Magnetospheric Research with Advanced Techniques, Proceedings of the 9th COSPAR Colloquium

195 Pages·1998·8.856 MB·English

by  R.L. Xu and A.T.Y. Lui (Eds.)

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
esenihC 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 ,seuqinhcet )3( active experiments ni space, dna (4) numerical simulation and theoretical
.gniledom This meeting was held after eht 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 Solar-
lairtserreT Physics Program. These exciting new results and the ongoing discussions of innovative approaches to
cifitneics 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 lla over the world participated, from countries of Austria, Belgium, Canada,
,anihC Finland, Germany, Japan, Korea, Russia, Sweden dna the United States. A total of 39 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 eht 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 era 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 ni eht 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 ,iQ Daheng Wang, dna 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 ecaferP
,gnijieB gniwolla eht stnapicitrap to gain na noisserpmi no eht long historical tradition dna recent progress of .anihC
We are grateful to W.-H. pI for his help during the beginning of this Colloquium, dna to .S Grzedzielski for his
encouragement during preparing this Colloquium Monograph.
We deeply etaicerppa eht effort ofD. .N Baker, .P .A ,tdrahnreB .J .F Carbary, C.-L. Chang, .J Chen, .S .P Christon,
.R .B Decker, .R .E Erlandson, .W .R Hu, .Y .Q ,uH .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 ni improving the quality of the manuscripts published in this Monograph. In particular, the Associate Editor
of this Monograph ieL ,iL sevresed laiceps noitnem for her major contribution in bringing together the materials for
this Colloquium Monograph for publication by Elsevier Science ni early .8991
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 Co-
Chairman 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 South-
Eastern 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 ,esruo~o 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 lednereaH
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 Solar-
Terrestrial 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 ground-
based 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
gninepO sserddA of eht RAPSOC tnediserP ot eht RAPSOC muiuqolloC 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 - 12 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
INVESTIGATION OF A SUBSTORM FOLLOWING AN EXTENDED
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
ehT1 Johns Hopkins ytisrevinU Applied Physics ,yrotarobaL Laurel, MD 20723, ASU
tnemtrapeD2 of ,scisyhP University of ,dnalyraM College Park, MD 20742 ASU
ehT3 Institute of Space dna Astronautical Science Kanagawa 229, Japan
lairtserreT-raloS4 Environment ,yrotarobaL Nagoya ,ytisrevinU Toyokawa 442, Japan
,CSAR5 Kyoto ,ytisrevinU ,ijU Kyoto 611, Japan
,CFSG/ASAN6 ,tlebneerG MD 20770, ASU
tnemtrapeD7 of ,scisyhP University of Alberta, ,notnomdE Canada G6T 792
soL8 Alamos National ,yrotarobaL soL Alamos, NM ,54578 ASU
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
10 A.T.Y. 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
1995 beF 8-9 (39-40) Solar Wind Observations
February 9, 1995 at 0431 UT DNIW
O
o~,~
Wind -40
(193,-44,-18)
-30
,--"r: : .... _. .... 1
002 190)/ ~v9of I -'~0~10 -20 -30
I I/{ t t ' / ' """e ' I I I
msgX ~ ~ \ ~.~._~
// \ \ I"640 7G-- liatoeG 671~';~ r 1
Low Altitude CIS ,,,. IMP-8 12:00 18:00 00:00 00:6O 12.'00
- Freja ~_ aa'~-13,21, )92 2/8/95 8-PMI2/9/95
-DMSP 1"" ~ ho, . . . . . 1 , , ~,.~(cid:12)9 .- ~ (cid:12)9 ' i . . . . . i (cid:12)9 ' ''' ' 4
Ground Station Network + 04
- CANOPUS
SuperDARN msgY 2/8/951 2:00 18.0o 2/9/95o o:oo o6:oo 12:00
-
Fig. 2 The solar wind parameters as
Fig. 1 Spacecraft locations at substorm onset 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
noitagitsevnI of a Substorm gniwolloF na dednetxE Interval of Northward IMF
zB( 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 component was quite strong, in the neighborhood of --4-
Y . .
21 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 301 MW) before the southward
turning of the IMF. Afterwards, the epsilon parameter reached to large values (near 501 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 of-500 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) ..CITAVRESBO ,.-.,
5 Ikli i , , , , i . . . . . I ' IJ~az=' ' ', I L_' ' '
"~ o . . . . . . . . . . . .
i a I : ~ : : : .' : ', : ' (cid:12)9 ' ' ' ..... L
'10 ~ " , , - . , ,
"~'104
~ 10 e
"~ -140
Jl~
-15o
-160 J i
00:00 06.00 L.. 12:00 18:00 oo oo
(cid:12)9 i (cid:12)9 , (cid:12)9 i , , , i (cid:12)9 , , i , (cid:12)9 (cid:12)9 1 , , ,
IKSEuHcF . . . .
BACK ~ I 054 Tn
, . , ....
02 00 03:00 04:00 05:00 06.'00 07"00 08"00
Fig. 3 Shown here are the Bz component in the
solar wind, Akasofu's e parameter, the AKR
index (in units of V/m/~(kHz) observed by
GEOTAIL), and magnetograms from Canopus
21 A.T.Y. iuL 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). At-0300 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 at-0300 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 about-65 ~ (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 17 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 of-120 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 (GOES-
7), 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 at-03
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 of a Substorm Following an Extended Interval of Northward IMF 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
1995 Feb 9 (40) Geosynchronous Observations
~o ~ ,.; --. . . . . . . . . , ,
101
105
Z 10-1 - | ~. "
104
.-:~176176 ' , P~,A,I 7
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tit" 4
04:00 04:30 05:00 05:30 06:00
Fig. 4 Geosynchronous observations. Fig. 5 Geotail observations
~.? --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 at-0435 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 X-
line. 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 (--10 nT) seen at this time, the
plasma in the plasma sheet boundary layer remained rather undisturbed, just like before the dropout at