Geotail observations of energetic ion species and magnetic field in plasmoid‐like structures in...

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 102, NO. A6, PAGES 11,409-11,428, JUNE 1, 1997

Geotail observations of energetic ion species and magnetic field in plasmoid-like structures in the course of an isolated substorm even

Q.-G. Zong •, B. Wilken •, G. D. Reeves 2, I. A. Da.glis •'3, T. Doke 4, T. Iyemori s, S. Livi •, K. Maezawa •, T. Mukai 7, S. Kokubun 8, Z.-Y. Pu •, S. Ullaland •ø J Woch 1 R. Lepping TM and T Yamamoto 7 • ß • • ß

Abstract. On January 15, 1994, the ion spectrometer high energy particle- low energy particle detector (HEP-LD) on the Japanese spacecraft Geota.il observed five quasi-periodic energetic ion bursts in the deep tail (X=-96 RE). These bursts were associated with plasmoid-like structures in the magnetic field components. In addition, three multiple TCR groups were identified in the interva.1. The observations in the distant tail occurred during a time interval of substorm activity which also produced multiple injections in the geosynchronous orbit region. The HEP-LD observations show that B• bipolar plasmoid-like structures a, re associated with tailward flowing particle bursts. However, ea,rthward flowing particle bursts are predominantly associated with bipolar signatures in B u. In addition, an oxygen burst was seen in the back of a plasmoid (postplasmoid) which showed both By and B• bipolar magnetic field signatures. The oxygen burst lasted for 23 min, and the density ratio (O/H) reached 15% for the HEP-LD energy range (in the same plasmoid, this ratio was approximately 1% before the oxygen burst). The oxygen burst exhibited a strong beam-like structure which occupied only 6 •0 7% of the full soiid angle (4•r). We suggest that energized oxygen ions of ionospheric origin travel downtail in the narrow postplasmoid-plasma sheet which tra, ils the plasmoid. Furthermore, we suggest that the magnetosphere dissipated larger qua. ntities of energy during this very intense substorm event by ejecting multiple relatively s•na, ll plasmoids rather than through the formation and ejection of a single large pla.smoid.

1. Introduction

One of the most important topics in magnetospheric research is the study of magnetotail dynamics associated with magnetospheric substorms. Many phe-

1Max_Planck_institut fiir Aeronomie, Katlenburg-Lindau, Germany.

2 Los Alamos National Laboratory, Los Alamos, New Mexico. a Now at Institute of Ionospheric and Space Research, National

Observatory of Athens, Greece. 4Department of Physics, Nagoya University, Nagoya, Japan. 5WDC-C2 for Geomagnetism, Faculty of Science, Kyoto Uni-

versity, Kyoto, Japan. 6Advanced Rese•irch Center for Science and Engineering,

Waseda University, Tokyo, Japan. 7Institute of Space Astronautical and Science, Kanagawa,

Japan. 8Solar-Terrestrial Environment Laboratory, Nagoya Univer-

sity, Toyokawa, Japan. 0 Department of Geophysics, Peking University, Beijing,

China.

løUniversity of Bergen, Bergen, Norway. llNASA Goddard Space Flight Center, Greenbelt, Maryland.

Copyright 1997 by the Axnerican Geophysical Union.

Paper number 97JA00076. 0148-0227/97/97 J A- 00076 $09.00

nomena associated with substorms have been inter-

preted with the concept of a near-Earth X line and subsequent release of plasmoids [Hones, 1979; Hones and McPherron, 1994]. Plaslnoids have been identified in the distant magnetotail by magnetic and energetic particle signatures observed by a number of different spacecraft, for instance, ISEE 3 [Hones et al., 1984; Tsurutani et al., 1984; Baker et al., 1987; Scholer et al., 1984a, b; Richardson and Cowley, 1987; Richard- son et al., 1987; Nishida et al., 1986; Slavin et al., 1989; Moldwin and Hughes, 1993], Galileo (Earth fly- by) [Kivetson et at., 1993], IMP-8 [Slavin et at., 1990; Moldwin and Hughes, 1994a], and the Geotail spacecraft [Frank et al., 1994a, b; Machida et al., 1994a, b; Nagai et al., 1994; Lui et al., 1994].

Plasmoids have been found to be highly correlated with geomagnetic substorm activity [Hones et al., 1984; Baker et al., 1987; Slavin et al., 1989; Nagai et al., 1994]. Slavin et al. [1992] and Moldwin and Hughes [1993] found a nearly one-to-one correlation between large isolated substorms in the near-Earth region and signatures consistent with the passage of a plasmoid or a traveling compression region (TCR) in the distant tail. Over 84 % of the plasmoid events occurred between 5 and 60 rain after a substorm onset [Moldwin and Hughes,

11,409

11,410 ZONG ET AL.: ENERGETIC IoN SPECIES IN PLASMOIDS

1993]. The 15-30 min delays for plasmoid propagation to X=-100 to-200 RE was also demonstrated previously by Baker et al. [1987] and Slavin et al. [1993] using su- perposed epoch analysis on plasmoids and regression analysis on TCRs, respectively.

It also has been pointed out that the delay time is typically less than 20 rain between substorm onset and the time of the zero crossing in Bz bipolar events at X=-100 RE in the tail [Nagai et al., 1994].

Indirect observational evidence for a plasmoid passage in the distant tail has also been inferred from magnetic field signatures observed in the lobes. When a plasmoid passes downtail in the center region of the magnetotail, the magnetic flux in the tail lobes is tem- porarily compressed as the cross-sectional area of the lobe is reduced. This results in a transient increase

in the lobe field strength and a deflection of the magnetic field vector, first northward and then southward. Such signatures have been called TCRs [Maezawa, 1975; Slavin et al., 1984, 1989, 1990, 1993, 1994; Murphy et al., 1987; Owen and Slavin, 1992]. The relative amplitude of a TCR, AB/B, is usually in excess of 1%, often 5% ~ 10%. The duration time of a TCR, AT, can last from several minutes to half an hour [Slavin ½t al., 1984, 1993].

A bipolar signature in the By component of the magnetic field was first observed in a flux rope structure by $ibeck et al. [1984]. Furthermore, Moldwin and Hughes [1992] had found that 34% out of a total of 366 plasmoids had bipolar signatures in the By component. In fact, the authors found no differences between the set of "By" plasmoids and "Bz" plasmoids and concluded that they may be member s of a single class of events.

Substorm phenomena are a global electrodynamic response of the coupled magnetosphere-ionosphere system to the dynamic forces of the solar wind. A siln- ulation study of forced reconnection processes during magnetospheric storms and substorms was made by Lee et al. [1985], which was based on a constant driving force caused by the solar wind. In this case, plaslnoids will occur intermittently and repeatedly every 2-4 hours. Richard et al. [1989] presented a two-dimensional MHD computer simulation highlighting the relevance of the magnetic-island-coalescence instability for magnetotail reconnection. In particular, their simulation suggest that plasmoid formation can occur through the coalescence of several magnetic islands rather than directly through a large scale reconnection at a neutral line. However, there is so far no observational evidence to support this view.

In other substorln models the existence of a plasmoid does not imply that the formation of a near-Earth neutral line (NENL) causes the substorm. Rather, they suggest that production of a plasmoid may be one of the consequences of some other substorm onset process which'occurs at some earlier time [e.g., Kan et al., 1991; If an, 1993; Lui et al., 1988; Lopez and Lui, 1990].

None of the previous studies of plasmoids addressed (1) the occurrence and flow of energetic ion bursts in By bipolar plasmoids, (2) the occurrence and flow of energetic oxygen bursts in B• or By plasmoids, and (3) sequence of multiple energetic particle bursts with plasrnoid-like signature in conjunction with a single substorm event (possibly with multiple onsets).

This paper reports on observations made in the course of a substorm by combining data from Geotail HEP- LD (energetic ions) and MGF (magnetic field), geosynchronous spacecraft, and ground-based magnetometer. Five plasmoid-like structures (three "By" and two "B•" type plasmoids) and three groups of multiple TCRs (total of 16 TCRs) were identified by Geotail measurements during a probably isolated substorm on January 15, 1994. The average time interval between two suc- cessive plasmoids (plaslnoid centers) is found to be 1 to 2 hours. Furthermore, the observations show that the two B• bipolar plasmoid-like structures are associated with tailward flowing particle bursts; the By bipolar plasmoid-like structures, on the other hand, are associated with earthward flowing particle bursts.

The HEP-LD composition measurements showed that energetic protons were the dominant species in all plasmoid events except for one of the tailward flowing plasmoids in which the oxygen flux was dramatically increased. This oxygen ion burst exhibited a particularly confined beam structure which occupied only 6 ~ 7% of the total solid angle (4•r).

It should be mentioned that this substorm on Jan-

uary 15, 1994, occurred during a period of enhanced magnetospheric activity associated with the passage of a cotorating interaction region (CIR). Throughout the event, the solar wind speed was high and the IMF Bz was always negative (details are given in Table 2).

2. Instruments and Satellites

In this paper we use energetic ion measurements from the HEP-LD instrument on board the Geotail spacecraft. This particle spectrometer employes time-of- flight (T) and energy (E) detection systems which de- termine the mass of incident nuclear particles. Its novel concept combined with the sectored spin plane of the spacecraft allows the iina.ging of flux distributions over the complete unit sphere in phase space. The energy ranges for hydrogen, helium, and oxygen are approxilnately 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 out carbon, nitrogen and oxygen. For the purpose of this paper the CNO group is referred to as oxygen because this species is considered the major constituent. Detailed information on the HEP-LD instrument is given by •Vilken et al. [1993] and Doke et al. [1994]. Furthermore, we use magnetometer measurements from the magnetic field experiment

ZONG ET AL.' ENERGETIC ION SPECIES IN PLASMOIDS 11,411

Geotail HEP-LD 15.01.1994 MPAe/UoB Mode: s/E+T. ed B. At=30 sec WU

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Figure 1. Overview of Geota. il HEP-LD energetic particle and MGF magnetic field observatio]•s from 0000 to 0700 UT on January 15, 1994. The oxygen ion burst. is marked in black. Fronl the top the panels show: Integral counting rates for hydrogen and helium; anisotropy A, (A - 1 for the entire period), and look direction (or sector) c• for the maxin]u]n flux of hydrogen (cross- hatched vertical bars); GSE colnponents of the magnetic field and its magnitude (in nanotesla).

(MGF) on board Geotail, which makes high resolution vector field measurements. The MGF instrument is de-

scribed in detail by Kokubun et al. [1994]. Energetic electron data in the gcosynchronous orbit

were obtained from the synchronous orbit particle all- alyzer (SOPA) on board three Los Alamos.spacCcraft with name code 1989-046, 1990-095 and 1991-080. The SOPA instrument consists of three nearly identical sili- con detector telescopes, pointed a.t 30 o 90 o a. nd 120 o to the satellite 's Earth-centered spin axis. A single spacecraft rotation requires • 10 s, and in that time, 64 cuts of the unit sphere are ta. ken by the three telescopes. The SOPA energy range for electrons and protons extends from 50 keV to 1.5 McV [Belian ½t al., 1992].

3. Observations :.:

3.1. Geotail Particle and Magnetic Field Observations

Figure 1 gives an overview of the HEP-LD energetic particle a. nd MGF lnagnetic field measurements for Jan-

uary 15, 1994, 0000 to 0700 UT while Geotail was in the distant magnetotail (X6sz = -96.2 Rz, •,sz = 7.2 Rz, Z6sz =-4.6 Rz).

The first panel in Figure i shows count rate versus time profiles for ali species accumulated over particle mass (labeled "All Ions") and for oxygen ions (labeled "CNO Ions"). The All Ions curve, which is lnos[ly dolninated by protons, shows a sequence of five particle bursts between 0000UT and 0700UT (labelled P1, P2, P3, P4, and P5, respectively). Between 0247 and 0310 UT an intense oxygen burst is seen with count rates close or equal to the All ions rates.

The second panel shows count rate profiles for protons and hcliuln ions. Both count rates tend to follow

the All Ions profile. It should be noted that the proton and helium plots show a transient decrease between 0300 and 0310 UT at the time when the oxygen rates peaked.

The third panel depicts the first-order azimuthal proton anisotropy A = (M-m)/(M+m), the values M and m denote maxinmln a, nd minirotan count rates, respec-

!1,412 ZONG ET AL.' ENERGETIC ION SPECIES IN PLASMOIDS

tively. The sha.ded boxes indicate the position of the sector or look direction ct for which the maximum rate

was observed. The first particle burst, named "P 1," shows clearly a tailward look direction of t!m instrument (i.e., earthward flow) at the time when the peak rate was detected. The bursts "P4" and "P5" show a similar earthward flow direction whereas the bursts "P2"

and "P3" are a. ssociated with ta. ilward flowing particles. The la.st panel in Figure I shows the magnetic field

components (in GSE) and the total field magnitude obtained froin the MGF instrument (units in nanoteslas) during the interval 0000-0700 UT, January 15, 1994. The streaming energetic pa, r[icles and diamagnetic decreases in the magnetic field during the events indicate that Geota.il was in the PSBL or plasma sheet when the five particle bursts occurred at 0055-0115 UT (P1), 0220-0320 UT (P2), 0430-0500 UT (P3), 0551-0607 UT (P4), and 0625-0632 UT (P5). Before and between the particle bursts the magnetic field components are al- most consta. nt, B• is about 10 nT, By and B• are very sma.ll and the ion count rate is at background. This implies that Geotail was indeed in the northern lobe at those times.

HEP-LD sorts three-dimensional energetic particle data in a total of 192 contiguous angular bins (12 polar intervals over 180 o and 16 azimuthal sectors in the spin plane) which cover the unit sphere completely. The 16 polar-azimuthal angle frames in Plate I show the evolution of the particle intensity distributions for the event on January 15. Each frame is summed over 26 rain and the total time interval corresponds to that of Figure 1.

In Plate 1, the abscissa is the azimuthal angle marked Tail Dusk, Sun, Dawn, Tail, for tailward, duskward, sunward, dawnward, and tailward look directions. E ma,rks the earthward direction. The ordinate is the polar angle from 0 ø to 180 ø, 0 ø direction is equal to the q-Z direction of the spacecraft (equivalent to north N in GSE). The particle count rates per angular bin are color coded as shown on the right side. The sinall display under each frame represents an angle-angle plot with reduced angular resolution (for detail, see Wilkan at al. [1993]).

Pla.•e ! a.mplifies •he fac• •ha,• the energetic pa.r•i- cle bursts a.ppea.red in•ermi•ent, ly and repea.•edly a.nd cha.nged flow direction. They appea.r •o form "energetic ion s•rea.ms", since •hey are confined •o ra.•her na.rrow a.zimu•ha.1 a.nd pola.r angles. During •he interva.ls P1, P4, a. nd P5 •he ion bursts a.re moving ea.rthwa.rd; in con•ra.s•, •he in•erva.ls P2 and P3 show •a.ilwa.rd flow.

The LEP pla.sma. observa.fions in Figure 2 show •ha.• •he pla.sma. ions were s•rongly a.niso•ropic and were flowing in •he sa.me direction as •he energetic pa.rficles in ghe five differen• plasmoid-like s•ruct, ures, in con- •ra.s• electrons were isotropic. In •hese s•ruc•ures, bo•h plasma. ions a.nd electrons were obviously energized (Figure 2). De•a.ils of •he Geo•ail/LEP ins•ru- men• a.re given by Mukai ½t al. [1994]). The occurrence of isotropic energized electrons is •a.ken a.s evidence for

closed field line structures with in the pla.smoid [Scholar at al., 1984b; Scholar, 1986]

Inspection of Figure 1 suggests that four of the five bursty intervals can be identified as pla. smoids (P1, P2, P4, P5) a.nd one exhibits the more complex structure of a "plasmoid flux rope" (P3). All five events can be dis- tinguished from simple crossings into the quiet plasma sheet boundary layer or pla.sma. sheet by (1) the aP- pearance of compressed magnetic fields as a precursor of the event, followed by a sharp decrease in the total magnetic field intensity; (2) bipolar excursions in the B• or By field components together with the presence of impulsive increases in the energetic proton or oxygen ion flux. The ion bursts seem to be embedded in the

plasmoids. P1 plasmoid event. The first event, named P1, is

bipolar in the By component. It lasted from 0055 UT to 0115 UT as can be seen in Figure 1. The magnetic field measurements display an obvious (+/-) bipolar signature in the By component with a peak-to-peak amplitude of 4.0 nT. In coincidence with this waveform in By, the B• component shows a small peak of 3.0 nT. At the time when the By bipolar signature passed through the inflection point (a.t 0100 UT) the total lnagnetic field intensity dropped [o its minionurn value. These features agree with the morphology of a By bipolar plasmoid described by Moldwin and Hughes [1992, 1993, 19943, b]. The authors point out that such a "By bipolar" plasmoid has very similar characteristics as a "cla.ssic" B• bipolar plasmoid. The HEP-LD measurelnents show an energetic particle burst of earthward streaming H and He ions associated with the variation of the magnetic field when the plasmoid pa.ssed Geotail. This indicates that the ion burst was embedded in the plasmoid. No oxygen ions were detected in the tilne interval of the P1 event.

This event with a. bipolar excursion in the By component can be explained in two different ways: (1) A plasmoid with its symmetry axis orientated along the Z axis (GSE) propagating downtail or earthward over the spacecraft; (2) a plasmoidal structure with its symmetry axis along the X direction and propagation in the north-south direction ra. ther tha:n downtail as is nor-

really presulned [Moldwin and Hughes, 1992]. Froin Figure 1 and Plate I follows that the energetic

particle peak count rates occurred when the instrument was looking tailward, indicating a clear earthward flow. Therefore the conclusion is that this event is consistent

with a topology as described in a) which moved earthward. The sinall peak observed in the B• component with a value of/xB• - 3.0 nT exceeds the lnagnitude of the B• field in the lobe by ovdr 200%. These observations support the picture of a three dimensional plasmoid structure which deviates considerably from the usual two-dimensional classical plasmoid.

Another possible wa,y to explain the P1 event is that the spacecraft encountered the pla.sma sheet earthward of a distant neutral line. This would result in northward

ZONG ET AL.: ENERGETIC ION SPECIES IN PLASMOIDS 11,413

! !

! !

11,414 ZONG ET AL.: ENERGETIC ION SPECIES IN PLASMOIDS

GEOTAIL LEP PLOT DATE 1994. 1.15 Editor' B Mesh' HI

P1 P2 P3 P4 P5

..... , ..... I ..... ' ..... I ..... ' ..... 1: ..... ' ..... ,: ...... , .... , ..... , ..... ' ..... 01:00 02:00 03 O0 04 O0 - 05:00 06:00 07:00

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5 10 50 100 500 1000

Figure 2. Geota.il/LEP energy-time spectrograin of electrons and ions observed during the tilne interval 0000 to 0700 UT on January 15, 1994. Electron and ion data in four azimuthal directions are shown. The energy ra. nge of the spectrolneter is 8 eV to 38 keV for electrons a.nd 32 eV/Q to 39 keV/Q for ions.

Bz, ea.rthward plasma flow and earthward streaming energetic ions. However, such a. scenario is inconsistent with the observed (+/-) By wa. veform and isotropic energized electrons.

P2 plasmoid event. The second event P2 lasted froIn 0220 to 0320 UT. From 0210 to 0220 UT, as a

precursor of this pla.smoid, the total magnetic field intensity Bt was coinpressed froIn 8 to 10 nT (the Bx a.nd Bz components showed an increase as well) prior to the arrival of the pla.smoid a.t 0220 UT. This ma.y be caused by a. compression of the lobe magnetic field and lobe plasma. density in front of the moving pla.smoid. The relative increase in the field ma.gnitude is ,,• 20%. In the plasmoid the Bs and B• co•nponents show (+/-) bipolar signa.tures. The variations a.re ABu=15 nT a.nd ABz=9.6 nT. The inflection points of the Bu a.nd Bz bipolar signattires occurred at 0243 and 0226 UT, re-

spectively. There is no indication for a core field at the time of the inflection points.

This kind of pla.smoid ( bipolar signa.tures in the Bs a.nd Bz component), wa.s first investigated by Mold- win and Hu#hes [1992]. Inspection of Figtire 1 shows that during this event, the energetic particle peak count rates occurred when the instrument was looking sunward, that is, there wa.s a. clear tailward flow a. nd/or a. downtail propaga.ting structure. In pa,rticular, the ma.g- netic field of this event is consistent with a. "closed loop" pla.slnoid with a. symmetry axis roughly in the y-z plane forming a.n angle of approxima.tely 450 with respect to the +z axis [Moldwin and Hu#hes, 1992]. The inflection points of the Bz a.nd Bs bipolar signa.tures a.re not in coincidence as shown in Figtire 1, tha. t is, 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 event (from 0220 to 0320 UT), embedded in this particle burst is a short intense increase in the oxygen flux from 0247 to 0310 UT. The particle burst followed the profile of the disturbed magnetic field. The oxygen spike started 27 min after the onset of the particle burst and lasted for 23 min. The oxygen burst seems to be closely correlated with the strong negative By component which developed after a strong positive excursion. 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 a.t 0320 UT the By component was essentially negative, while Bz fluctuated with small amplitudes around zero. This is indicative for the fact that Geotail was traveling through a postplasmoid- plasma sheet (PPPs).

Another important point to be noticed is that the Bz component was about +1 nT before the arrival of the plaslnoid and assumed a value of-1 nT for a rather long time interval after the main body of the plasmoid had passed and even when the spacecraft moved in the northern lobe. This is additional evidence for (1) a continuing existence of a neutral line earthward of Geotail, (2) the connection of the post-plasmoid configuration directly to this neutral line, and (3) curved lobe field lines caused by the huge size of the plasmoid in the x-z plane.

Oxygen burst in P2 event. As mentioned above, a strong oxygen ion burst was detected from 0247 to 0310 UT in the post-plasmoid region. It started 27 min after the onset of the P2 event and lasted for 23 lnin.

Figure 3 shows distribution functions f(v) for protons, helium and oxygen ions obtained during the P2 event. The top panel, displa.ys distribution functions f0 for the time interval 0220 to 0246 UT which is void of oxygen ions. The distributions for the oxygen burst fl froin 0247 to 0310 UT are shown in the bottom panel.

The distributions f0 are straight lines in velocity space with a.pproximately parallel slopes indicating (1) equal mean velocities for the three species, (2) a species dependent temperature T m T(A) (with A denoting the particle mass). (3) proportionality of the distribution functions, that is, they can be normalized by the re- spective densities.

During the oxygen burst, however, the oxygen distribution increased substantially at small velocities as documented in Figure 3. The high-velocity tail of the distribution function, beyond 2500 kin/s, remained nearly unchanged. On the other hand, the slopes for hydrogen and helium became somewhat harder by the same factor. The oxygen peak in Figure 3 occurred at a velocity of about 1700 km/s which corresponds to 250 keV in energy space.

Although many plasmoid/fiux rope observations have been made by various satellites, this is the first time that a very large oxygen ion burst flowing tailward has

Geotail HEP-LD DE. Ed_B. Mode: stE.T. D•(.. IkV

I0'1

iO-I

io-J

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15.01.1994 02:47-03:10 UT X•M.2, Y-?.2. Z--4.6

v, Particle Speed (Kin/s)

Figure 3. Distribution fi•nctions f(v) for suprathermal H +, He and oxygen ions versus the ion speed v' (top) before the oxygen burst; (bottom) during the oxygen burst.

been found in a plasmoid. As shown in Plate 1, the O + beam was restricted to a rather small solid angle (15 o by 15 o in azimuthal and polar angle). These na, rrow beams of oxygen ions are similar to oxygen bursts found in the distant magnetotail in association with multiple flux ropes [Wilken et al., 1995].

The P3 Plasmoid Event. The third event P3 was observed by Geotail from 0430 to 0500 UT, about 70 min after P2. Inspection of Figure 4 reveals the occurrence of two plasmoidal disturbances (P3(a) and P3(b)) at the beginning and at the end of the event interval. Two closely spaced TCR signatures can be seen at about 0441 UT, approximately half way between the two plasmoidal structures.

The first disturbance (P3(a)) lasted from 0430 to 0440 UT and shows a very strong (+/-) bipolar signature in the B• component with a peak-to-peak variation of 12.4 nT. The bipolar waveform in B• is associated with monopolar peaks in By (approximately-5 nT) and B., (approximately +10 nT). The inflection point of the B• bipolar signature a.t 0432 UT coincides with the maximum values in the total field strength Bt, and h• the B., and By components.

The strong axial field at the structure's center with a duration of 74 s equals or even exceeds the field strength


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UT 0420 0430 0440 0450 0500 X -96.2 -96.1 -96.1 -96.1 -96.1 RE Y 7.1 7.0 7.0 7.0 7.0 GSE Z -4.6 -4.6 -4.6 -4.6 -4.6

Figure 4. Twelve-second averages of Geota.il energetic particle and magnetic field data from 0420 to 0500 UT for the P3 event, consisting of the duoblet p3(a) and p3(b). The inflection points of the B• bipolar signatures are centered a.t a transient maximuln of the magnetic field strength and the particle intensity. Two TCR events passed between 0440 and 0450 UT.

in the adjacent lobes. Such an intense lna.gnetic field is an important prerequisite for the center of a "force free" (i.e., J x B = 0) flux rope. The observed signatures in the field components thus appear to be more indicative for a flux rope topology than for that of a classical plasmoid with a rather weak core field [Moldwin and Hughes, 1991, 1993; Slavin et al., 1995].

A principal axis analysis (PAA) was performed to study the morphology and topology of this flux rope structure following the procedure proposed by Sonnerup and Cabill Jr. [1967]. In this analysis, the Bx* axis is defined by the direction of lnaximuln variance for the magnetic field during the time interval of inter- est. The other two axes, By* and Bz*, are oriented along directions with interlnediate and miniinure field variations, respectively. Figure 5 displays hodograms for the lnagnetic vectors in the PAA coordinate sys- rein. The hodograms were colnputed from 40 lnagnetic field vectors observed when the flux rope crossed the spacecraft between 0431:06 and 0433:03 UT. The lower frame in Figure 5 delnonstrates the large variation in amplitude and the rotation of the vector in the Bx*-By* plane. The directions of the principal axes in

GSE are described by the following eigenvectors: Bx*:(- 0.6692,-0.7344, 0.1132); By*:(-0.5631, 0.6006, 0.5677); Bz*:(0.4849,-0.3161, 0.8154).

In GSE coordinates, the eigenvector Bx* describes to a first approximation a core field in the x-y plane with an angle of 450 in the (-x,-y) sector.

The quality of the flux rope is reflected by the high value of the ratio r = 10.5 for the intermediate to the

•ninimuln eigenvalues [Sonnerup and Cahill Jr., 1967; Lepping and Behannon, 1980]. This analysis is indeed strongly suggestive of this plasmoid having a flux rope- type core field configuration [Lepping et al., 1995; Slavin et al., 1995].

As in the case of the plaslnoid P2, the magnitude of the B• component is about +1 nT prior to the plasmoid {from 0425 to 0430 UT). After the passage of the plasmoid the value of the B• component is about -1 nT for more than 10 min. The extended interval of a

southward Bz following the plaslnoid is caused by the reconnection of lobe field lines.

It. should be noted that in Figure 4 the B u component was predominant negative throughout the P3(a) event, in fact, B u was negative for the entire period considered


Principal Axis Analysis ?rinclpal Axis Coordinates

Bx,,By,,Bz,, maximum,intermediate,minimum 940115

10.0

5.0

o.o

-1o.o

10.0

5.0

• 0.0

-5.0

-10.0 -5.0 0.0 5.0 10.0

43103

43303

-10.0 • , , , -o.o A.o olo

By,

Figure 5. PAA hodograins of the magnetic vector froin 0431:0• •o 0433:03 VT (P3(a) event)in the minimum

* * B• variance coordinate system (B•.,B u, ) with the axes corresponding to the maximum, intermediate and min- iinum variances, respectively. B and E denote the begin and end of the traces.

in this paper (0000 to 0700 UT). This is consistent with a twist of the tail field caused by the IMF B u component which is indeed detected by IMP data (see Table 2).

The second part of the P3 event, called P3(b), occurred from 0450 •o 0455 UT as a relative small struc-

ture. There is vague evidence for a (+/-) bipolar signature in the Bu colnponen[ with a peak-to-peak value of 5 nT. The inflection point of the bipolar waveform occurred at 0452 UT, at the same time the total magnetic field intensity asstuned a relative maxilnum.

The energetic particles associated with the two P3 events are shown in Figure 4. Prior to the appearance of the P3(a) plasmoid energetic particles are absent, according •o the •nagnetic field observations, the spacecraft was traveling in the north lobe. However, when the core field of the plasmoid passed the spacecraft, from 0431 to 0433 UT, a short isolated proton spike was detected with a spread in time which roughly matched the core field in duration. The flow direction in the

plasmoid relative to the spacecraft is roughly duskward. Another possibility •o explain the event P3(b) is that the spacecraft entered the topology of a postplasmoid plasma sheet.

On the trailing side of the plas•noid, between 0435 and 0440 UT, the magnetic field recovered to normal lobe values and tailward streaming protons appeared indicating that Geotail crossed the PSBL on the exit to the northern lobe.

The P4 and P5 Plasmoid. From 0551 to 01532

UT, about 1 hour after the plasmoid doublet P3 (a/b) two relatively small and closely spaced plasmoids (P4 and P5) were observed. High-resolution magnetic field profiles are displayed in Figure 6.

Geotail MGF 15.01.1994 6 sec aver

_

By 0 ••P•'"""-••---• .................... .• -

.

Bz 2

ß •-• ...... ..

Bt 6

- I

UT0545 06• 0615 0630 0645 X -96.1 -96.1 -96. l -96.1 -96.1 Y 6.9 6.9 6.8 6.8 6.8 Z -4.7 -4.7 -4.7 -4.7 -4.7

Figure 6. Six-second averages of Geotail ma,g•etic field data fi'ozn 0545 to 0•45 I•T •or the P4 and P5 events. Pd is a Bs bipolar structure, P5 exhibit a Bs ";V" figure type signature. The inflection points of the Bs bipolar waveforms coincide approximately with the minimum of the total magnetic field intensity.

11,418 ZONG ET AL.' ENERGETIC ION SPECIES IN PLASMOIDS

The magnetic field variations in P4 show a rather asylnmetric (+/-) bipolar excursion in B u with a peak- to-peak value of 8.3 nT. The lnean of the very disturbed Bz component is unipolar with a peak value of about 3 nT. The P5 event, separated froIll P4 by about 30 rain, exhibits a "W" type shape (-/+/-)in the Bu colnpo- nent (5 nT peak-to-peak value) similar to observations discussed by Lui et al. [1994]. As in the cases of Pl and P4, the B, component is unipolar in nature with a.n amplitude of 2 nT. Such variations in the Bu and B, components have been discussed in the literature [Richardson eta!., 1987, 1989; Fairfield eta!., 1989]. This kind of wa.ve structure is interpreted in terms of a "two-loop" plasmoid. Th• (-/+/-) variation in the Bu field component is thought to result from a passage though the trailing half of the first pla.smoid followed by the usual (+/-) signature of the second plasmoid.

In both cases (P4 and PS), the energetic particle fluxes are confined to the plaslnoidal structures. The flux levels are small compared to intensities kequently observed in the plasma sheet; the composition is dom- inated by protons. Within the rather limited counting statistics different phases of the plaslnoids can be discussed:

Plasmoid P4: During the three main phases (leading edge 0553 to 0556 UT, center 0556 to 0558 UT, and trailing edge or boundary layer 0558 to 0605 UT) the particle flow appears to be field-aligned bidirectional or unidirectional earthward.

Plaslnoid P5: For the short tilne of the event, the particles seem to stream along the magnetic field lines in an earthward direction.

As for P1, it cannot be completely ruled out that the observed signatures are the result of a PS encounter earthward of a distant neutral line, however, the clear bipolar By waveform is evidence for the existence of a complex three-dimensional configuration passing out the spacecraft.

3.2. Ground Observations

Figure 7 shows magnetograms from four groundsta- tions for Jan.15, 1994 with clear evidence for the development of a substorm. The station Leirvogur was near local midnight (at 1.7 MLT). The isolated substorm event with potentially multiple onsets is manifested by the evolution of a sharp decrease in the H colnponents in the Leirvogur and Kiruna recordings. The substorm started at about 0100 UT ( growth phase) and reached a. maximum deviation in the H component of 640 nT at about 0235 UT as indicated in the Leirvogur magne- tograin.

The occurrence of the plasmoid/fiux rope-like events (P1 through P5) described in section 3.1 are marked in Figure 7. The plasmoids or plaslnoid-like events P2, P3, P4, and P5 a. re obviously related to this substorm event. The first event P 1 (froin 0055 to 0115 UT) rea. ched Geotail when the westward auroral electrojet of

11i00.0

10700.0

10300.0

12000.0

112200.0

11800.0

900.0

•5oo.o

lOO.0

18950.0

• 16900.0

•ee6o.o 0 2 4 6 8

JAN. 15, 1994 (UT)

Figure 7. Traces of ma.gnetic recordings of the H or X component froin four ground based stations. Heavy vertical bars mark local lnidnight, the events P! to P5 are also indicated. A black triangle marks the beginning time of oxygen burst. Leirvogur station, close to local midnight, documents substorm activity colnmenc- ing with a growth phase at 0100 UT.

this substorm just began to develop. Thus it is possible that P1 event was trailing a preceding substorm disturbance (see also section 3.3).

The rather well developed plaslnoid P2 passed Geo- tail between 0220 and 0320 UT which coincides with

the leading edge of the expansion phase in the ground magnetograins even if an estimated transfer time for the plasmoid of about, 10 rain is folded in. The oxygen burst in the P2 event was observed froin 0247 to 0310 UT when the H component in Leirvogur reached its maxilnum deviation of 640 nT.

The P3 plaslnoid, froin 0430 to 0455 UT, and the doublet P4 (0551 to 0607 UT)/P5 (0625 to 0632 UT) appeared early and late in the substorm recovery phase, respectively.

The D component in Leirvogur exhibits a very large bipolar variation (Figure 8). Such fast, (+/-/+) oscillations are called "transitional" D bays [Kamide, 1988]. The peak-to-peak variation in this event is extremely large (approximately 1200 nT) and is alldost twice as large a.s the variation of the H component. It is sug- gested that such "transitional" bays can be explained


1700.0

• t3oo.o

• 900.0 .,.,,

500.0

11500.0

• 11100.0

o

• 10700.0

10300.0

1300.0

,• 9oo.o

,• 5oo.o

100.0

•' 2950.0

• 2850.0

2750.0 o 2 4 6 8

JAN. 15, 1994 (UT)

Figure 8. The same as Figure 7, but for the D (or Y) components.

in terms of the spatial development of the current system associated with tl•e auroral electrojet in the westward expanding auroral bulge [Rostoker, 1966; I(amide, 1988]. It should be noted, however, that this sudden change in the D component did not occur at the onset time of the substorm; ratlmr, it occurred in the main- phase of the substorm. It is worth noting that this "transitional" D bay is very well correlated with the second plasmoid P2. The very large oxygen ion burst was observed during the first excursion of the D bay (cf. Figure ,1).

3.3. Geosynchronons Observations

Differential flux versus time profiles of energetic electrons from three Los Alamos geosynchronous satellites with the SOPA instrument are shown in Figure 9 for the first 9 hours of January 15, 1994. The five p!asmoids are marked in the figure. The spacecraft 'designation, the approximate conversion to local time (LT), and the energy channels are given on the right side of each panel.

The particle data indicate the development of an isolated substorm on January 15 with a moderate level of activity starting a few minutes before 0220 UT. The event was embedded in a period which was characterized by a nearly constant Kp = 4. This disturbed interval was caused by a CIR and lasted already for about 9 days since January 11.

At the onset of the event the three geostationary spacecrafts were located 1524 LT (S/C 1989-046), 0042 LT (S/C 1990-095), and at 0654 LT (S/C 1991-080). The spacecraft S/C 1990-095 detected all injection of electrons close to 0250 UT. The effect of the injection was also noticed by the satellite 1991-080 in the vicinity of 0700 LT.

The appearance of the first plasmoid P1 (from 0055 to 0115 UT) has no correspondence effect in the energetic electron observations.

The plasmoid event P2 is related to a clear intensification in the electron flux commen•:ing at 0220 UT (0040 and 0700 LT). Even the spacecraft located at the frontside of the magnetosphere (at 1525 LT) detected some fluctuations in the electron flux.

The initial phase of the substorm event in the geostationary orbit is not very well documented because of a data gap in the measurements at local midnight (tlm data gap in the S/C 1990-095 located at 0040 LT lasted from 0040 UT until 0250 UT). However, the spacecraft 1991-080 at the dawnside (0700 LT) observed an injection in all energy chammls. With reference to the bottom panel in Figure 9 the following phases of tlfis injection can be distinguisl•ed: (1) a step like dispersionless flux increase occurred shortly after 0210 UT in the low-energy chalmels, for higher energies the flux remained essentially constant. (2) About 30 minutes later, at 0240 UT, a sharp plus/minus impulsive flux variation oc9urred simultaneously in all electron energy channels, (3) At 0247 UT, at the minimum of the rather short flux variation, the mean intensity in all channels started to increase by a coinmon factor of about 6 with a slow rise time of approximately 60 minutes. (4) An impulsive injection with dispersion is superimposed on the beginning of the general flux increase. Drift echoes of this injection pulse can be identified in the four lowest- energy channels. In the highest-energy channel, 225 to 315 keV, the leading injection pulse is missing although a echo appears to be present.

After the first injection and the related drift echoes in the S/C 1991-080 observations, some of the energy channels in the S/C 1990-095 spectrometer at about 0200LT show quasiperiodic flux oscillations or multiple injections with a period of about 45 min. At about 0700 UT the activity dampened out. The phasing of the five plasmoidal events in Figure 9 indicate that there is no convincing correlation with these oscillations.

The spacecraft at local midnight (S/C 1990-095) re- sumed data transmission at 0250 UT, apparently at the onset of the intensification at local midnight. There is some evidence for multiple dispersionless injections between 0250 and 0330 UT followed by a period of flux oscillations due to temporal and/or spatial structures which ended at 0630 UT when a new data. gap started.

Out of the five plasmoid events (P1 to P5) only the event P2 is clearly associated with a major activity in the geostationary orbit and on the ground. The events P3, P4, P5, and the TCRs were detected at the G'eotail

11,420 ZONG ET AL.' ENERGETIC ION SPECIES IN PLASMOIDS

LANL Geosynchronous Energetic Electron Data January i 5. 1994

lo 8

LT - UT - 10.! M

• •SO-?SIceV k. 10 7 ' ? .............. .•. •--•

•%5Q-•25•ev

• $0 •

20 • •01

LT-•- 1.8m

•50-•5kev -' I

• •o I

2o I zo $

LT - UT ß 4J m

• •5o-?s•ev • 10 7 •75-105k•V

•0 4

x0 S 0 x 3

U•ve• •me

Figure 9. Differential electron •ata from the Los Alamos Na.tionM Laboratory (LANL) spacecrafts 1989-046, 1990-095• an• 1991-080. Energy ra.n•es• the international satdlite •es/•na.tion• a.n• the appro•ima,t.e conversion from universal time (UT) to •eo•raphic loca,1 time (LT) are •iven a,t the right of each panel. The beginning ti•ne of tl•e event P1 to P5 is indicated. Black triangle marks the onset of the oxygen burst, in P2.

position in the decay phase of the injection event. The first plasmoid event P1 passed Geotail more than an hour before the main activity started at geosynchronous distance. The conclusion is that P1 is not caused by this isolated substorm event, in fact, P1 maybe a remnant of a. preceding substorm or disturbance (a weak substorm had occurred about 2 hours prior to the current event).

3.4. Evidence for traveling compression regions (TCRs)

Traveling compression regions (TCRs) are identified by several-minutes long compressions in the field magnitude, bipolar variations in B• with a,n inflection point coincident with the maximum in Bx and Bt, and by an extended interval with Bz < 0 which is frequently observed after the passage of a TCR.

The bipolar variation in the Bz component is the result of the lobe field draping over a plasmoid in the X - Z plane. The extended interval with southward Bz trailing a TCR is caused by continuing reconnection of lobe field lines after the plasmoid was released [Slavin et al., 1984, 1989, 1990, 1993, 1994].

A total of 16 TCRs passed Geotail in the decay phase of the substorln. They appeared in the intervals between plaslnoids which were characterized by ra,ther s•nooth lnagnetic fields (positive Bx) and absence of energetic particles. This is rather good evidence for a spacecraft position in the north lobe.

The TCRs are divided into three groups according to the tilne of their appearance relative to the plasmoids: TCRs of group A appeared between the plasmoids P2 and P3. TCRs between the double structures of plasmoid P3 and in the interval between P3 and P4 are

grouped in B and C, respectively. High-resolution magnetic field data. in Figure 10 doc-

ument the magnetic signatures of TCRs in group A and C (TCRs of group B are •na. rked in Figure 4). Each TCR is characterized by impulsive excursions (several minutes in duration) in the field components; B• and Bt show a.lways compressional effects.

The paralneters of the 16 TCR events a.re listed in Table 1. The average duration of the TCRs was found to be about 2 min 28 s. The average compression relative to the background lobe field was 6.6%, and the


G½otail MGF 15.01.1994 6 $e•: aver

By.t -2

ß

.!

Ii

B! •o

UT 03•0 0400 0410 0420 0430 X -96.2 -96.2 -96.2 -96.2 -96. I

Rœ Y 7.1 7.1 7.1 7.1 7.0 Z -4.6 -4.6 -4.6 -4.6 -4.6

4 MOF-•m I i I i I - --

"',,k ..... ............... '-"-' .............. BI i• UT 0500 0510 0520 X -96.1 -96.1 -96.1 Y 7.0 7.0 6.9 RE Z -4.6 -4.6 -4.6 GSE

Figure 10. Magnetic field data (6-s averages) displayed in GSE coordinate. 14 TCRs of (top) group A and (bottom) group C are marked.

average peak-to-peak bipolar amplitude in the Bz component was 1.0 nT. Those values are in good agreement with the survey of Slavin ½t al. [1990]. As emphasized in Table 1, all 16 cases show without exception a first- northward-then-southward waveform in the Bz component.

Further insight into the structure of TCRs can be gained from examining the magnetic field in principal axes coordinates using the same technique as described for the P3 pla. smoid event. Figure 11a displays about 7 min worth of magnetic field data in principal axes coordinates for a TCR in group A which was observed in the time interval 0411:01 to 0417:55 UT.

The directions of the principal axes (i.e., the eigenvectors) in GSE are given a.s follows: (1) the maximum variation direction, Bx*=(0.5923,-0.7124, 0.3765); (2) the intermediate variation direction, By*=(-0.7434,- 0.6633,-0.0857); (3) the minimuln variation direction, Bz*=(-0.3108, 0.2291, 0.9225).

The ratio of the intermediate to minimum eigenva. lue is 12.1, indicating a rather well developed TCR event [Sonnerup and Cahill Jr., 1967; Lepping and Behannon, 1980]. The maximum relative field compression was • 15% and the duration AT • 7 min.

Another well developed TCR of group C occurred in the interval 0505:03 to 0508:36 UT. Tile result of the

principle axis analysis, based on 3 rain and 30 s the variation data. of magnetic field vectors, is displayed in Figure 11b (right). The directions of the principal axes (i.e., the eigenvectors) in GSE are (1) the maximum variation direction, Bx*=(0.47O3,-0.8783, 0.0403); (2) the intermediate varia. tion direction, By*=(-0.8142,- 0.4233, 0.3974); and (3) the minimum variation direction, Bz*=(0.3320, 0.2221, 0.9167).

In this case the ratio of the intermediate to minimum

eigenvalue was as high as 83.2. The relative mnplitude of a TCR, AB/B, is in excess of 12% and the duration AT was 3 rain and 33 s.

As shown in Figure 11, the hodograms provide views of the TCRs magnetic field variations in two orthogonal planes (beginning and end of the traces are marked with "B" and "E," respectively) and allow an estimate for the direction of motion. The minimum variance directions

for both of the two TCR events are essentially along the GSE Z axis; tile intermediate and maxilnum variance directions were mainly along the GSE X and Y axes. This may imply that the related plasmoids were moving mainly in the GSE X and Y direction with a relatively slow velocity in the GSE Z direction.

Viewed in this coordinate system, the simple nature of TCRs is apparent: the magnetic field drapes around the plasmoid. As shown in Figure 11, the intermediate and maxilnum variance of both TCRs rotates clockwise.

A previous study of TCRs [Slavin et al., 1989] found mainly counterclockwise rotating hodograms which may be indicative for the different three-dimensional struc-

ture or velocity of the plasmoid. The fact that the hodograms of these two TCRs

(one from group A, the other from group C) have similar minimum variance direction and equal direction of rotation indicates that the plaslnoids have similar characteristics (velocity, direction of motion, etc.)

Table 1. TCRs Signatures of 0000-0700 UT January 15, 1994

Tixne AInplitude Duration • s Comment B

0347:15 0.97 nT 2 min 49 s 10.0% NS

0348:59 1.45 nT 1 min 58 s 5.4% NS/- 0353:57 0.40 nT I rain 24 s 2.6% NS/- 0415:34 1.50 nT 6 rain 54 s 10.0% NS 0421:14 0.69 nT 2 min 16 s 6.1% NS 0426:02 0.88 nT 2 rain 05 s 3.4% NS 0427:38 0.62 nT I rain 05 s 4.2% NS

0440:36 0.85 nT 54 s 7.5% NS/- 0441:30 2.14 nT I rain 54 s 11.5% NS 0456:42 1.13 nT 2 rain 13 s 10.% NS 0459:40 1.10 nT 1 rain 50 s 3.4% NS

0502:44 0.43 nT 2 •nin 38 s 3.0% NS/- 0507:02 2.17 nT 3 rain 33 s 12.3% NS 0509:00 0.96 nT 2 rain 30 s 4.6% NS 0517:06 0.60 nT 3 rain 20 s 7.5% NS 0523:00 0.33 nT 2 rain 08 s 4.3% NS

Ns/-:Bz began at negative value.


Table 2. Observation Signatures of January 15, 1994, Events

Parameter P1 P2 P3 P4 P5

Time (UT) Duration (rain)

Geotail/MGF Bx

By

Sz

Bt

Geotail/HEP-LD H+ ions He ions

Oxygen Ions Burst He/H O/H Flow

Geotail/LEP Electron [on

Electron Flow Ion Flow

0055-0115 UT 0220-0320 UT 0430-0455 UT 0551-0607 UT 0625-0632 UT 20 60 25 16 7

decreasing decreasing decreasing decreasing decreasing bipolar, bipolar, Peak, bipolar, 'W' Type, 5By=4.0nT 5By=lSnT 5By=6.0nT 5By=8.3nT 5By=5.0nT Peak, bipolar, bipolar, Peak, Peak, 5Bz=2. lnT 5Bz=9.6nT 5Bz= 12.4nT 5Bz=6.4nT 5Bz=2.2nT

decreasing decreasing decreasing decreasing decreasing

burst burst burst burst burst

increasing burst increasing increasing increasing no strong beam no no no 28.5% 13.3% / 4.8% 2.9% 10.7% 19.0% 1.1% 1.9% / 15.0% 0.48% 1.1% 10-4% earthward tailward tailward earthward earthward

burst burst burst burst burst burst burst burst burst burst

isotropic isotropic isotropic isotropic isotropic earthward tailward tailward earthward earthward

Geostationary Spacecraft 1989-046(LANL) s•nooth disturb disturb slnooth smooth 1990-095(LANL) no data M. injection recovery smooth slnooth 1991-080(LANL) s•nooth injection s•nooth smooth smooth

Groundbased B Field

H or X component negative bay xninimum recovery recovery recovery D or Y component disturb bipolar recovery recovery recovery Substorm Phases growth expansion recovery recovery recovery

IMF

IMF By (nT) IMF Bz (nT)

-3.0 -5.5 -6.5 -3.9 -3.0 -2.0 -5.0 -4.0 -0.6 -0.5

Solar Wind

Solar wind speed 675 690 690 631 623 vBz(nTkm/s) 1350 3450 2760 379 312 v2Bz (nT (kin/s) 2) 9.11x10 • 23.8x105 19.0x10 • 2.39x105 1.94x10 •

010 • (10IS e = vB•sin4• ergs -1) 2.13 4.26 4.26 2.28 2.11

Again, these results are generally consistent with traveling compression regions being lobe field-draping signatures caused by a plasmoid moving in the PS be- low the spacecraft [Slavin et al., 1984, 1989, 1993]. In- spection of Figure 1 shows that energetic particles were essentially absent in all 16 TCRs. This implies that Geotail traveled indeed in the northern lobe sufficiently far from the plasma sheet and its boundary layer. The plasmoid responsible for the TCR signature was not thick enough to engulf the spacecraft.

4. Summary of Observational Facts The characteristic features of the five plasmoids ob-

served with Geotail and of the observations in the

geosynchronous orbit and on the ground collected in the course of the substorm on January 15, 1994 are listed in Table 2 to provide a synoptic view of this substorm event. IMP 8 data for the IMF and solar wind speed and values for the solar wind magnetosphere energy coupling functions (vBz), (v 2Bz) and [Perreault and Akasofu, 1978; Akasofu, 1980] are also shown in Table 2. As can be seen the values of the cou-

pling function e exceed 10 ls ergs -1 which is considered to be a threshold value for the development of substorm activity.

The observations can be summarized as follows:

1. Five plasmoids and three groups of multiple TCRs (total of 16 TCRs) were identified in the deep tail in the course of an isolated substorm on Jan. 15, 1994. These


Princ{pal Ax{s Analysis Principal Axis Coordinates

Bx,.By,.Bz,. maxlmum.lntermedla re.minimum

, , ß • (I) 04111:01.-0417:SS

, I , { , i , i ,

(b) M31:03-0•3:0•

&o

5.5

5.0

•.• ' io' •' •1o' •i•'

--•.5 ''' '' ''

-2.O 4.0

-•5 ß -4.0 -•.0

Figure 11. Geota.il magnetic field observat. ions (6-s averages) are displayed in principal axes coordinates for a typical TCR in (left,) group A and in (right) group C. B:;, By, B z axes correspond to the maximum, intermediate and minimum variances, the beginning and end of the trace are marked with "B"and "E," respectively.

multiple structures imply that the magnetosphere dissipated large amounts of energy during this intense substorm by ejecting multiple and relatively small plasmoids rather than by the for]nation and ejection of a single large plasmoid.

2. The average repetition rate for the five observed plasmoids was approximately one plasmoid every 1.,5 hours; if including sixteen TCRs, the repetition rate was every 20 min.

3. Each plasmoid was associated with an energetic ion burst.

4. The energetic particle flow was tailward in the classic Bz (+/-) bipolar plasmoids P2 and Pa(a); the energetic particle flow was predominately earthward in the By (+/-) bipolar event. s (P1, P4, PS).

,5. Geotail detected a large oxygen burst in the postplasmoid phase of event P2 as defined by the Bz waveform. At this ti•ne the bipolar variation of the By component reached its negative phase. The oxygen burst appeared in close coincidence with the injection pulse observed in the geostationary orbit. The O/H ratio reached a maximum of 1,5.0% in the energy range cov- ered by HEP-LD during the oxygen burst. This value was much higher than values usually found in the dis- rant tail (a.pproximately 1% [Lui et al., 1994])

6. The O + beam occupied only a small fraction of 6 to 7 % of the total solid angle (4•r).

7. In all plasmoids the •SBy had rather large values, and the •SBy were larger than the IMF By, except for P3 (see Table 2).

8. The events P2 and P3, although rather different in timescale, represent clear features of classic plas-

moids: bipolar signatures in Bz associated with tailward pla.sma flow and energet. ic particles st. remning. In contrast, the events P1, P4, and P5 exhibit bipolar By co]nponents and the plasma and energetic particle population move earthward.

9. The duration of the plasmoids P2 and P3 and the events P4 and P5 are 60, 25, 16, and 7 min, respectively (the event "duration" is defined by the duration of the energetic particle pulse which is approxilnately equal to the total time interval of the disturbance in the magnetic field). Plasmoid P1 (duration 20 min) was released about 1 hour before substorm onset and is

therefore probably not related to this substorm event. 10. All these structures occurred in the conditions

of high-speed solar wind streams and southward IMF magnetic field (int. eraction of the magnetosphere with an interplanetary CIR).

5. Discussion

5.1. Oxygen Ion Burst Detected in the Plas- moid P2

It is now well known that ionospheric-origin O + ions are an important and occasionally dominant part of the near-Earth plasma population [Lennartsson and Sharp, 1982; Da#lis and Azford, 1996]. In addition to numer- ous studies of the abundance of ionospheric O + ions in the plasma sheet [e.g., Lennartsson and Sharp, 1985; Da#lis et al., 1994], there have been reports on both cold and energetic O + beanis streaming tailward [Can- didiet al., 1982; Ipavich ½t al., 1985] during substorms. Recently, field-aligned tailward flowing O + beams in the energy range 3-5 keV were observed in the inner plasma sheet [Frank et al., 1996]. Outflowing ionospheric O + ions with energies up to 40 keV have been observed by the Viking satellite at altitudes about 8000 km [Lundin and Eliasson, 1991].

Oxygen ions have been observed in plas]noid/flux rope structures in the distant magnetotail but the relative abundance was comparable to the solar wind abundance [Lui et al., 1994]. However, during the present January15 even[, in the ti]ne interval 0247 to 0310 UT, a very large oxygen ion burst with a beam-like angular distribution was found in the back part of the plasmoid P2 (see Figure 3). The O + beam was restricted to a rather small spatial structure, and the flow direction was tailward.

The description of the observations in section 3 emphasized already that the oxygen burst occurred in the negative phase of a By variation and very soon after the injection onset in the geostationary orbit at 0247 UT. How did the oxygen get into the plas•noid? A conceivable process is schematically illustrated in Figure 12. Low-energy oxygen ions are extracted from the ionosphere as the result of the substorm activity. The energy of the ions is of the order of 1 to 10 keV. These ions leave the ionosphere along open field lines and [ravel tailward until they reach the reconnection region (near- Earth neutral line). After the acceleration process they


E- 1-2keV •xõ

i

I •xõ •;H•O' I

! i I i I I I I I i NENL I ! I

~ lO0 Re

• 15eol'ail

i He'

I j(O')=O

I

I

I

Figure 12. Sche•natic diagram of a pla,smoid with high oxygen abundance detected in the distant tail.

move with the high speed plasma ejected both earthward and tailward of the NENL [MJbius et al., 1987]. Geotail was tailward of the reconnection region and observed the ions (protons, helium, and oxygen ions) as they moved in a tailward direction.

The geomagnetic activity during the P2 event was rather pronounced in Leirvogur (the ABu=640 nT, /kBr• m 1200 nT). However, the Kp index was only 4- which is an indication for a relatively localized substorm event (with single or multiple onsets). Previous studies have shown that a large portion of field-aligned currents in the polar magnetosphere is carried by outflowing ions instead of precipitating electrons [L•lons and Samson, 1992; Lennartsson et al., 1985; Wablund ½t al., 1992; Da91is and Axford, 1996], thus providing a linkage between the ionospheric acceleration region and regions of field aligned currents [e.g., Lundin et al., 1994].

Before t, he arrival of the oxygen ions Geot, ail was passed by a pla.smoid (P l) as indicated by t, he magnetic field perturbation in Figure 1. Rapid field line reconnection at the near-Earth neutral line eventually result, ed in the reconnect, ion of lobe field lines form-

ing field lines disconnect, ed-from-Earth (in the literature t, his phase is oft, en termed "formatted" plasmoid, that is, the plasmoid morphology has already been formed in two dimension). These field lines are connected to the interplanetary field in the deep tail and the resulting magnetic t, ension causes the pla.s•noid to move downtail with high speed. However, the postpla.smoid field lines are still connected to the near-Earth neutral line. The

energized ionospheric oxygen ions emerging from the NENL travel downtail between the very narrow postplasmoid separa.trix and follow the plasmoid. They are detected by the HEP-LD sensor in the distant tail (Fig- ure 12). Figure 12 shows t, ha.t the oxygen is delayed by about 25 rain with reference to the appearance of the protons. This delay is probably due to the slow mot, ion of the ionospheric oxygen before they become part of the acceleration process in t, he reconnect. ion region. Assuming a. mean ext, raction energy of 2 keV for the oxygen the observed delay of 25 rain mnounts to a NENL position between 30 and 40 Rs in the tail, which is quite reasonable.

Solar wind conditions similar t,o those observed dur-

ing all these plasmoid-like events (i.e., nega. tive Bz and high solar wind speed throughout all events), would have promoted ionospheric extra.c.tion and acceleration of ionospheric O +. With regard to high solar wind speeds, it has been shown by Lundin et al. [1995] that the energy of outflowing ionospheric ions is in close rela.t, ion to the solar wind velocity, irrespective of the sign of B•. Furthermore, Daglis [1997] has argued that high- speed solar wind strea•ns during sout, hward IMF seem to enhance the efficiency of ionospheric ion acceleration and extraction. The argument is based on the strong correlation between ext, remely large values of the PC index [Troshichev et al., 1979], which is considered a, sta- tistical measure of the dawn-dusk electric field, and ex- t, raordinarily large abundance of O + in the inner plasma sheet [Daglis et al., 1995a, b].

5.2. Multiple Plasmoid For•nation The phenomenon of substorms as a •neans of energy

release in the magnetosphere may occur in a manner that is in some way analogous t,o a dripping faucet. Such a substorm model has been proposed by Hones [1979].

The classic picture of a. substorm and pla.smoid for- marion assumes the existence of an X line (NENL) tailward of the dipola. rization region and earthward of the thinning region where the Bz component is essentially nullified. Eventually, part of the plasma sheet is pinched off as a. single plasmoid. This plasmoid moves down the tail and out of the magnetospheric system at high speed and dissipates in this way a large amount of the energy stored in the magnetotail. The resid- ual plasma sheet snaps back toward Eart, h injecting energetic plasma into the inner •nagnetosphere [Hones, 1979; Baker et al., 1994]. In this paper, we propose to modify t, his single-plasmoid model in order to explain the •nultiple-plasmoid formation in the course of the present substorm event,. The magneto•neter t, race at Leirvogur, discussed in section 3.2, shows a negative bay in the H co•nponent lasting from about 0200 to 0530 UT. The first injection in the geostationary orbit occurred at 0210 UT and in the distant tail (position of Geotail) the event started at, 0220 UT. The sequence of plasmoids (P3, P4, P5) and TCRs following P2 is an indication for the repeated development of NENLs in the course of the recovery phase of the substorm event. Inspection of Figure 9 shows that no convincing one- to-one correlation can be established for the multiple plas•noids observed by Geotail and the injections observed by the geostat, ionary spacecraft except for P2. However, it is nevertheless conceivable that the chain of plasmoids and TCRs is related to the possible multiple injections between 0300 and 0500 UT in Figure 9. The lack of clear correlation may be due to unknown para•neters in the plasmoid formation and motion.

Previous simulation studies, assuming both constant and time-varying driving forces delivered by the solar wind, indicated the formation and subsequent convec- tion of magnetic islands (pla,smoids) in the magnet, otail.


The plasmoids occur intermittently and repeatedly every 2 to 4 hours [Lee et al., 1985]. Lyon et al. [1981] also showed that a near-Earth X line could occur repeatedly, the time interval between the formation of two succes-

sive near-Earth X lines was found to be 40 min. It also ha.s been demonstrated that various combinations

of the solar wind speed (V) a. nd the south component B, of the interplaneta. ry magnetic field (IMF) such as VB,, V2B, and e - t, B2sin4•l• are fairly well correlated with the geomagnetic indices AE(AU, AL), Dst and the total energy dissipation rate UT of the magne- tospl•ere [Perreault and Akasofu, 1978; Akasofu, 1980; Iyemori et al., 1979; Bake,' et al., 1981]. All the values for the coupling parameter e quoted in Table 2 for this substorm event exceed the critical limit for the neces-

sa. ry energy input. Total 21 pla.smoids and TCRs are observed in 7 hours in the observations, this gives a high repetition rate of one pla.smoid every 20 min. The repetition rate may depend on the conditions of the solar wind and the IMF. For comparison, the formation of tnagnetic islands at the dayside magnetopause tends to occur repeatedly every 8 •nin tip and Jin, 1991].

6. Conclusion

On January 15, 1994, five quasiperiodic plasmoid- like s[ructures (P1 to P5) associa[ed with energetic ion bursts and [l•ree groups of multiple TCRs were observed by the Geotail spacecraft in the deep tail (X=-96 R•) in the time interval of an isola. ted substorm event. This

observations may imply that the magnetosphere dissi- pa. ted large quantities of energy during all intense substorm by ejecting a sequence of relatively small plasmoids rather than through the formation and ejection of a single large plasmoid.

All these pla.sn•oid-like structures occurred in the conditions of high-speed solar wind streams and southward IMF magnetic field. The size of individual plasmoids (assuming in approximate equal velocity) in a multiple ejection process decreases with time after substorm onset. A decrease in size fi'mn the first to the nth TCR

in a given series of multiple TCRS is also found by a previous study [Slavin et al., 1993]. This also generally agrees with the decreasing values of the solar wind- magnetosphere energy coupling functions (vBz),(v 2 Bz), •tlld • - uZ 28i• 4 •lo 2 (cf. Table 2). These results suggest that the solar wind and the IMF have a close con- trol on the ma.glmtospheric dynamics and on the rate of magnetic reconnection. In the magnetotail, the for- marion of the plasmoids seems to be basically a. forced reconnection process, in which plusmolds can be formed intermittently and repeatedlyl

The observations show that Bz bipolar plasmoid-like structures are associated with tailward flowing particle bursts. However, earthward flowing particle bursts are mainly observed for bipolar signatures in By. The particle bursts are embedded in these magnetic structures which indicates that the ion dynamics are dmninated

by a reconnection site located tailward or earthward of the spa. cecraft. The events P2 and P3, although differen[ in timescale, represent clear features of cla,s•c plasmoids: bipolar signature in Bz associated with tailward plasma flow and streaming of energetic particles. This obviously implies that reconnection is an important process for substorm dynamics; the release of multiple plasmoids down the tail is one of the major ways of •nagnetospheric energy dissipation during an intense substorm.

In addition, an oxygen burst was seen in the largest of the plaslnoids which showed both By and Bz bipolar magnetic field signa, tures. These oxygen ions of ionospheric origin were accelerated to high energies in a reconnection region and, as a result, tra,veled downtail in the narrow postplaslnoid-plas•na sheet which trailed the plasmoid. In this way, the ionospheric oxygen ions may play an ilnporta.nt role in the substorm dynanfical processes.

Finally, the possibility that the pla.smoida.1 signatures P1, P4, and P5 result from the existence of a, distant neutral line tailward of the spacecraft ca. nnot be completely ruled out. However, the clear bipolar By waveform together with the occurrence of isotropic energized electrons in these structures make thein most likely be closed field line structures.

Acknowledgments. The Editor thanks Mark B. Mold- win and another referee for their assistance in evaluating this paper.

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S. Livi, J. Woch, B. Wilken, Q.-G. Zong, Max- Planck-Institut ffir Aeronomie, D-37191, Katlenburg-


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T. Mukai, T. Yamamoto• Institut e of Space Astronautical and Science 3-1-1, sagamihara, I(anagaWa 22,9g Japan. (e- mail: mukai@gt!isas.ac.jp )

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G. D. Reeves, Los A!amos National Laboratory, NIS-2, MS=D436 Los Alanms, NM87545. (e-mail: [email protected] v)

S. U!laland, University of Bergen, Bergen, Norway. (e- mail: [email protected] )

(Received July 16, 1996; revised December 20, 1996; accepted December 23, 1996.)

Geotail observations of energetic ion species and magnetic field in plasmoid‐like structures in...

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