is shown in the Figure 4. The geometry is identical for all of the three apertures of the IBS instrument. A cylindrical (3D) simultaneous overrelaxation (SOR) algorithm is used to solve the stray electric field (see Press et al. (1986)).
,B
Cross-section A-A
el ~2.5 mm I~ ~)~~~ I
1 2 mm
_
"~ \ ~ \ 15 m m ~
mm . "B
~
hv
Cross-section B-B
~+_j175~ j~jJ-
!
15mm 1
1
-
Fig. 4. The geometry of the IBS entrance aperture. On the left side of the figure the aperture is seen from straight above the front aperture plate. The cross-sections A-A and B-B are marked with dashed lines. The numerical values of the parameters of the geometry of the IBS entrance aperture are marked in the figure. The figure has been published earlier in Paper II.
The equations of motions are written for cylindrical aperture geometry and the ion trajectories in the stray field area have been solved using the fourth order Runge-Kutta method (see Press et al. (1986)). The initial values for a hemispherical routine, introduced in the previous section, are calculated using the output values of the stray field routine given above. For more details about solving the ion trajectories outside the IBS instrument and combining the stray field routines to the hemispherical ones see Paper II.
80
J.H. Vilppola et al.
RESULTS OF SIMULATIONS In Papers I and II, we have studied the ideal case corresponding to an instrument with perfect geometry, and in cases where linear misalignments in each main coordinate axis x, y, and z occurred. Also the effects of indentations of the hemispheres were considered in Paper I. These results are not presented here. Thus, only the results of the simulated new asymmetric case introduced in the theory section are included in this report. The values of the parameters for the laboratory ion beam used in the simulations are given in Paper II. The requirements of the stray field calculations used in the simulations are fulfilled using the potential mesh parameters shown in Paper II. Also other parameters used in the simulations are given in Papers I and II. The symbols of the IBS instrument parameters as well as their values used in the simulations are presented in Figures 2, 3 and 4. In the actual IBS, the center of the hemispheres is 2 mm above the plate tangential to the edges of the hemispheres. Thus, the real reverse angle of the IBS flight model is only about 178 ~ instead of 180 ~ which is the definition of hemispherical analyzer. This means that the [3-angles used in our simulations can be used for the actual IBS only after a correction of 2mm/100mm rad has been subtracted. The simulations were made for the ideal case, and for the case of an asymmetric inner hemisphere where the value of 50~m for the parameter 5&Ras introduced in Eq. 2. is used. In every simulation for each ion beam setting (a,~,E) 250 ions were injected. Since there are 51 different values for each of these variables, a total of 33162750 (=51x51x51x250) ions per simulation were injected. An exception had to
Fig. 5. The energy/alpha (E/o~ in text) distribution for an asymmetric case. )~ = number of particles transmitted per bin square. The key shows the gray scales for the g-numbers. The bin widths are ~5E=0.12% and 80~=3.0~ The energy is (E-Eo)/Eo where Eo is the nominal kinetic energy. An ion with nominal kinetic energy has a spherical orbit exactly in the middle of the hemispheres (r=-0.100 m). The dashed cross shows the bin square where the transmission has its maximum. The dashed lines show the bins, which have been used to produce the profile plots at the upper and right sides of the plot. The gray scales in the plot have been chosen so, that the inner black ring shows the bin squares with the full width at half maximum (FWHM) values.
Simulations of a Spherical Section Electrostatic Analyzer
81
Fig. 6. The energy/beta (E/f5 in text) distribution for an asymmetric case. The bin widths are 5E=0.12% and 513=0.12~ Energy, )~, key, profile plots and dashed lines are similar to the ones shown in figure 5.
be made when the cut through at IBS response function was studied for the parameters ~, ~ and E. To avoid excess random noise the beam intensity was increased by a factor of 10. The simulation was made once for each parameter, yielding a total number of 19507500 (=3x51x51x2500) injected ions per each studied case. In Figure 5 a two-dimensional E/o~ distribution for the asymmetric case is presented. The corresponding E/f5 distribution is shown in Figure 6. In Figures 5 and 6 a cut through for invisible parameters are used.
The values of all cut through parameters are proportional to the indices which give maximum transmission using statistical averaging instead of the real maximum. Otherwise the random noise might effect the cut selection as can be seen in profile plot selections where the actual maximum has been used. The definition of the cut through for an invisible parameter using statistical averaging is later called a cut through method. An other way to present the distributions is that the invisible parameter is summed over all its values from the three-dimensional data matrix. This method of handling the invisible parameters is called the white spectrum method. In this article the white spectrum method figures are not presented since they do not give any significant new addition to the results attained using only the cut through method. RESULTS OF CALIBRATIONS In the calibrations commandable laboratory high voltage power supply was used to control the voltage between the hemispheres. All other functions were accomplished using the flight electronics installed on the IBS instrument. In the calibrations the proton beam was accelerated using a constant 10 kV voltage (10006V to be exact) source. Energy variations result from different voltages between the hemispheres. For each set of the parameters (E,a,[3) the transmitted ions were counted. The counts measured have to be corrected for of the detector dead time given by CM c r = l_cu T , (3)
82
J.H. Vilppola et al.
Fig. 7. Energy/alpha (E/a) distribution obtained for the calibration data. The bin widths are 8E=I.0V and 8a=5.0 ~ The energy is given as voltage between the hemispheres. 10 volts in the figure is about 1.9% when comparing energies here to the ones shown in figures 5 and 6. g, key and profile plots are similar to the ones in figures 5 and 6. Note that all g-numbers are corrected using a dead time correction given in equation 3.
where cr is the true counts per second, cM is the measured counts per second and T is detector dead time. T marks the time during which the detector is inoperative after each single event. The dead time correction T for flight electronics used in the calibrations is 0.30 ps. In Figure 7 a two-dimensional E/o~ distribution calculated using calibration data is presented. The corresponding simulation distribution is presented in Figure 5. A two-dimensional E/f5 distribution
Fig. 8. Energy/beta (E/f3) distribution composed using the calibration data. The bin widths are 8E=I.0V and ~513=0.2~ Parameters g, key, energy and profile plots are similar to the ones in figure 7.
Simulations of a Spherical Section Electrostatic Analyzer
83
calculated using calibration data is presented in Figure 8. The corresponding simulation distribution is shown in Figure 6. Cut throughs for invisible parameters are used in Figures 7 and 8. The values of all cut through parameters are chosen by defining the maximum of the parameters using separate measurements before the actual calibration measurements. Thus, the results of the simulations and the calibrations are comparable. The white spectrum method cannot be used in the case of the calibrations. The measurements needed to collect the required data set for the white spectrum method cannot be accomplished in a reasonable time. DISCUSSION AND CONCLUSION The Hemispheres of the IBS Instrument The hemispheres of the IBS flight model were manufactured at VTT Automation in Espoo, Finland. The hemispheres were built to meet the 25pm tolerance limit given in Papers I and II, and they were measured using a touch sensor before assembly in order to check that the quality requirements were fulfilled. The hemispheres cannot be measured after they have been assembled, and therefore the errors arising during the assembly phase cannot be detected directly. There is a slight possibility that the hemispheres are tensed and some asymmetry is present in the hemispheres of the IBS flight model for that reason.
E/t~ Distributions The double curved shape of the E/c~ distribution shown in the calibration data plot of Figure 7 was unexpected. The simulation result shown in figure 5 confirms the opinion that there might exist some asymmetry in the hemispheres of the IBS flight model. The asymmetry in the hemispheres is not necessary exactly the same as described in the theory section but definitely there are some similarities between the geometry of the hemispheres of the IBS flight model and the assumed geometry given in the theory section. The similarities in figures 5 and 7 support this conclusion. If the assumption is made that the asymmetry is similar in the simulations and in the IBS flight model, then the magnitude of the asymmetric factor ~)Z~asintroduced in Eq. 2. is approximately the same 50pm as used in the simulations. The small counter-clockwise rotation appearing in the E/a distribution shown in the calibration data plot of Figure 7 can be explained using the methods given in Paper II. A similar rotation in the E/a distributions of the simulations occurs as a result of misalignments along the direction of y-coordinate. The magnitude of the misalignment responsible for the rotation in Figure 7 is about 10pm. This is below the limit given in Papers I and II. This conclusion can be reached by comparing Figure 7 here to Figure 7 of Paper II. Other Distributions The similarities in other distributions when comparing simulations of an ideal and an asymmetric case with the calibration results supports the conclusions given above. The butterfly effect given in Figure 8 of Paper II can be seen in the calibration results, as well as in the new simulation results. The E/f5 distributions given in Figures 6 and 8 also supports the conclusions given above. The energy resolution is approximately the same for the simulations and calibrations. The same conclusion holds for the angle 13 resolution. In the calibrations the measurements are made for E/f5 distributions with several cut through values for the angle c~ which varied from-80 ~ to 80 ~ with 10 ~ steps. Other cases than the one given in Figure 6 are not yet simulated. Because of the lack of calibration data discussed earlier which prevented the usage of the white spectrum method a new simulation might be required in the verification of the calibration data. At least some minor checks with specific cut through values for the angle c~ should be
84
J.H. Vilppola et al.
accomplished. Also new calibrations for the two side apertures are required before the IBS response function is fully understood. ACKNOWLEDGEMENT The authors would like to acknowledge the Academy of Finland,' University of Oulu and NASA for financial support of the project. The authors would like to thank Dr. J. T. Gosling, Los Alamos National Laboratory for his valuable comments and suggestions. REFERENCES G. Arfken, Mathematical Methods for Physicists, Academic Press, Inc., San Diego 1985. S. J. Bame, D. J. McComas, D. T. Young, and R. D. Belian, Diagnostics of Space Plasmas, Rev. Sci. Instrum., 57, 1711 (1986). J. T. Gosling, J. R. Asbridge, S. J. Bame and W. C. Feldman, Effects of a Long Entrance Aperture Upon the Azimuthal Response of Spherical Section Electrostatic Analyzers, Rev. Sci. Instrum., 49, 1260, 1978. J. T. Gosling, M. F. Thomsen and R. C. Anderson, A Cookbook for Determining Essential Transmission Characteristics of Spherical Section Electrostatic Analyzers, Los Alamos National Laboratory Report LA- 12962-MS, 1984. I. S. Grant and W. R. Phillips, Electromagnetism, John Wiley & Sons Ltd., 1976. D. J. McComas and S. J. Bame, Channel Multiplier Compatible Materials and Lifetime Tests, Rev. Sci. Instrum., gg, 463 (1984). D. J. McComas, J. R. Baldonado, S. J. Bame, and B. L. Barraclough, Channel Electron Multiplier Compatibility with Viton and Apiezon-L Vacuum Grease, Rev. Sci. Instrum., 58, 2331 (1987). NASA, A Proposal for the Plasma Science (PLS) Investigation for the Cassini Orbiter Spacecraft, Volume 1: Investigation and Technical Plan. W. H. Press, B. P. Flannery, S. A. Teukolsky and W. T. Vetterling, Numerical Recipes, The Art of Scientific Computing, Cambridge University Press, 1986. M. R. Spiegel, Theory and Problems of Vector Analysis and an Introduction to Tensor Analysis, McGraw-Hill, New York 1959. J. H. Vilppola, J. T. Keisala, P. J. Tanskanen and H. Huomo, Optimization of Hemispherical Electrostatic Analyzer Manufacturing With Respect to Resolution Requirements, Rev. Sci. Instrum., 64, 2190 (1993). J. H. Vilppola, P. J. Tanskanen, H. Huomo and B. L. Barraclough, Simulations of the Response Function of a Plasma Ion Beam Spectrometer for the Cassini Mission to Saturn, Rev. Sci. Instrum., 67, 1494 (1996).
A NUMERICAL, DOUBLE-LAYER SOLUTION FOR PLASMA CONTACTORS COLLECTING ELECTRONS IN ELECTRODYNAMIC TETHER APPLICATIONS A. J. Li and D. W. You
Center for Space Science & AppfedResearch, Academia Sinica, Beijing 100080, P.R. CHINA
ABSTRACI"
Tethers in space can be used for a wide variety of applications. In this case, plasma contactors play an important role. A plasma contactor is a special device which can provide electric contact of the ends of a tether with the ambient plasma. Plasma contactors based on the hollow cathode technology have been used as electron emitters or collectors for tethers, and neutralizers for spacecraft charging control. Our work has been placed an emphasis on the laboratory simulation of the double layer that exists between two plasmas, the plasma plume generated by a hollow cathode and the simulated ionosphere plasma. The plasma contactor used as a current collector was numerically computed according to the space charge limited double layer model. When the plasma contactor is used in electro-dynamic tether application, where a substantial current can be collected, the ignited mode of electron collection is needed. In this case, We numerically solve the Poisson's equation in one dimension considering the thermal electrons in the double layer and taking streaming electron ionisation into account. The calculated resuks are presented in comparison with the collisionless double layer theory and the experimental data. INTRODUCTION The electrodynamic tether system(Martinez and Hastings, 1987) has a great potential to provide great electric power for space applications. A tether can be used as a power generator to deliver the power to a load, however, if the load on the tether is replaced by a DC power supply, the tether will act as a thruster that can increase the orbital energy of the system, so the whole system can be applied as an effective energy storage system. In either case, a complete circuit that includes the space plasma, the tether and the contactors is needed and the contactors acting as space plasma brushes should improve the electrical efficiency by reducing the impedance. Therefore, plasma contactor(Williams and Wilbur, 1990) is a plasma producing device which is conceived to enhance the electron collection and emission between the tether and ambient plasma. Hollow cathodes(Sigfried,1982; Deninger et al., 1987) have been found to be particularly well suited to the plasma contactor use because of the lower parasitic power consumption and better feasibility. 85
A. J. Li and D. W. You
86
A key issue in the plasma contactor research is to enhance the electron current collection and reduce the impedance of the contactors for improving the power generation efficiency. Because in the electron collection mode plasma contactors usually has high voltage drops, so only this mode attracts more attention. Our work will focus on the electron collection mode. Langmuir(Langmuir, 1929) pointed out that between two quasineutral plasmas at different potentials, a double- layer(DL) occurs. The DL consists of a layer of positive charge and a layer of negative charge. The DL region is governed by a space-charge-limited process. Wei and Wilbur(Wei and Wilbur, 1986) introduced the first model of DL produced by the expansion of a high-density plasma into another plasma. This model describes spherical DL as spherical diodes. A current Ji of ions flows outward from the inner sphere having radius ri, and a current Jo of electrons flows inward from the outer sphere having radius ro. The outer surface is at zero potential, and the inner radius is at Vi. The particles leave each surface in a space-charge-limited manner. All components are assumed to have very low temperature compared with the voltage drop. Given the ratio ri/ro and the voltage drop Vi, the currents Jo and Ji can be calculated. This model, which treats the electrons as collisionless and unmagnetized, is able to describe many of the experimental observations. However, to further fit the experimental data, an improved model that includes ionization and thermal electrons is proposed in this paper. THEORY Following the discussion of Wei and Wilbur(1986) and using the same definitions, we can write the Poisson's equation to describe the potential variation in the DL region V 2V =- e--(n+ -n ) *0
-
,
(1)
where the space charge density is determined by the density of both accelerated and trapped charged particles. For the spherically symmetric case, Eq.(1) can be simplified to 1 d
r 2 dr
(r 2 &~_~)_
__~_o ( n + _ n
).
(2)
In Eq.(1) and (2), V is the potential at a spherical surface of radius r, where the ions from the inner surface have a density ni and velocity ui, and the electrons from the outer surface have a density no and velocity Uo. Then the ion current Ji and the electron current Jo are J o = 4 r c r 2 n o e U o and di - 4 r c r 2 n i e u i 9 (3) Because the particles are assumed to have been drawn from the boundaries at zero initial velocity, these velocities are given by u o - x/(2e / m o ) V and u i - x/(2e / mi)(Vi - V), (4) where mo and mi are the masses of the charged particles drawn from the outer and inner surfaces, respectively. From Eq.(3) and (4), we can write the stream charged particle densities as no =
do
a n d r/i =
4nr 2e4(ze / m o ) v for electron and ion densities.
(5)
di
4nr 2e4(2e / m i ) ( V i
- V) '
In addition to the above two terms that were used in Wei and Wilbur' model, now we consider the thermal electrons trapped on the inner side, which can be written as _
?leth
r"2
-'~?leth
(Vi) exp[
e(V-Vi)
-kTe
]"
(6)
Numerical, Double-Layer Solution for Plasma Contactors Collecting Electrons
87
Since the density of ions trapped on the outer side is low it can be neglected. Finally the density of accelerated ions created with zero velocity by ionization due to the streaming electrons (thermal electrons are not assumed to ionize) can be given by
@ niz =
,, ,Z t , Vl ]
,
(7)
r 2 x/2e[V(rl) - V(r)] / m i
where the rate of ion creation is given by niz (r,V) - cr(V)n n (r)n e (r)v e (V), (8) with vo(V) the electron velocity and cy the ionization cross section(Rapp and Englander-Golden, 1965) depending on the kinetic energy of the accelerated electrons. The density of neutral particles depends on the atoms expanding from the plasma contactor because the ambient pressure is very low. When a gas atom is ionized, a new electron is also created. This electron is supposed to be thermalised quickly and contributes to r~th. Then, we can rewrite Eq.(1) as r 21 drd (r2 --h-g-,-U-,,, ~ 0 d) V= - e (n i _ no) _ -~o (niz-neth) "
(9)
Using the definitions given by Wei and Wilbur(1986): qk- V / V i , p - ln(r / ro) , ]o - (Jo / 4rccoVi3/2)x/mo / 2e ,and a - (Jo / Ji)x/mo / mi , Eq.(9) can be rewritten as
(10)
d2qk dO 1 --~p2 + -~p - jo ( x[-~
(11)
1 )+aexp(_b(l_q~))_CjoS(q~), a 4 1 - q~
eri e eVi 3 where a - ~oVi net h (V i), b - -~e ' c - n n (r i )• % ,
l. l n%.. ~r and s(~b)-ri
(12)
cr(rl)
(13)
~oo r~r124~k(r1)_qk(r) dr1
We use boundary conditions ~b= 1 at p = p; = ln(~ / ro), and ~b= 0 at p = 0 = ln(ro / ro), d ~ = 0 at p - Pi where ~b- 1, and d__~_ 0 at p - 0 where r dp
(14) 0.
(15)
dp
Now, if To, Vi, ri, ro, r~(Vi) and nn(Vi) are given, we can solve the DL problem based on our model. RESULTS AND DISCUSSION Using Wilbur's experimental data(Wilbur, 1987) xenon in the plasma contactor, Vi=55V, the xenon atom density of n, r2=3.6• the thermal electron density of 1.0x1013/m3, and 4eV of the electron temperature of inner plasma, the problem is solved by obtaining the numerical solution of the Eq.(11) combined with the boundary conditions Eq.(14) and Eq.(15). Both the normalized current from outer surface jo and the normalized current ratio tx are calculated and shown in Figure 1 and Figure 2. Using Wilbur's experimental data, for ro--0.128m and ri/ro=0.61, JCE =-250mA, corresponding to jo=4.48, our model gives jo=3.60 while Wei and Wilbur's model gives jo---3.07. So our model is more coincide with the experiment data.
88
A. J. Li and D. W. You o
"~
2
1
- ~
Wilbur's Model
?
"~ 1.5
ca 0.5
0
Z:
~
Wilbur's
Model
.~08
I.. =
.~
T
0 9 --Jr
~
0.6
0
~
.5
-0.5
~
0.4 0.3
-1 0
r
'~"
~0
O0
0
o
0
0
Sphere radius ratio (ri/ro) Fig. 1 Effect of radius ratio on current magnitude
I
l
I
I
I
I
I
I
I
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0.2t
Sphere radius ratio hlro
Fig.2 Effect of radius ratio on counterflowing current ratio
The at is the normalized current ratio(ct=l is the Langmuir condition). In both models(Wei and Wilbur's and this paper), cx
Section II Active Experiments
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M O D I F I C A T I O N N A T U R A L AND M A N M A D E EM E N V I R O N M E N T DUE TO I O N O S P H E R I C PLASMA BARRIER T R A N S P A R E N C Y FOR G R O U N D B A S E D T R A N S M I T T E R EMISSION V.N.Oraevsky ~ , Yu.Ya.Ruzhin i , V.S.Dokukin : , Kh.D.Kanonidy i , B.P.Singh 2 , and G.S.Lakhina 2 1
IZMIRAN, Troitsk, Moscow Region, 142092, Russia," E-mail.
[email protected]; tel. (095) 334-02-91
2 Indian Institute of Geomagnetism, Bombay, India
ABSTRACT On investigation of natural and manmade electromagnetic (EM) environment in the near Earth space, it is important to examine the sources of EM and ways of transportation of wave energy. One of the possible candidate for observing the waves above plasma barrier is the mechanism of ballistic penetration which was originally proposed by Oraevsky and Lisitchenko (1971) to understand the results of laboratory experiments. To check that idea in space plasma, we have carried out above Dushanbe heating facility three in situ experiments on the appropriate Active Plasma Experiment(APEX) satellite's orbits.The results of that experiments are discussed below. INTRODUCTION In the past, there have been several ionospheric modification experiments, e.g. at Arecibo, EISCAT etc. employing the ground based heater with high power emission of frequency (.oj, as well as diagnostic radars (Carlson and Duncan, 1977~ Ganguly and Gordon, 1983; Isham et el., 1990). Some of these experiments had anomalous signals ( observations of different plasma lines) from topside of the ionospheric density profile where cob= 6Ope(c0pe'the local Langmuir frequency). Several mechanisms have been put forward to explain the results of these experiments, but no single mechanism could fit the observations quite well. EXPERIMENTS The APEX (APEX project., 1992; Oraevsky and Triska, 1993) satellite has been in orbit since 18 December 1991. The orbit of the APEX satellite has apogee at 3050 km, perigee at 440 km and inclination of 82.5 o
I
~Heated
/
i.."" K I
L.,"]
Fig.
plasma
To carry out the active experiments, specific APEX orbits, which cross magnetic field flux tube heated at base (200 km ~ [ or ~\-. - ' ~ - ~ O r b l t 1042 altitude) by the Dushanbe heater facility were chosen. The geometry of the magnetic field lines ( model IGRF-90 ) above the heating facility at Dushanbe, the APEX orbits and the =] ~/ "\ IGRF-90 " ground based diagnostic facilities at India are shown in Zl ~/ \,model fleld lines L J---- ~ / ~ - . :,:-7 , Figure 1. Orbits number N 1013 and N 1025 are the _ '~ - -_- ~ ~ o ~---__ k \ k)rblts trajectories for the perigee pan of the orbit (satellite was moving from south to north direction) and orbit number N ~'-"""-~ "', ,/",, "'-~,. 10 2 5 1042 are for the apogee part of the APEX trajectory (satellite AZ b g ,{"'. TZ was moving from north to south).The last orbit, whose height Jalpur'~. < / / [ /':%( above the magnetic equator is about 3000 km, is practically in Gulmarq / ,ooo the magnetic meridian plane ( better than 100 km accuracy ) Dushanbe heater of Dushanbe facility. The ionospheric heater at Dushanbe (Latitude ~ =38.57 ~ N, Longitude /t =68.77 ~ E, and L=1.45 1 APEX heating experiments diagram ) was operated at the frequency of 6 MHz in continuous or 91
92
V . N . Oraevsky et al.
pulse mode with ON/OFF interval of 2s/2s (or 8s/8s) with mean power of 15 KW ( in continuous mode).The choice of this heater frequency close to 6 MHz depends on the maximum plasma frequency of F2 - the main ionospheric layer. The heater frequency has to be smaller than the latter by at least 1-2 MHz to study the wave phenomena above plasma barrier. The state of the ionosphere was monitored by digital ionosonde. Figures 2a,b show calculated electron density profiles above Dushanbe for case of orbits 1013 and 1042 respectively.APEX satellite has full set of diagnostics for the plasma and fields parameters. In this paper we shall use the in situ data of plasma measurements and the HF electric field measurements in the frequency range from 0 to 10MHz on the APEX satellite
9oo
Du~th,anbe
I ~ u s I-1 r n Ic~.e I " 7 . C ; , 3 .'cu 2 1 6:oo u-r IRI --gO
15.0,.3.92
04-:00 UT IRI ~gO
900
500
.5,00
,,,
N
~m--3
~~,
' .... ~~;. ~ _ 1
......
,,<-~-.1
o-
......
,o-
Fig. 2a Electron density profiles above Dushanbe for Fig. 2b Electron density profiles above Dushanbe for APEX orbit 1013 APEX orbit 1042 The dynamic spectra of the electric field component of the high frequency (HF) noises observed on the board of APEX by PRS-3 experiment (Klos et al. ) is shown on Figure 3 and 4. The frequency range of the spectrograph is from 0.1 -10 MHz, which covers the main parameters of the ionosphere, e.g. electron plasma frequency and gyrofrequency ( or its harmonics ). The time taken for one sweep of the full frequency range is 2.56 s.
Fig. 3 The dynamic spectra of the electric field component of the high frequency (I-IF) noises observed on the board of APEX by PRS-3 experiment -orbit1013 Figure 3 shows the dynamic spectra from 0.1-10 MHz for the orbit number N 1013 from 15:45 to 16:10 UT on March 12, 1992. The geographical coordinates, latitude ~b, longitude,,1, and height h for the satellite positions are shown. The relative intensity scale for the electric field is shown on the left side. The horizontal arrow shows the latitude of the Dushanbe. The heater at Dushanbe was continuously ON from 15:45 to 16:00 UT, and it was operating in the pulse mode with ON/OFF interval of 2s/2s during rest of the period. The upper part from left to right shows the typical image of the equatorial anomaly ( the variation in sttu of the plasma frequency). When the satellite is out of the equatorial anomaly region, one can see the effects of heater near the 6 MHz line shown on the Figure. Usually it was not observed for similar conditions of observation when the heater was not operative
