multi-point in-situ and ground-based observationsrunov et al.: plasma sheet at auroral onset x - 3 9...
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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. ???, XXXX, DOI:10.1029/,
Multi-point in-situ and ground-based observations1
during auroral intensifications2
A. Runov,1
V. Angelopoulos,1
X.-Z. Zhou,1
I. O. Voronkov,2
M. V.
Kubyshkina,3
R. Nakamura,4
C. W. Carlson,5
H. U. Frey,5
J. McFadden,5
D.
Larson,5
S. B. Mende,5
K.-H. Glassmeier,6
U. Auster,6
H. J. Singer7
A. Runov, Institute of Geophysics and Planetary Physics, UCLA, 3845 Slichter Hall, Los
Angeles, CA-90095, USA. ([email protected])
1Institute of Geophysics and Planetary
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Abstract. Analysis of in-situ and ground-based observations is performed3
to establish the precise timing of observed signatures during two successive4
auroral activations. The first, minor, activation was interpreted as a pseudo-5
breakup, while the second, major one, was classified as a substorm. Timing6
of observations aboard four of the THEMIS probes situated in the plasma7
sheet in the tail-aligned conjunction indicates initial activity at XGSM ≈-8
Physics, UCLA, Los Angeles, CA-90095,
USA
2Athabasca University, Edmonton,
Alberta, Canada
3St. Petersburg State University, St.
Petersburg, Russia
4Space Research Institute, Austrian
Academy of Sciences, Graz, Austria
5Space Science Laboratory, University of
California, Berkeley, CA, USA
6Institut für Geophysik und
Extraterrestrische Physik, Technische
Universität Braunschweig, Germany
7NOAA Space Weather Prediction
Center, Boulder, CO, USA
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15RE. Magnetic field variations, tailward fast flows, and signatures in par-9
ticles behavior observed by two THEMIS probes in the mid-tail plasma sheet10
suggest magnetic reconnection as the source of the first activation. The sub-11
storm onset, detected 6 min after the pseudo-breakup, was found to be as-12
sociated with the rapid decrease of the magnetic field strength, dipolariza-13
tion, and increase of plasma density and pressure, i.e., signatures of the cross-14
tail current reduction (disruption), observed in the near-Earth plasma sheet15
at XGSM >-10RE. Thus, in this case, reconnection in the mid tail preceded16
the near-Earth current reduction. A scenario based on a model of the near-17
Earth breakup triggered by the fast Earthward flow, generated by preced-18
ing reconnection, is proposed.19
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1. Introduction
Although it is generally accepted that active auroral phenomena, such as substorms20
[Akasofu, 1964] and pseudo-breakups [see, e.g., Aikio et al., 1999], are counterparts of21
dissipative processes in the magnetotail plasma sheet, the concrete causal relationship is22
still unclear.23
It is established that the magnetospheric substorm involves processes on different tem-24
poral and spatial scales: magnetotail stretching with a time scale of tens of minutes during25
a growth phase [e.g., Voronkov et al., 2003; Petrukovich et al., 2007], a rapid dissipation26
process with a time scale of 1 - 3 min [e.g., Sergeev et al., 1996b], a breakup, and tens27
of minutes-long processes during a recovery phase [e.g., Voronkov et al., 2003]. Substorm28
manifestations in the ionosphere include an intensification and an equatorward motion of29
an auroral arc during the growth phase, an explosive-like brightening of the arc, and a30
rapid development of vortex structures with poleward expansion of those structures dur-31
ing the breakup. Substorm onset is commonly identified by ground magnetic signatures32
of the currents associated with auroral arc intensification, including irregular variations33
in the 40 - 150 s range, called Pi2 pulsations [see, e.g., Kepko et al., 2004, and refer-34
ences therein]. The term “pseudo-breakup” is used to describe an auroral disturbance35
that does not exhibit significant poleward expansion and generally ceases in 5 - 10 min-36
utes [Voronkov et al., 2003]. Pseudo-breakups are associated with the same type of Pi237
pulsations as substorm breakups [Aikio et al., 1999].38
Substorm phases may also be identified by magnetic field variations at geosynchronous39
orbit. The magnetic field on the night side becomes more tail-like during the growth phase,40
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which results in an increase of the Earthward magnetic field component at geosynchronous41
orbit. The breakup (onset of the expansion phase) corresponds to a reconfiguration of42
the magnetic field to a more dipolar-like configuration (dipolarization) [e.g., Nagai , 1982].43
It was also found that the dipolarization occurs first in a longitudinally localized sector,44
expanding westward and eastward. Thus the growth-phase signatures may co-exist with45
the dipolarization just after onset [Nagai , 1991].46
Substorm breakup is associated with a sudden decrease in the tail current intensity47
and the formation of field-aligned currents (FACs) flowing downward on the dawnside48
and upward on the duskside of a substorm meridian and forming the substorm current49
wedge [McPherron et al., 1973; Nagai , 1987]. In-situ observations have shown that the50
cross-tail current reduction (often referred to as Current Disruption, CD) usually starts51
in the near-Earth magnetotail at |X|
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supports the scenario of transient instability of a thin current sheet, forming at the end of64
the growth phase in the magnetotail plasma sheet, resulting in a spatially localized high-65
speed plasma flow with a temporal scale of 1 - 3min [Sergeev et al., 1996b]. The Earthward66
high-speed bursty bulk flow (BBF), originating in the mid-tail plasma sheet, may cause the67
cross-tail current reduction (disruption) in the near-Earth magnetotail, due to the pileup68
of the northward magnetic field during BBF’s breaking in the transition region between69
tail-like and dipolar magnetic fields [Hesse and Birn, 1991; Haerendel , 1992; Shiokawa70
et al., 1998]. More recently, it was shown by modeling that an Earthward BBF may trigger71
nonlinear ballooning instability at the inner edge of the plasma sheet [Voronkov , 2005].72
The model predicts a jump-like increase of the plasma pressure, collapse-like dynamics73
of the magnetic field (a rapid reduction of the cross-tail current) in tens of seconds, with74
subsequent tailward expansion of the unstable region.75
It is crucial therefore to pinpoint the time and the spatial location of the very first76
signatures of the activity. For the analysis, the following three requirements are essential:77
1) multi-point observations in the magnetotail, to prob the mid-tail and the near-Earth78
plasma sheet simultaneously; 2) simultaneous ground-based observations with high time79
resolution; and 3) precise mapping between the magnetotail and the ionosphere. The last80
is indeed critical and difficult to do since the statistical models of the magnetospheric81
magnetic field [e.g., Tsyganenko, 1995] may not provide a precise solution during active82
periods. Event-oriented models [cf., Kubyshkina et al., 1999, 2002] may give more adequate83
mapping. Also, the flow bursts in the plasma sheet are found to be localized in the north-84
south within
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Thus a tail-aligned spacecraft constellation is needed to record the timing and location of86
the initial activity.87
The timing of signatures during auroral activations is the primary goal of the THEMIS88
mission [Angelopoulos , 2008; Sibeck and Angelopoulos , 2008]. THEMIS employs five iden-89
tical satellites (hereafter termed “probes”) on orbits enabling recurrent probe alignments90
parallel to the Sun-Earth line (probes within YGSM±2RE from each other), referred as the91
major conjunctions. The probes thus monitor tail phenomena simultaneously from ∼10 to92
∼30RE downtail, while mapping magnetically to a network of ground-based observatories93
(GBOs) recording substorm onsets.94
In the present study, we analyze the in-situ measurements along with the images ob-95
tained by the THEMIS GBOs during a set of auroral intensifications between 0130 and96
0205UT on March 1, 2008. The main aim of this study is to establish the exact timing97
of the signatures, observed in the mid-tail (-17> X >-23RE), the near-Earth (X∼10RE)98
plasma sheet, and at geostationary orbit and associate them with distinct auroral inten-99
sifications (a pseudo-breakup and a substorm) detected by GBOs.100
2. Instrumentation
Each THEMIS probe carries identical scientific equipment [Angelopoulos , 2008]. Data101
from the following instruments aboard the THEMIS probes are used in this study: 1)102
the Flux Gate Magnetometer [FGM, Auster et al., 2008], providing DC magnetic field103
measurements with a temporal resolution of 1 vector per spin (∼3 s, spin-fit data set) and104
4 vectors per spin (fast survey mode). The fast survey mode was maintained during the105
entire event. 2)The Electrostatic Analyzer [ESA, McFadden et al., 2008], providing ion106
and electron distribution functions in the energy range from 5 eV up to 25 keV with a107
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time resolution of 1 distribution function per spin in the fast survey mode. 3)The Solid108
State Telescope [SST, Larson et al., 2008], detecting high-energy (25 keV - 1MeV) ion109
and electron fluxes with a time-resolution of 1 distribution function per spin in the fast110
survey mode. The set of THEMIS Ground-based Observatories provides complimentary111
data including images from the all-sky cameras (ASI) with a time resolution of 1 image112
per 3 s and magnetometers that record the 3-axis variations of the magnetic field at a 2Hz113
frequency [GBO, Mende et al., 2008].114
Additionally, the magnetic field data from the two NOAA Geostationary Operational115
Environmental Satellite (GOES 10 and GOES 12) [Singer et al., 1996] were used in this116
study. The time resolution of the GOES magnetometers is 1 vector per 0.5 s.117
3. Event overview
The auroral activity was registered by the THEMIS all-sky camera array starting at118
0148:42UT. Figure 1 shows mosaic images from the all-sky cameras in Eastern Canada,119
with foot points of four THEMIS probes (P1 - P4) and two geosynchronous satellites120
(GOES-10 and GOES-12) determined using T96 model [Tsyganenko, 1995]. The UT of121
the aurora onset registrations are listed in Table 1. The first, minor, intensification was122
detected at the easternmost station (GBAY). The aurora expanded westward to TPAS.123
No poleward expansion was detected, which allows us to classify this intensification as a124
pseudo-breakup. The next, major, intensification onset was detected first at 0152:27 at125
SNKQ. The auroral activity expanded azimuthally and poleward and may, therefore, be126
classified as a substorm.127
The keogram from the all-sky camera located at Thykkvibaer, South Iceland, shows128
continuing equatorward motion of the aurora until 0156UT (growth phase) with a local129
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intensification at 0149UT (not shown; E. Woodfield, private communication). Thus de-130
spite a lack of data eastward of GBAY, the minor intensification onset at 0148:42 seems131
to be the very initial activation.132
Figure 2 shows the history of the integrated brightness (image totals) of all-sky images at133
a set of stations located from east (GBAY) to west (GILL). The first minor intensification134
is pronounced at the GBAY record (bottom panel). The integrated brightness jumped at135
0148:42UT then decreased, but did not drop down to the pre-intensification level.136
Figure 3 shows the solar wind velocity (GSM coordinate system), dynamic pressure, and137
IMF GSM components from the WIND spacecraft (X=198.6, Y =-37.2, and Z=-41.4RE),138
along with original and band-passed filtered (40 - 150 s) magnetograms from the set of139
geomagnetic stations between 280 and 300◦ geographic longitudes. The vertical dashed140
lines indicate times of the two auroral onsets: the minor one, detected at 0148:42UT at141
GBAY, and the major, observed at around 0155:00.142
The solar wind velocity was high (Vx ≈-750 km/s) and with large variations of Vy and143
Vz (up to 150 km/s). However, since the solar wind density was on average 2.5 1/cm3,144
the dynamic pressure was modest, varying between 2 and 3 nPa. At the first onset, Vy145
changed from 0 to ∼60 km/s, and Vz reversed from ∼20 to ∼-20 km/s. At the second146
onset, Vy reversed from ∼60 to -25 km/s, and Vz - from -20 to 50 km/s. The IMF Bz was147
southward (-6 nT) during both the minor and the major disturbance onsets. The IMF By148
reversed from dawnward (negative) to duskward (positive) at around the first onset and149
from duskward to dawnward at around the second one.150
Ground-based magnetometers in GBAY, DRBY (54.5 mlat, 6.4 mlon), and, LOYS (50.6,151
357.2) detected positive By variations and distinct onsets of Bx pulsations in the PI2 range152
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between 0148:40 and 0149:00UT. CHBG, situated westward, detected By variations at153
∼0151 and the distinct Pi2-pulsations onset at 0156:30UT. No distinct magnetic field154
variations were detected by KUUJ until 0155:30UT. A negative Bx bay at the second155
onset was registered.156
Figure 4 shows positions of the five THEMIS probes (P1/THB, red; P2/THC, green;157
P3/THD, cyan; P4/THE, blue, and P5/THA, magenta) and GOES 10 (asterisk) and158
GOES 12 (plus) satellites. The THEMIS probes were in the pre-planned major con-159
junction: P1 - P4 were at 5.9< YGSM
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Vz (see Figure 3) and, most likely, were solar-wind induced. The two innermost probes,176
initially staying at a Bx level of 20 nT, exhibited a gradual increase of Bx up to 40 nT177
starting at 0110UT. The increase of Bx was accompanied by a decrease of By from -10 nT178
at P3 and -17 nT at P4 down to -30 nT at P3 and -40 nT at P4. This extra-flaring (up179
to ∼45◦) indicates an unusual state of the magnetosphere during our interval of interest.180
The geosynchronous satellites GOES-10 and GOES-12 detected a gradual increase of the181
radial magnetic field component (Be) starting at 0115UT. The plasma sheet state at182
P1 and P2 locations (X=-22.7 and -17.4RE, respectively) changed abruptly at around183
0149:00UT, when first P2 (at 0148:40UT) then P1 (0149:30UT) detected an arrival of184
fast (Vx ≈-500 km/s) tailward-duskward bulk flow accompanied by a sharp variation of185
By (negative at P2 and positive-then-negative at P1) with an amplitude of 15 nT and a186
bipolar north-then-south variation of Bz. The tailward flow was detected within ∼2.5 min187
by both probes. The two near-Earth THEMIS probes (P3 and P4) detected only minor188
(of ∼5 nT amplitude) variations of Y and Z magnetic field components starting at about189
0148:30UT (see Fig. 12). Bx as well as the total field increased continuously, indicating190
a build-up of the total current in the plasma sheet.191
At 0154:40UT, P2 detected a large negative variation of By, with an amplitude of192
20 nT, a positive variation of Bz with an amplitude of 5 nT, and the onset of tailward-193
dawnward bulk flow. P1 detected an intensification of the tailward flow at 0154:40UT and194
significant By and Bz (both negative) variations at 0155:10 and 0155:25UT, respectively.195
The situation in the near-Earth plasma sheet also changed dramatically: at about 0154:50,196
P4 (the most Earthward probe) detected an intensification of the ion flow and a rapid197
decrease of Bx and |By|, accompanied by an increase of Bz modulated by ∼5 nT-amplitude198
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variations. Similar signatures were detected by P3 starting at 0155:30UT. GOES-10 and199
GOES-12 started to detect a dipolarization (increase of Bp) with a decrease of Be and200
variations in Bn at about 0155:00UT, i.e., roughly simultaneously with the magnetic field201
variations detected by P4.202
4. Detailed Data Analysis
4.1. Dynamics at -23< X
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grams. Prior to the onset, P2, situated near the neutral sheet, detected By=-3.54±1.04 nT,219
and Bz=2.08±0.31 nT.220
At around the first onset, P2 started to detect ion and electron energization with the221
enhanced flux of super-thermal and thermal tailward streaming ions. At the same time,222
some increase of Bx (of ∼10 nT amplitude), the negative By (of 20 nT amplitude) and the223
bi-polar north-south Bz (of ∼10 nT amplitude) variations were detected by P2/FGM. The224
particle density decreased down to 0.05 cm−3, the bulk flow velocity reached -500 km/s,225
the plasma pressure first increased then decreased, while the magnetic pressure increased,226
and total pressure (Pt = Pi + Pe + B2/2µ0), calculated from both ESA and SST inputs,227
also first increased then decreased. The pressure variations indicate plasma and magnetic228
field compression on the front of the tailward flow burst. The dawnward By, along with229
the positive Bx , the southward Bz, and the tailward bulk flow may be interpreted as a230
signature of the quadrupolar magnetic field due to the Hall effect [e.g., Runov et al., 2003],231
suggesting the reconnection-related origin of the tailward fast flow. The flow burst contin-232
ued for about 2 min, which is a typical time scale for elementary dissipative processes in233
the plasma sheet [Sergeev et al., 1996b; Runov et al., 2008]. The observed bulk velocity of234
500 km/s is about 70% of the Alfven speed, calculated for B=10nT (the average magnetic235
field strength at the first tailward flow onset) and the density of 0.1 cm−3.236
The same sequence of signatures was observed by P1 about 40 s later than by P2, but237
the By variation is bipolar (first positive then negative) with an amplitude of 15 nT. This238
variation of By may hardly be explained by the Hall effect. Likely, the spacecraft crossed239
a part of a complex 3-D current system with a local vertical current filament, transported240
by the tailward plasma flow.241
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Figure 8 shows the Φ-spectrograms of SST and ESA ion fluxes along with normalized242
fluxes in several energy channels during 0148:00 - 0149:30 UT at P2 and during 0149:00 -243
0150:00 at P1 plotted versus UT. Signatures observed by both probes are similar, includ-244
ing a pronounced high-energy (>30 keV, SST) flux anisotropy with an enhanced flux in245
the duskward (270◦) direction, gradually turning to the tailward (180◦) direction, accom-246
panied by the spectral density increase at around 0148:40UT (i.e., at the onset marked247
with the vertical dashed bar) at P2 and at 0149:20UT at P1. The thermal ion flux in the248
tailward (180◦) direction increased at about 0148:45UT at P2 and at 0149:30UT at P1.249
The Inverse Velocity Dispersion (IVD) [Sarafopoulos and Sarris, 1988] of the high-energy250
ions was observed by both probes: the flux of ∼60 keV ions exhibited the increase earlier251
than the fluxes of 90, 130, and 200 keV ions. An onset of the 60 keV ions flux increase was252
observed at P2 at about 0148:25UT, i.e., 15 - 20 s prior to the auroral onset. P1 observed253
the increase of the 60 keV ions flux at about 0149:10UT.254
A similar IVD was detected by P1 during the second tailward flow burst between 0154:00255
and 0155:30UT (not shown). The ∼60 keV ion flux began increasing at 0154:45UT.256
However, no pronounced velocity dispersion (either direct or inverse) was detected at P2,257
which rapidly crossed the current sheet during the second tailward flow interval.258
The thermal ion flux at Φ ≈180◦ (tailward bulk flow) increased between 0148:45 and259
0148:50 at P2 and between 0149:25 and 0149:30 at P1 without any visible dispersion. This260
lag time suggests tailward propagation velocity of 800 km/s, which is 40% faster than the261
observed tailward bulk velocity.262
Figure 9 presents pitch-angle distributions (PAD) of SST and ESA electrons, obtained263
by P2 between 0147:00 and 0150:00UT and P1 between 0148:00 and 0151:00UT. P2264
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detected a pancake-like PAD of high-energy electrons until 0147:20UT, staying at the265
neutral sheet. Between 0147:20 and 0148:25UT the bi-directional (0 and 180◦) SST-266
electrons PAD was observed. At 0148:25UT (about 20 s prior to the first auroral onset),267
an enhancement of the 0◦ and 180◦ super-thermal electron flux was detected. The PAD268
changed again to the pancake-like with a transient enhancement of duskward-directed269
(270◦) flux between 0148:25 and 0148:40UT. After 0149:10UT, the high-energy elec-270
tron flux was mainly anti-parallel to the magnetic field. Thermal electrons with energies271
between 0.5 and 21 keV exhibited a drop in the flux between 0147:20 and 0148:40UT.272
The flux increased up to a maximum at about 0149:00UT, remaining isotropic. After273
0149:10UT, P2 detected the bi-directional thermal-energy electron flux with a somewhat274
larger density in the anti-parallel direction. The low-energy (0.1 - 0.5 keV) electron flux275
was detected to be bi-directional (0 and 180◦) between 0147:10 and 0148:20UT, during276
a local maximum of the magnetic field. Between 0148:20 and 0148:40UT, P2, staying277
at Bx=3nT, detected a mainly anti-parallel low-energy electron flux. The flux decreased278
between 0148:40 and 0149:00UT, coinciding with a minimum (maximum value) of the279
negative By variation. A local increase of first a parallel, then a bi-directional low-energy280
electron flux, was detected between 0149:00 and 0149:15UT.281
Similar signatures were observed by P1 starting at 0149:00UT. It is interesting to note282
an increase of bi-directional high-energy electron flux, coinciding with the positive By283
variation. Another interesting feature is the observation of an enhanced flux of parallel284
thermal energy electrons coinciding with an increase of mainly anti-parallel flux of the low-285
energy electrons between 0149:10 and 0149:45UT. Since the probe was in the southern286
part of the sheet detecting the tailward bulk flow, these observations may be interpreted287
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as signatures of low-energy electron inflow and thermal-energy electron outflow, expected288
from the Hall reconnection model [e.g., Hoshino et al., 2001].289
Thus summarizing the observations at -23< X
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magnetic field and the plasma pressure jump were detected first by P4, then by P3, this310
front propagated tailward (see Figure 4) at a velocity of 150 km/s.311
Notable features, common for P3 and P4 observations between the two onsets, should312
be highlighted. 1) the plasma and total pressure were gradually decreasing starting at the313
first onset, while the magnetic pressure was gradually increasing. 2) the plasma density314
decreased starting at about 0152UT until the step-like increase. This density decrease is315
also seen in ion and electron spectrograms. The density decrease was preceded by a local316
increase of the super-thermal ion flux, visible in SST Φ-spectrograms, with a spectral317
maximum gradually shifting from 0 (360◦ to 180◦, the latter corresponds at P3 and P4 to318
Earthward flowing ions). It is also worth noting a tailward (0◦) ion beam detected by SST319
aboard P4 at 0153:00UT, corresponding to local variations of the magnetic field, plasma320
and total pressures.321
Figure 12 presents magnetic field measurements at P3 and P4 in the field-aligned frame322
along with magnetic field variations measured by two geosynchronous satellites GOES-10323
and GOES-12 in the PEN coordinate system. The two dashed bars mark the first and324
second onsets. The main magnetic field (Bfacz) at P3 and P4 grew continuously until325
the second (major) onset, while the behavior of Bfacx, directed vertically, and Bfacy,326
directed duskward along YGSM , changed at around the first onset. Namely, Bfacy at both327
probes experienced a positive (i.e., duskward) variation with an amplitude of about 7 nT328
between the two onsets. This variation may be interpreted as a signature of a field-329
aligned current (FAC) sheet underneath the probes. The sign of variation suggests the330
anti-parallel direction of the current, i.e., outward FAC. Bfacx started wiggling without a331
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pronounced growth at the first onset, and exhibited a large positive variation up to 20 nT332
(dipolarization) starting at the second onset.333
GOES-10, mapped to the ionosphere close to P3 and P4 (Figure 1), started detecting334
a large-amplitude (up to 27 nT), two-peak variation of the eastward component (Bn) of335
the magnetic field at about 0148:45UT (first onset). Again, interpreting the eastward Bn336
variation as a signature of outward FAC, flowing above the satellite, the pair of P3/P4 and337
GOES-10 were bracketing the FAC in the north-south direction. The earthward compo-338
nent, continuously increasing prior to the first onset, also exhibited significant variations,339
first a transient decrease, coinciding with the first maximum of Bn, then an increase,340
followed by a decrease, coinciding with the second maximum of Bn. No significant varia-341
tions of the polar Bp were detected until the dipolarization at the second (major) onset.342
GOES-12, mapped about 10◦ westward of GOES-10, detected a rather minor variation of343
Bn about 20 s after the first onset. A drop of Be and strong dipolarization, accompanied344
by large-amplitude variations of Bn were detected by GOES-10 and GOES-12 roughly345
simultaneously at the second onset.346
4.3. Timing of observed signatures
Table 2 summarizes timing of the observed signatures. The very first indication of an347
activity was observed by P2 (event #1 in Table 2), located at X=-17.4RE at 0148:25UT,348
i.e., 17 s prior to the minor auroral intensification detected by the all-sky camera at GBAY349
(event #4). Spacecraft, located in the near-Earth plasma sheet, detected signatures of an350
upward FAC (event #3) a couple of seconds prior to the minor auroral intensification.351
The second, major, intensification, observed at KUUJ (event #17) was associated with352
the dipolarization and the current reduction, observed at geostationary orbit (events #10,353
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11) and at X=-8.1 and -9.3RE (events #11, 14, 17) . The tailward flow, accompanied by354
significant magnetic field variations, was observed by P2 at X=-17.4 (event #12) and by355
P1 at -22.7RE (event #15) roughly simultaneously with the dipolarization at X=-8RE,356
suggesting independent processes acting in the near-Earth and mid-tail plasma sheet.357
5. Discussion
Before the discussion, it is important to remember that the main aim of this work is358
to establish the precise timing of the observed signatures, and locate the first indication359
of the activity in the magnetotail. All the observations suggest a possible scenario of the360
activity development, which is described at the end of the Discussion.361
5.1. When and where did the activity start?
The analysis of in-situ measurements reveals that the first indication of the activity in362
the plasma sheet was detected by P2 at X=-17.4RE about 20 s prior to the minor auroral363
intensification and the onset of Pi2 pulsations, detected on the ground. It should be noted,364
however, that according to the T96 model, P2 mapped eastward of GBAY, where the first365
auroral brightening was detected (Figure 1). Application of the event-oriented magnetic366
field model [Kubyshkina et al., 1999] does not change the longitudes of the spacecraft foot367
points, but shifts their latitudes ≈2 degrees equatorward. Thus the 20 s delay should be368
treated with the caution: it is a time delay between the energetic particles’ burst arrival369
in the field of view of the detector aboard P2 and the aurora appearance in the field of370
view of the GBAY all-sky camera. The activity in the plasma sheet might start earlier371
than its signatures were detected by the spacecraft.372
D R A F T August 27, 2008, 10:35am D R A F T
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X - 20 RUNOV ET AL.: PLASMA SHEET AT AURORAL ONSET
Activated prior to the first minor onset, the plasma sheet at X =-17 - -23RE remained373
in an active state (see Figures 6 and 7). Thus the time of the first activity indication,374
detected by P2, gives a rough estimate of when the plasma sheet became activated and375
may be adopted as the time-zero of the entire event, including both onsets.376
The first indications of the activity observed by P2 (see Table 2, #1, 2) were the377
enhancement of the bi-directional (0 and 180◦ PA) flux of the energetic (E >30 keV)378
electrons and the appearance of a tailward-directed super-thermal ion flux. The inverse379
velocity dispersion (IVD) was observed during the tailward burst of high-energy ions: the380
enhancement of the 60 keV ion flux was detected about 20 s earlier than that of the 200 keV381
ions. The IVD of high-energy ion bursts is a frequently observed phenomenon in the382
distant magnetotail [e.g., Sarris et al., 1996]. Also, observations of the energetic ion IVD in383
the near-Earth plasma sheet were reported [Lui et al., 1988]. Two alternative explanations384
of the IVD were suggested: 1) a spatial model [Sarafopoulos and Sarris, 1988], implying385
that the spacecraft cross the plasma sheet toward the plasma sheet boundary layer (PSBL)386
in the vicinity of X-line (low energy ions are carried deeper inside the PS, while higher-387
energy ones remain on the outer lines on the PSBL); 2) and a temporal model [e.g.,388
Taktakishvili et al., 1993], suggesting particle acceleration by an inductive electric field389
in the course of magnetic reconnection (tearing mode instability). It was shown that390
particles of higher energy are “born” in a later stage of the instability development than391
smaller-energy ones [Taktakishvili et al., 1993]. Since in our case the ion IVD was observed392
deep inside the plasma sheet (see Figures 7 and 8), the temporal explanation seems more393
suitable. The model gives us the possibility to make a quantitative estimation of the394
distance between the acceleration region (AR) and the spacecraft location. Let us assume395
D R A F T August 27, 2008, 10:35am D R A F T
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RUNOV ET AL.: PLASMA SHEET AT AURORAL ONSET X - 21
that all ions are protons and the length of the AR is of 10∗1/ωpi=7210 km (ni=0.1 cm−3)396
[e.g., Pritchett , 2001]. Setting an instability growth time equal to the time between397
the onset of the flux increase and the flux maximum (≈30 s, see Figure 8), B0=40nT,398
b0z = Bz(ts)/B0=0.25, where ts is an instance of the maximum flux, and using equation399
(14) of Taktakishvili et al. [1993], we obtain the estimation of the critical distance from400
the source, within which the IVD may persist, Sc=5RE. Comparing the lag time between401
the flux maxima of 60 and 200 keV ions (≈20 s) with the model calculations [Fig. 3 in402
Taktakishvili et al., 1993], the distance between the AR and the probe is estimated to403
be about 1 - 3RE . Thus the activity started at X ≈-15RE. Observed signatures of the404
Hall-related magnetic field and the electron inflow, mentioned above, suggest collisionless405
magnetic reconnection as a driver of the activity. Since no Earthward flow was observed,406
reconnection, if any, was strongly transient: no quasi-steady tailward retreating X-line407
was formed.408
5.2. What happened in the near-Earth plasma sheet?
The observations at the two near-Earth THEMIS probes (P3 and P4) and at the geosyn-409
chronous satellites show a continuous increase of the magnetic field until the onset of major410
activation (substorm). Both THEMIS probes were above the neutral sheet, at the Bx-level411
of 40 nT. This may explain why no distinct signatures of an earthward outflow, expected412
from reconnection, were observed by P3/P4: the earthward flow might be collimated in413
a narrow flow channel in the vicinity of the neutral sheet. Alternatively, the earthward414
outflow might be broken at larger geocentric distances.415
Transient FAC signatures with a pattern similar to a localized current wedge, i.e., FAC416
out of (into) the ionosphere at the dusk (dawn), can be observed at the leading edge of417
D R A F T August 27, 2008, 10:35am D R A F T
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X - 22 RUNOV ET AL.: PLASMA SHEET AT AURORAL ONSET
the fast flowing plasma [e.g., Sergeev et al., 1996a]. Shear in the magnetic fields and flows418
between the leading edge region and the main body of the fast flow produces an FAC .419
Simultaneous in-situ and ground-based observations support this current wedge pattern420
associated with bursty flows [Kauristie et al., 2000; Grocott et al., 2004]. The FAC region421
on the duskside of a bursty bulk flow, where the current is flowing out of the ionosphere,422
has been identified consistently with the electron precipitation region in auroral images423
[Nakamura et al., 2001b; Sergeev , 2004]. In our case, the outward FAC was detected by424
P3, P4, and GOES-10 between the first intensification and the substorm onset (Figure 12,425
Table 2, #3). Since the FAC started to be observed about 6 min before the increase of426
Bz (dipolarization) and decrease of Bx, it can not be attributed to the current wedge427
[McPherron et al., 1973; Hesse and Birn, 1991]. P3, P4 and GOES-10 were located in428
the duskward sector and mapped westward of the initial auroral intensification. These429
FAC signatures may be hypothetically interpreted as a remote sensing of the Earthward430
fast flow resulting from reconnection [Nakamura et al., 2001b, 2004a]. (Note that this431
interpretation does not imply that the Earthward fast flow reached X ≈-8RE , where the432
FAC was detected; see Fig. 7 in [Nakamura et al., 2001b].)433
Signatures observed by P3 and P4 at around the second (major) onset, including first a434
plasma density and pressure decrease, then a rapid jump-like increase, dipolarization and435
reduction of the magnetic field simultaneously with bursty ion and electron energization,436
may be interpreted in terms of the cross-tail current reduction or disruption (CD) [Jacquey437
et al., 1991, 1993; Ohtani et al., 1992]. Note that the term “CD” is used to describe the438
observed complex of signatures without a reference to any particular CD model. The439
aforementioned density decrease may indicate thinning of the current/plasma sheet prior440
D R A F T August 27, 2008, 10:35am D R A F T
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RUNOV ET AL.: PLASMA SHEET AT AURORAL ONSET X - 23
to the CD. The dipolarization first was detected by GOES-12, mapped onto the field441
of view of SNKQ (Figure 1), where the substorm onset was observed, then, roughly442
simultaneously, by GOES-10 and P4 and, finally after about 40 s, by P3. Therefore,443
the dipolarization expanded azimuthally and tailward. The plasma pressure jump was444
observed first by P4, then by P3. Thus the pressure front propagates tailward at a velocity445
of 150 km/s, which is consistent with the previous CD observations [Jacquey et al., 1993]446
but larger than the reported velocity of tailward retreat of a dipolarization [Baumjohann447
et al., 1999]. The tailward flow, detected by P1 and P2 roughly simultaneously with the448
CD, was generated by the local source, presumably reconnection acting independently449
from the CD.450
5.3. A scenario
According to the integrated brightness from ground-based images (Figure 2) the first451
intensification may be interpreted as a precursor of the second, major, intensification (sub-452
storm). The precursor onset was most likely caused by magnetic reconnection on closed453
field lines, creating tailward (detected by P2 and P1) and Earthward (missed) outflows.454
The FAC, detected by P3, P4, and GOES-10 may hypothetically indicate the presence of455
the Earthward outflow from the reconnection site. The earthward flow, broken between456
X=-15 and -10RE, may lead the magnetic field pile-up and trigger an instability in the457
near-Earth plasma sheet [Haerendel , 1992; Shiokawa et al., 1998]. If the plasma/current458
sheet was thinning prior to the CD (the density decrease) then an instability, triggered459
by the Earthward flow, seems more adequate than the pile-up effect only. The jump-460
like increase of the plasma pressure indicates a non-linear evolution that predicted by461
the coupling model of a near-earth breakup based on the nonlinear ballooning instability462
D R A F T August 27, 2008, 10:35am D R A F T
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X - 24 RUNOV ET AL.: PLASMA SHEET AT AURORAL ONSET
triggered by an Earthward fast flow [Voronkov , 2005]. More comprehensive data analysis463
and modeling, exceeding the scope of the present work, are needed to define the exact464
mechanism of the instability causing the major auroral intensification.465
6. Conclusions
We have analyzed simultaneous spacecraft and ground-based observations during two466
successive auroral intensifications: minor (pseudo-breakup) and major (substorm) de-467
tected in 6 min. The data from the tail-aligned THEMIS constellation and the geosyn-468
chronous satellites were used. The main results of the data analysis are the following:469
1. The observed pseudo-breakup was caused by a dynamic process generating the in-470
ductive electric field at X ≈-15RE. The first signature of the activity in the magnetotail471
plasma sheet was observed at X=-17.4RE about 20 s prior to the auroral intensification.472
The character of the observed tailward bursty bulk flows, associated magnetic field vari-473
ations, and the particle behavior suggest transient magnetic reconnection as the source.474
The activity in the mid-tail plasma sheet started earlier than that in the near-Earth475
plasma sheet.476
2. The substorm onset detected 6 min following the pseudo-breakup was associated477
with the rapid reduction of the cross-tail current, the magnetic field dipolarization, and478
the jump-like increase of the plasma pressure, observed in the near-Earth plasma sheet at479
-9.4≥ X ≥-6.6RE.480
3. Signatures of the field-aligned current were observed in the near-Earth magnetotail481
between the pseudo-breakup and substorm. These signatures are hypothetically inter-482
preted as a remote sensing of the transient Earthward flow, resulting from reconnection.483
D R A F T August 27, 2008, 10:35am D R A F T
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RUNOV ET AL.: PLASMA SHEET AT AURORAL ONSET X - 25
This interpretation suggests that the instability in the mid-tail plasma sheet (reconnec-484
tion) triggers the subsequent cross-tail current reduction (disruption), observed in the485
near-Earth plasma sheet.486
Acknowledgments. This work was supported by NASA grant NAS5-0299 and Ger-487
man Ministry for Economy and Technology and the German Center for Aviation and Space488
(DLR), contract 50QP0402. The work of I.O.V. was supported by the Natural Sciences489
and Engineering Research Council of Canada (NSERC) and the Canadian Space Agency490
through e-POP and contracts to Dr. A. Yau and Dr. M. Connors at the University of491
Calgary and Athabaska University. We acknowledge J. Wild and E. Woodfield, providing492
all-sky images from Thykkvibaer, Iceland. We thank V.A. Sergeev, T. Nagai, A. Roux,493
C. Jacquey, M. Ashour-Abdalla, R. Walker, and M. El-Alaoui for fruitful discussions, B.494
Kerr, P. Cruce, and A. Prentice for help with the software and paper edition.495
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X - 32 RUNOV ET AL.: PLASMA SHEET AT AURORAL ONSET
THEMIS GBO/ASI
2008-03-01/01:48:50
P1P2
P3P4
G10G12
GBAY
KUUJ
SNKQ
CHGBKAPU
PINA
TPAS
GILL
RANK
P1
P2
P3P4
G10G12
GBAY
THEMIS GBO/ASI
2008-03-01/01:54:30
KUUJ
SNKQ
CHGBKAPU
PINA
TPAS
GILL
RANK
Figure 1. THEMIS all-sky camera observations at 0148:50 UT (minor intensification)
and at 0154:30 UT (major intensification) on March 1, 2008. Foot points of THEMIS
probes, GOES-10 and GOES-12 are shown by circles, asterisk, and plus, respectively.
D R A F T August 27, 2008, 10:35am D R A F T
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RUNOV ET AL.: PLASMA SHEET AT AURORAL ONSET X - 33
Table 1. Geomagnetic coordinates and timing of minor (4th column) and major (5th
column) onsets observations
GBAY 60.73 23.08 01:48:42 01:55:36
KUUJ 66.89 13.23 01:48:51 01:55:21
CHBG 59.57 3.62 01:49:39 01:56:33
SNKQ 66.45 356.99 01:50:54 01:52:27
GILL 66.18 332.78 01:51:18 01:54:21
TPAS 63.27 323.80 01:51:21 01:54:30
0
2•104
4•104
6•104
01•10
42•10
43•10
44•10
45•10
4
02000
4000
6000
8000
10000
02•10
4
4•104
6•104
8•104
1•105
0130 0140 0150 0200 0210
0
5.0•103
1.0•104
1.5•104
hhmm2008 Mar 01
GILL
SNKQ
CHBG
KUUJ
GBAY
Ima
ge
To
tals
,A
rbit
rary
Va
lue
s
Figure 2. Integrated brightness of all-sky camera images at several stations from East
(bottom) to West (top).
D R A F T August 27, 2008, 10:35am D R A F T
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X - 34 RUNOV ET AL.: PLASMA SHEET AT AURORAL ONSET
WINDVx, km/s
Vy, km/s
Vz, km/s
Pd, nPa
B, nTBx
By
Bz
B, nT
GBAYδB, nT
DRBY
B, nT
δB, nT
LOYS
B, nT
δB, nT
KUUJ
CHBG
B, nT
B, nT
δB, nT
Figure 3. From top to bottom: GSM components of the solar wind bulk velocity,
solar wind dynamic pressure, X (blue), Y (green), and Z (red) GSM components of
the magnetic field from the WIND spacecraft (time shifted using the solar wind velocity
and the distance between WIND X=198.6RE and X=0); Magnetograms (components
and band-pass filtered X-component) from GBAY, DRBY, LOYS, KUUJ, and CHBG
stations. Here and in the other figures dashed vertical bars mark the time of the two
auroral onsets (events #4 and #16, see Table 2).
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RUNOV ET AL.: PLASMA SHEET AT AURORAL ONSET X - 35
P1
P2
P3
P4
G10
G12
Figure 4. THEMIS and GOES 10 and 12 spacecraft positions (GSM) at 0150 UT on
March 1, 2008
D R A F T August 27, 2008, 10:35am D R A F T
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X - 36 RUNOV ET AL.: PLASMA SHEET AT AURORAL ONSET
P1
P2
P3
P4
G10
G12
BpBeBn
BpBeBn
B, nT
Vi,
km/s
B, nT
Vi,
km/s
B, nT
Vi,
km/s
B, nT
Vi,
km/s
B, nT
B, nT
VxVyVz
VxVyVz
VxVyVz
VxVyVz
Figure 5. Spin-averaged (3 s) magnetic field and ion velocity (GSM) observed by
THEMIS P1 - P4, and the magnetic field (PEN) observed by GOES 10 and GOES 12
satellites during 0100 - 0210 UT on March 1, 2008. Note that the Bn component is
eastward in the PEN coordinate system
D R A F T August 27, 2008, 10:35am D R A F T
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B,
nT
GS
M
SS
T i
+
E,
keV
ES
A i
+
E,
keV
SS
T i
+
φ, o
ES
A i
+
φ, o
N,
cm-3
Vi,
km
/s
G
SM
P, n
Pa
SS
T e
-
E,
keV
ES
A e
-
E,
keV
P1 (THEMIS B)
Vz
Vy
Vx
Pp
Pm
Pt
DE
FeV
/cm
s
sr
eV2
DE
FeV
/cm
s
sr
eV2
Figure 6. Summary of P1 (THEMIS B) observations between 0140 - 0205UT. From top
to bottom: GSM components of the magnetic field; Omnidirectional ion ET spectrogram
from SST (30 - 500 keV); Omnidirectional ion ET spectrogram from ESA (0.1 - 30 keV);
Φ-angular ion spectrogram from SST (Φ is the azimuth of the detector looking direction
in the probe spin plane, see text for the details); Φ-angular ion spectrogram from ESA;
Ion number density, ion bulk velocity, and ion scalar pressure (all three quantity are
calculated from ESA and SST distributions); Omnidirectional electron ET spectrogram
from SST (30 - 500 keV); Omnidirectional electron ET spectrogram from ESA (0.05 - 30
keV). Vertical bars indicate the two auroral onsets (events #4 and #16, see Table 2).
D R A F T August 27, 2008, 10:35am D R A F T
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B,
nT
GS
M
SS
T i
+
E,
keV
ES
A i
+
E,
keV
SS
T i
+
φ, o
ES
A i
+
φ, o
N,
cm-3
Vi,
km
/s
G
SM
P, n
Pa
SS
T e
-
E,
keV
ES
A e
-
E,
keV
P2 (THEMIS C)
Vz
Vy
Vx
Pp
Pm
Pt
DE
FeV
/cm
s
sr
eV2
DE
FeV
/cm
s
sr
eV2
Figure 7. Summary of P2 (THEMIS C) observations between 0140 - 0205UT. The
same format as in Figure 6
D R A F T August 27, 2008, 10:35am D R A F T
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Φ
SST FLUX
160
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DE
Fe
V/c
m
s sr
eV
2
PA, e
lec
tro
ns
P1
B,
nT
E>30 keV
0.5
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RUNOV ET AL.: PLASMA SHEET AT AURORAL ONSET X - 41
P3 (THEMIS D)
B,
nT
GS
M
SS
T i
+
E,
keV
ES
A i
+
E,
keV
SS
T i
+
φ, o
ES
A i
+
φ, o
N,
cm-3
Vi,
km
/s
G
SM
P, n
Pa
SS
T e
-
E,
keV
ES
A e
-
E,
keV
Vz
Vy
Vx
Pp
Pm
Pt
DE
FeV
/cm
s
sr
eV2
DE
FeV
/cm
s
sr
eV2
Figure 10. Summary of P3 (THEMIS D) observations between 0140 - 0205UT. The
same format as in Figure 6
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B,
nT
GS
M
SS
T i
+
E,
keV
ES
A i
+
E,
keV
SS
T i
+
φ, o
ES
A i
+
φ, o
N,
cm-3
Vi,
km
/s
G
SM
P, n
Pa
SS
T e
-
E,
keV
ES
A e
-
E,
keV
Vz
Vy
Vx
Pp
Pm
Pt
DE
FeV
/cm
s
sr
eV2
DE
FeV
/cm
s
sr
eV2
P4 (THEMIS E)
Figure 11. Summary of P4 (THEMIS E) observations between 0140 - 0205UT. The
same format as in Figure 6
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P3 P4
PEN
Bfa
c_z,
nT
Bfa
c_x, nT
Bfa
c_y,
nT
dB
, nT
dB
, nT
GOES 10
GOES 12
Figure 12. The magnetic field at P3 (red) and P4 (blue) in the field-aligned coordinate
system; magnetic field variations observed by GOES 10 and GOES 12, PEN coordinate
system: Bp is directed poleward, Be - Earthward, and Bn - eastward.
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Table 2. Timing of the signatures, observed by the four THEMIS probes (P1, P2, P3,
and P4), GOES 10 (G10) and GOES 12 (G12) satellites, and by ground based observa-
tories (GBO)
# UT SC/GBO signature
1 0148:25 P2 high-energy electron flux increase
2 0148:25 P2 ∼65 keV ion flux increase
3 0148:40 P3, P4, G10 dBy (FAC)
4 0148:42 GBAY minor auroral onset (precursor)
5 0148:45 GBAY Pi2 onset
6 0148:45 P2 dBy and dBz
7 0148:50 P2 Tailward flow
8 0149:30 P1 Tailward flow, dBy, and dBz
9 0152:21 SNKQ Auroral onset
10 0153:30 G12 Dipolarization
11 0154:20 G10, P4 Dipolarization
12 0154:30 P1 Tailward flow
13 0154:30 P2 dBy, dBz
14 0154:40 P4 Drop of Bt, dVy, and dVz
15 0155:00 P2 Tailward flow
16 0155:21 KUUJ Auroral onset
17 0155:50 P3 Dipolarization, drop of Bt, dVy and dVz
D R A F T August 27, 2008, 10:35am D R A F T