JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 106, NO. A9, PAGES 18,843–18,855, SEPTEMBER 1, 2001
Substorm and convection bay compared:
Auroral and magnetotail dynamics during convection bay
V. A. Sergeev,1 M. V. Kubyshkina,1 K. Liou,2 P. T. Newell,2 G. Parks,3
R. Nakamura,4 and T. Mukai5
Abstract. Using observations from eight spacecraft and a ground network, we study two
subsequent bay-like disturbances on December 10, 1996, initiated by southward
interplanetary magnetic field intervals, one being a classic substorm and another one a
convection bay. Both events showed enhanced convection and Dst decreases as well as Pi2
pulsations in the auroral zone. Contrasting to the well-defined substorm signatures of the
first event (poleward auroral expansion, substorm current wedge, strong particle injection
to 6.6 R E ) resulting from energy loading/unloading and near-Earth reconnection in the
tail, these signatures were virtually absent during the convection bay (CB). Distinctive
features of the CB event were the same as those during the Steady Magnetospheric
Convection intervals: (1) wide double oval at the nightside; (2) thick plasma sheet, relaxed
lobe field, and enhanced magnetic flux closure (large B z ) and multiple bursty earthward
flows (BBFs) in the midtail; (3) sporadic narrow soft injections to 6.6 R E ; (4) auroral
streamers associated with both BBFs and narrow injections. We emphasize the
development of multiple and sporadic auroral streamers which start at the poleward oval
boundary, propagate equatorward (in 3– 8 min) and end with a long-duration bright spot
in the equatorward oval. We conclude that the plasma sheet and auroral dynamics during
the convection bay was formed by sporadic narrow (a few R E wide) plasma streams
(plasma bubbles) which transported the plasma sheet material from the distant
magnetotail reconnection regions to the inner magnetosphere and may significantly
contribute to the magnetospheric circulation on the nightside. We modeled the nightside
tail configuration using magnetotail magnetic observations and low-altitude particle
boundaries to show that at the beginning of the convection bay the increase of magnetic
flux tube volume with distance was small in the midtail. Therefore the “pressure crisis” in
the tail was significantly reduced during the convection bay, and the efficient earthward
transport by sporadic narrow plasma streams was probably able to balance the
magnetospheric circulation to avoid the large-scale instability of the magnetotail.
1.
Introduction
The magnetosphere is a driven system powered by the energy supplied from the solar wind. Different types of the magnetospheric response to the enhanced energy input to the
magnetosphere have been identified. The substorm (the most
studied and most frequent type of response, timescale about
1–2 hours) represents a large-scale magnetotail instability. It
starts with a global slow magnetic energy storage in the tail. To
get this substorm growth phase, there should be an imbalance
between the magnetic flux transport from dayside to the tail
and the return earthward convection in the plasma sheet. The
1
Institute of Physics, St. Petersburg State University, St. Petersburg,
Russia.
2
Applied Physics Laboratory, The Johns Hopkins University, Laurel,
Maryland.
3
Space Sciences Division, Geophysics Program, University of Washington, Seattle, Washington.
4
Max-Planck-Institut fur extraterrestrische Physik, Garching, Germany.
5
Institute of Space and Astronautical Science, Sagamihara, Kanagawa, Japan.
Copyright 2001 by the American Geophysical Union.
Paper number 2000JA900087.
0148-0227/01/2000JA900087$09.00
reason of this imbalance could be the “pressure crisis” in the
tail [e.g., Erickson, 1992] which makes impossible the earthward contraction (convection) of closed plasma sheet tubes in
the standard tail configurations.
This loading phase of a substorm usually turns suddenly to
the explosive energy release as the magnetic reconnection
starts at the near-Earth neutral line [Baker et al., 1996]. The
basic ground signatures of this expansion phase are the poleward auroral expansion, Pi2 pulsations, and the development
of DP 1–type current system (westward auroral electrojet and
associated specific variations outside the auroral zone) [Rostoker et al., 1980]. The latter feature is due to the growth of the
substorm current wedge connecting the active sector of ionosphere with the current disruption region in the tail by the
field-aligned currents [Baker et al., 1996]. The DP 2 (twinvortex) current system accompanies moderate and strong substorms during all phases manifesting the enhanced magnetospheric convection [Kamide and Kokubun, 1996] as long as the
intense energy supply from the solar wind is continuing.
Such behavior is sometimes not observed, and the observations reported show a variety of deviations from the classic
substorm scheme. During continuous strong energy input the
magnetotail may stay in nearly steady state for a long time,
exceeding a few substorm timescales. Such time intervals are
known as Steady Magnetospheric Convection (SMC) events
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(see a review by Sergeev et al. [1996a]). The magnetic configuration during SMC events was argued to be stable but specific, with less “pressure crisis” in the midtail portion of the tail
allowing plasma to convect earthward more easily. The observations also showed that nightside convection is not a laminar
process: a lot of transient mesoscale features including multiple bursty earthward flows (BBFs) in the plasma sheet (that is
typical for all conditions [e.g., Baumjohann, 1993]), magnetic
impulsive variations and propagating convection bursts in the
nightside auroral zone have been repeatedly identified during
the SMC events [Sergeev et al., 1996a]. The full picture of these
transient phenomena is not quite understood, particularly, because the information about the auroral signatures of these
transients is yet very scarce.
Sometimes during nearly steady energy input into the magnetosphere one can observe the sequence of substorms, but
these could be separated by comparatively long time intervals
ⱖ2–3 hours without loading of magnetic energy into the tail
[e.g., Yermolaev et al., 1999]. Enhanced bursty earthward convection is seen at these time in the plasma sheet with no
signatures of large-scale changes in the tail [Nakamura et al.,
1999; Yermolaev et al., 1999]. Like during the SMC events, the
magnetosphere could be in a “directly driven” mode during
these episodes between successive substorms.
A shorter type of events (⬃1–2 substorm timescales) has
been named convection bay [Pytte et al., 1978]. That kind of
event did not attract so much attention as the substorms and
our knowledge of convection bays is very incomplete and, in
some parts, confusing. The identification of convection bays
was always based on the absence of substorm expansion signatures (during enhanced convection-related DP 2 current system) rather than on some specific signatures inherent to the
convection bays. Pytte et al. [1978] reported a ⬃3– 4 hours long
convection bay event following after a substorm expansion
phase. It was not a pure SMC-like case because the authors
presented evidence of changing large-scale configuration
(magnetic field stretching) at the end of this event. Shorter (⬍1
hour) isolated bays in AE index have also been reported [Pellinen et al., 1982; Sergeev et al., 1998]. Both these events showed
intense growth (energy loading) phase, which, however, was
concluded only with very weak expansion phase (pseudobreakup). The growth-type related current system (DP 2)
provided the main contribution to the ground magnetic effects
during the above two events. No short-duration event was yet
reported in which the magnetosphere was basically in the directly driven mode without significant growth phase effects.
One such event will be presented in this paper.
Obviously, a large variety of observed responses put the
questions about the origin of such variant behavior of the
magnetotail and about additional factors (processes) which
control this variance under seemingly similar external conditions. One way to look at this problem is to compare the
observational signatures of subsequent isolated substorm and
convection bay events observed one after another with the
same extensive network of spacecraft and ground instruments.
It is just the purpose of our paper to study such a unique
sequence.
In this paper we emphasize two aspects. First, we pay special
attention to the global auroral dynamics during the convection
bay observed by Polar ultraviolet imager (UVI) instrument to
find the distinctive features of convection bays, to provide a
linkage between spacecraft observations in different tail regions and to form a picture of transient processes in the plasma
sheet. Second, as discussed above, an important controlling
variable may be the magnetospheric configuration, particularly, the radial variation of the magnetic flux tube volume in
the midtail. This aspect will be addressed using observations
from Geotail and Interball spacecraft in the tail and energetic
particle boundaries observed by DMSP- and NOAA-type
spacecraft, which all are used as the input for the modeling of
the magnetic field configuration at the beginning of the convection bay.
2.
2.1.
Observations
Overview of Observations
According to the Wind spacecraft measurements in the solar
wind on December 10, 1996, the solar wind velocity changed
between ⬃500 km s⫺1 (1600 UT) and ⬃600 km s⫺1 (2200 UT)
and there were two pressure pulses at 1658 and 1952 UT (see
Figure 1). (Here and afterwards the timing of Wind data corresponds to the corrected time taking into account propagation
from Wind position [73, ⫺42, 0] to X ⫽ 10 R E at solar wind
velocity). The IMF was large and positive before 1600 UT, B z
component changed to small negative values after 1600 UT. It
finally turned to the north at 2110 UT. There were two 0.5–2
hour long episodes of strong southward interplanetary magnetic field (IMF) B z and of resulting enhanced energy input to
the magnetosphere (see Eps3 ⫽ V B sin3 (␪/2) parameter in
Figure 1, which is known to give the best correlation with the
cross-tail potential drop [Boyle et al., 1997]).
As a consequence, there were two bay-like enhancements of
magnetic activity in the auroral zone commencing at ⬃1710
UT (weak growth phase actually started after 1610 UT) and at
⬃1900 UT (see a stack plot of magnetograms in Figure 1). The
ring current also showed the same response; the decreases of
Dst index are comparable in both events. The sudden impulses
corresponding to pressure jumps in the solar wind can be
noticed at 1658, 1941, and 2035 UT. Both events were marked
by significant Pi2 pulsation activity in the nightside auroral
zone (Lovozero) and midlatitudes (see Kakioka data in Figure
2).
However, there were also considerable differences allowing
to classify them as a classic substorm (first event) or as a
convection bay (second event). The tail observations show the
most drastic differences. The total pressure in the plasma sheet
(from Geotail located near [⫺25, 0, ⫺2.5]R E GSM, most of
time spent in the plasma sheet) as well as the lobe magnetic
pressure (from Interball 1 spacecraft at [⫺27, 8, 3]R E GSM
while being in the lobe) consistently show the strong loading/
unloading of magnetic energy in the tail during the first event;
see Figure 1 (bottom). A few strong tailward flow bursts (accompanied by north-south bipolar excursions of the magnetic
field) at Geotail which changed to the strong earthward flow
bursts and fluctuating positive B z in the dipolarized plasma
sheet after 1753 UT provide evidence of near-Earth reconnection in this event which progressed tailward during the substorm expansion phase (see a more detailed treatment of this
classic substorm event by Nagai et al. [1998], Nakamura et al.
[1998], and Haaland et al. [1999]).
On the contrary, no significant loading/unloading or fast
tailward flows were observed in the second event. Here the
earthward flow bursts (BBFs) and relatively large (ⱖ5 nT)
positive (but fluctuating) B z component are persistently observed at both growth and declining phase of auroral magnetic
Figure 1. Overview of activity on December 10, 1996. (a) Solar wind dynamic pressure (Pd) and (b) dayside
merging electric field estimate computed from solar wind velocity and magnetic field (Eps3 ⫽ V B sin3 (␪/2));
the universal time is corrected by the propagation at the solar wind speed. (c) Dst proxy (SYM index); the
onsets of sudden impulses are marked by triangles. (d) Stack plot of AE station magnetograms. (e) Total
pressure at Geotail and magnetic pressure at Interball (while in the lobe) compared with tail lobe pressure
(P 30 ) predicted at r ⫽ 30 R E using Fairfield and Jones [1996] regression, and identification of plasma sheet
regimes encountered by Geotail based on plasma ␤ parameter, IPS when ␤ ⱖ 1 and LOBE/BLPS when ␤ ⱕ
0.1. (f, g) B z magnetic field component and V x component of the plasma flow measured at Geotail. Vertical
dashed lines show approximate onsets of enhancements of auroral electrojets and of the Dst decreases.
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SERGEEV ET AL.: SUBSTORM AND CONVECTION BAY COMPARED
Figure 2. Band-pass filtered (40 –200 s) H component magnetograms (December 10, 1996) from Kakioka
(34⬚ corrected geomagnetic latitude, local midnight at 1500 UT) and Lovozero (63⬚ corrected geomagnetic
latitude, local midnight at 2100 UT).
activity. The plasma beta parameter was permanently large
(␤ ⱖ 1), indicating a thick plasma sheet.
The auroral observations show strongly enhanced precipitation in both events as well as considerable differences in their
morphology. As seen from auroral keograms made from Polar
UVI observations at two different local times (Plate 1), the
auroral oval looks rather narrow at the beginning of first event.
The main precipitation was observed between 66⬚ and 69⬚
before the classic poleward expansion phase (from low to high
latitudes) in this event. In the second case the oval was wide
before the main activity (between 65⬚ and 77⬚) and the main
site of activity was its poleward boundary. The situation here
resembles the recovery phase of the first (substorm) event
between 1820 and 1850 UT. The active auroras show some
southward motion from the poleward boundary, just opposite
to the substorm case. The details of auroral dynamics will be
studied in the next section.
Two geostationary Los Alamos National Laboratory
(LANL) spacecraft 1991-080 and 1994-084 (hereinafter referred to as L80 and L84) were favorably located at the nightside. They both observed (Figure 3) a strong energetic particle
injection in the first substorm (preceded by deep dropout at
L84 just at midnight), but only a few weak, soft and short
injections during the second event. At keV energies (Magnetospheric Plasma Analyzer (MPA) instrument) in both cases
one observes the increases of plasma pressures which could
correspond to the inward motion of the plasma sheet during
enhanced convection. The magnetic field inclination (␪) evaluated from the main axis of the electron anisotropy tensor
(reliable measurements are labeled with circles) showed a
strong magnetic field stretching (⌬⌰ ⱖ 20⬚) during the first
event, whereas only slight stretching (ⱕ10⬚) was noticed at the
beginning of the second event.
The midlatitude magnetograms (Figure 4) show the disturbance which is also different in both events. A few episodes of
substorm current wedge (SCW) growth are clearly seen in the
first event, starting near the meridian of Gnangara station
(GNA) at 1700 and 1716 UT and then expanding both west-
ward (at 1735 and 1748 UT) and eastward (1748 UT) (compare with the poleward expansion in Figure 3) [see Haaland et
al., 1999]. The SCW growth is recognizable as sharp positive H
excursion in the central SCW sector accompanied by positive
(negative) D excursions westward (eastward) of this meridian.
No such evident excursions are seen during the second event
where positive deviations of H component are weak (those at
1944 and 2038 UT actually correspond to the pressure increases in the solar wind, Figure 1). Positive D excursion dominates at near-midnight stations (GNA, AMS, CZT), and this
corresponds to the effects of the “partial DR current” (convection-related) system (see, e.g., Sergeev et al. [1998] for the
discussion).
To conclude, the first event is a classical substorm with all its
signatures, including the auroral expansion, growth of substorm current wedge and Pi2 activity, energetic particle injections to 6.6 R E , loading/unloading of magnetic energy in the
tail as well as the formation of the near-Earth neutral line and
its subsequent tailward retreat. The second event did not show
any of these substorm signatures (except for Pi2 pulsations in
the auroral zone). In the next section we study the auroral
dynamics emphasizing the localized sporadic auroral streamers, which give us a global view and will allow to link together
other observations available.
2.2.
Auroral Streamers and Related Phenomena
The auroral UV morphology during the most active half
hour of convection bay is illustrated in Plate 2 using a full time
resolution of the UVI instrument. (Movies showing auroral
UV dynamics for the whole event are available at 具http://sdwww.jhuapl.edu/Aurora/mpg_samples.html典.) The event
started as the activation of a small bright surge form in the
poleward half of the premidnight oval (at ⬃75⬚) at 1917 UT,
but it faded soon and the following activity differs much from
the typical substorm behavior. Being sporadic in time and very
structured in space (in local time), it was formed by specific
localized structures, auroral streamers, which started to develop from the poleward oval and propagated into the equa-
SERGEEV ET AL.: SUBSTORM AND CONVECTION BAY COMPARED
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Figure 3. Summary of observations at two nightside geostationary LANL spacecraft. (a) Polar angle (theta)
of main axis of electron temperature anisotropy tensor; values with large anisotropy (T ⬜ /T 㛳 ⱖ 1.13 or ⱕ 0.9)
are marked by circles, B field orientation predicted with T89 models are shown for comparison. (b) Anisotropy
ratio (T ⬜ /T 㛳 ). (c) Partial pressures of hot protons (0.13– 43 keV) and electrons (0.03–24 keV). All these data
are from L80 spacecraft. (d) and (e) Energetic electron (50 –315 keV) fluxes at L80 and L84 geosynchronous
spacecraft, respectively. The triangle in Figure 3d shows the data point used in the magnetospheric modeling.
torial oval. About 10 very distinct streamers have been identified during the convection bay event. Some of them were
clustered in time (like during the most active time period
between 1945 and 2000 UT), some appeared isolated, their
timing results are given in Table 1. There were also a few
confusing events in which stages B or C (see below) were
missing or could not be resolved for sure.
Three different stages could be identified during the
streamer development. Quite often its equatorward propagation is delayed by a few minutes after the initial brightening,
which takes place near the poleward border (phase A). Then
its active phase (B) follows when the streamer propagates
toward the equatorward oval. The bright spot in the equatorward oval is formed after the streamer hits the equatorward
oval. This spot can be eventually observed for as long as 10 –20
min (for example, the rests of streamer f could be recognized
until 2022 UT), and this is the phase C in our definition. The
duration of the active phase B varied between 3 and 8 min in
this event.
During the convection bay the Geotail spacecraft was right
near midnight; however, the time period of strong auroral
activity between 1945 and 2030 UT (Plate 1) does not look
more active in the spacecraft records. Geotail observed strong
flow bursts (with Ey ⱖ 2 mV m⫺1) at 1920, 1943, 2011, 2022,
2116, and 2148 UT. Five of these six bursts had associated
streamers in phase A or B (streamers a, c, h, i, j), as shown
by horizontal bars in Figure 5. As seen from Table 1 and Plate
2, these streamers were localized within ⬃1 hour from the
Geotail foot point (at 00 MLT). More detailed comparison will
be presented in a separate paper. From Figure 5 one could
clearly see the density and pressure drops and sharp increases
of B z in the central plasma sheet associated with impulsive flux
transport events.
3. Energetic Particle Observations and Magnetic
Configuration at the Beginning of Convection
Bay and Substorm
Between 1943 and 1948 UT, at the beginning of the convection bay, the NOAA 14 polar spacecraft crossed the southern
oval near the midnight meridian (see Figure 6). The fluxes of
both trapped and precipitated energetic particles were measured together with the precipitated energy flux of auroral
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SERGEEV ET AL.: SUBSTORM AND CONVECTION BAY COMPARED
Figure 4. Stack plot of midlatitude magnetograms of H component (solid lines) and D component (dashed lines). D component sign of stations in the Southern Hemisphere was inverted to facilitate the comparisons. The stations are arranged
according to the magnetic local time; their magnetic coordinates (latitude and longitude) are Chambon-la-Foret (CLF,
50⬚, 86⬚), Crozet (CZT, ⫺51⬚, 112⬚), Martin de Vives (AMS,
⫺47⬚, 144⬚), Gnangara (GNA, ⫺42⬚, 188⬚), and Canberra
(CNB, ⫺43⬚, 226⬚). Magnetic midnight is shown by the circles
under the magnetograms.
particles integrated over 0.3–20 keV. These observations carry
useful information concerning the magnetotail structure.
The regions of empty loss cone ( J 㛳 /J ⬜ ⬍⬍ 1) or isotropic
( J 㛳 /J ⬜ ⬃ 1) pitch angle distributions of energetic electrons
and protons may show us the mapping of regions in the equatorial current sheet where the ratio R C / ␳ is ⱖ8 (adiabatic
motion) or ⱕ8 (nonadiabatic pitch angle scattering), respectively. (R C is the magnetic field curvature and ␳ is the particle
gyroradius. See Kubyshkina et al. [1999] for more complete
discussion and references on that subject.) The equatorwardmost transition, the isotropic boundary, corresponds to the
ratio value R C / ␳ ⫽ 8. This gives us a formal rule to compute
the isotropic boundary position from the magnetospheric magnetic field model for the particle of given mass, energy, and
charge. In the current sheet, R C / ␳ ⬃ B 2Z /j, so the equatorial
position of the isotropic boundary is mostly sensitive to the
equatorial magnetic field (B Z component) and, to less extent,
the local current density. This makes the isotropic boundaries
to be an important source of information for the magnetospheric modeling.
Positions of isotropic boundaries are marked by dashed lines
in Figure 6. Although the detector which measured the
trapped flux of energetic protons was corrupted by penetrating
radiation, the isotropic boundary can still be determined with
a large confidence as the position of maximal precipitating flux.
(This is always the case when both precipitated and trapped
proton fluxes are measured; this was specially studied by Newell et al. [1998].)
We modeled the magnetic configuration at 1945 UT by using
a special technique (Hybrid Input Algorithm [Kubyshkina et
al., 1999]) which allows one to use as input the observations of
different kind if the formal rule is known how to compute them
from the model. We used magnetic field observations from
both Interball/Tail spacecraft (in the lobe) and Geotail (in the
central plasma sheet). We also used the isotropic boundaries
from NOAA 14 (the formal rule to compute it in the tail
neutral sheet was R C / ␳ ⫽ 8) and the orientation of electron
anisotropy tensor at geostationary L80 spacecraft (supposed to
be a proxy of the local magnetic field orientation). By varying
the intensity of both ring and tail currents as well as by allowing
the localized thinning of tail current and the tilt of plasma
sheet to be varied, we obtained a best fit model shown in
Figure 7. The model parameters included the current sheet
thinning at x ⫽ ⫺14 R E (D ⫽ 0.35), enhanced ring current
(I RC ⫽ 1.6), and unchanged tail current of the starting T89
Kp ⫽ 2 model (terminology is the same as given by Kubyshkina et al. [1999]). The additional tilt of the plasma sheet was
0.8⬚.
The modeling results give us a specific configuration with
nonmonotonous B z profile with local minimum of B z ⬃ 1 nT
at 12.5 R E , where the current density increased up to 5 nA
m⫺2, partly, because of thinning of the current sheet down to
0.8 R E .
One important observation which was not tried as input can
be used as an independent test of the model. There exists a
second region of anisotropic (empty loss cone) energetic electrons which was observed poleward of 66⬚ corrected geomagnetic latitude (Figure 6). Whereas the isotropic distributions
between 65⬚ and 66⬚ may correspond to a small B z (and/or
strongly enhanced current), the more poleward anisotropic
region implies a somewhat larger B Z down the tail (or depressed current or thick current sheet) so that energetic electrons are again nearly adiabatic and no more scattered in the
current sheet. This feature (including the poleward boundary
of isotropic region) agrees well with the model prediction as
shown in Figure 7. This assures us that the nonmonotonous B z
profile in the tail (with B Z minimum at 12 R E ) is a realistic
feature.
4.
Discussion
4.1. Tail Magnetic Configuration as a Factor
Controlling the Stability of Magnetotail
Steady convection time intervals exist when convecting magnetosphere operates without global changes of configuration,
that is, under the balanced flux transport from dayside to the
tail and back (in the plasma sheet). Such a mode has been
observed in the three-dimensional MHD simulations of magnetosphere under the continuous southward IMF made by
Ogino et al. [1994].
The logics of the “pressure crisis” concept [Erickson, 1992]
implies that the global growth of the tail current (energy loading) occurs when the return earthward convection is braked in
Plate 1. Keograms of auroral luminosity in LBHL (long) band constructed from Polar UVI imager data at ⬃22 MLT and ⬃24
MLT meridians, December 10, 1996.
SERGEEV ET AL.: SUBSTORM AND CONVECTION BAY COMPARED
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Plate 2. Time series of Polar UVI images at high time resolution for the time period of intense streamer
activity between 1940 and 2004 UT. Two subsequent frames in LBH (short) filter (LBHS) image follow after
two frames obtained with LBH (long) filter (LBHL), thus forming the cycle. Magnetic midnight (dawn, dusk)
is on the right (up, down) side of each image, respectively.
SERGEEV ET AL.: SUBSTORM AND CONVECTION BAY COMPARED
Table 1. Development of Auroral Streamers and Their
Ground Magnetic and Geostationary Effectsa
Poleward
Activation,
a
b
c
d
e
f
g
h
i
j
UT (CGLat, deg. MLT, hours)
Active Phase
B, UT
Phase C,
MLT, hours
1921 (74,23)
1936 (75,00)
1945 (74, 23.5)
1945 (72,22)
1948 (76, 00.5)
1951 (76,22)
2009 (74, 00.5)
2009 (71,23)
2022 (71,00)
2115 (75,23)
1925–1930
1936–1941
1948–1951
1949–1955
1949–1954
1957–2001
2010–2013
2010–2016
2022–2030
2115–2121
00.5
01
00.5
22
01
23
01
00
00.5
23.5
a
Abbreviations are CGLat, corrected geomagnetic latitude; MLT,
magnetic local time.
the tail plasma sheet by strong earthward plasma pressure
gradients, which eventually leads to the accumulation in the
tail of the large magnetic flux transported from dayside
magnetosphere. The tail-like magnetic configuration,
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namely, the fast increase of flux tube volume (V) with distance, is considered to be the main factor leading to the
convection crisis. The alternative configurations with small
radial gradient of V may be constructed, but they can not
give a realistic solution globally, because in that case of large
magnetic flux closure through the plasma sheet the length of
the tail will be unrealistically short [e.g., Schindler and Birn,
1982]. However, the “pressure crisis” problem is basically
sharp between 10 and 30–40 RE, and, if the configuration is
“crisisless” is that region, that will considerably relax the
problem of getting intense earthward convection.
The modeling results in Figure 7 indicate that this was the
case during the convection bay considered. Comparing the
pressure increase at the midnight meridian in the neutral
sheet computed from the pressure balance (by integrating
grad P ⫽ jyBZ from starting point at 30 RE, Pmod) and that
from the adiabatic state equation (PV␥ ⫽ const, ␥ ⫽ 5/3,
Padiab), we find nearly the same pressure increase values
between 30 RE and 12 RE for the convection bay configuration. Oppositely, for the standard T89 model, such comparison gives ⌬Padiab ⬃ 10⌬Pmod, which is usually taken as
indication of strong “convection crisis.” Our results indicate
Figure 5. Plasma sheet parameters measured at Geotail spacecraft: Horizontal bars shown in Figure 5b
corresponds to the observation of auroral streamers in the MLT sector of Geotail. Vertical dashed lines mark
the onset of flow bursts with [V ⫻ B] y ⬎ 2 mV m⫺1. Note the sharp decreases of both density and plasma
pressure following the flow burst onset. Both the total Vx (V TOT) and its part perpendicular to B (V PERP) are
shown on the V x panel. On the bottom panel, P PLASMA and P TOT are plasma pressure and plasma plus
magnetic pressure, correspondingly.
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Figure 6. Particle fluxes measured in the Southern Hemisphere at NOAA 14 spacecraft. (a) Total energy
flux of 0.3–20 keV protons and (b)–(e) differential fluxes of energetic particles (protons ⱖ 80 and ⱖ 250 keV
and electrons ⱖ 100 and ⱖ 300 keV). Vertical dashed lines on Figures 6b– 6e show the positions of the
isotropic boundaries.
that there is virtually no such problem during the convection
bay event.
Comparison of classic substorm and convection bay is consistent with that view. Indeed, the major difference between
two events was in their initial states. Before the substorm the
configuration was very stressed as indicated by enhanced total
pressure in the tail (Figure 1), by the inclined magnetic field
and dropout of energetic particle flux (Figure 3), and by the
narrow auroral oval (Plate 1). However, because of the action
of the preceding substorm, the midtail was dipolarized and the
configuration was still crisisless before the convection bay.
All basic features of configuration we had during the convection bay are similar to those found previously when studying
the steady convection events when the tail globally should be in
nearly steady state (see a summary in the review by Sergeev et
al. [1996a]). Indeed, the large width of the auroral oval, thick
plasma sheet, and large B z in the midtail plasma sheet (all
indicating a large magnetic flux closure through the midtail
plasma sheet) and the isolated region of energetic electron
precipitation corresponding to the local B z minimum at r ⬃ 12
R E are also the essential features of steady convection events.
A reduced total pressure in the midtail (smaller than lobe
pressure estimated from the solar wind dynamic pressure
alone) could be one more key signature of such configuration
(this is true in our case; compare total pressure and P 30 at the
bottom of Figure 1). Such reduced pressure has also been
noticed by Nakamura et al. [1999] and Yermolaev et al. [1999]
during episodes of intense plasma sheet convection continued
for 2–3 hours after substorms without any energy loading/
unloading signatures.
This set of features of steady convection episodes in the
plasma sheet indicate a direction in which one could search for
a formal parameter to predict whether the tail respond in a
steady or time-varying mode to the enhanced driving by the
solar wind. However, this may only be a necessary (but not
sufficient) condition. During both convection bay studied and
during steady convection events the convection appears to be
very bursty and localized, which demonstrate that two requirements (locally crisisless configuration in the midtail and intense
bursty convection) may both be important to close the global
circulation pattern in its plasma sheet segment.
4.2.
Auroral Streamers and BBFs
The north-south oriented (NS) active auroral forms are
known for a long time [Akasofu, 1968]. Previously, the NS
auroras have been identified as the features of early expansion
phase (when the westward surge is formed [Nakamura et al.,
1993]) or, more frequently, during the late expansion or recovery phase when the double oval is well formed [e.g., Rostoker et
al., 1987; Elphinstone et al., 1996; Henderson et al., 1998]. The
term “auroral streamer” was introduced by Elphinstone et al.
[1996] to capture the specific dynamics of these forms (propagation from poleward to equatorward oval) rather than their
orientation which may differ from case to case. This may be an
important distinction since sometimes these forms are oriented
more east-west than north-south (see, e.g., the streamer d in
Table 1 and Plate 2) and, oppositely, in ground observations
the observed (NS-oriented) part of the large-scale fold of an
auroral arc can be incorrectly identified as a streamer. As
follows from this discussion, the streamers can be observed at
vastly different conditions including different substorm phases
(see above), during convection bays (this study), as well as
during the steady convection time periods (recently, we studied
a number of SMC time intervals to confirm this; the results will
be presented in following publications).
Following Henderson et al. [1998] and Sergeev et al. [1999],
SERGEEV ET AL.: SUBSTORM AND CONVECTION BAY COMPARED
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Figure 7. (top) Magnetic field configuration in XZ plane modeled at 1945 UT with superimposed projections of the spacecraft (stars) and equatorial mapping of isotropic boundaries (dark circles). (bottom)
Modeled and standard (T89 model, Kp ⫽ 3) configurations are compared. Blank and hatched blocks in
equatorial B z panel show the observed and predicted positions of strong loss cone filling of 100 keV electrons.
we interpret the auroral streamer in terms of sporadic and
narrow fast-flow channel (fast plasma stream) which is initiated
in the distant tail (phase A), propagates earthward over large
distance (phase B), and finally intrudes (stops) in the nearEarth magnetosphere leaving here the locally enhanced pressure region (stage C). The physical processes as well as those
controlling the formation of auroras may differ at these three
stages reflecting very different plasma regimes in the distant
tail (A), through the midtail region (phase B) and in the inner
magnetosphere (phase C). Briefly, the magnetic reconnection
can be responsible for the stage A and be the origin of fast
plasma streams (BBFs and/or bubbles [e.g., Chen and Wolf,
1993, 1999; Sergeev et al., 1996a, 1996b]). The modified pressure profile formed after the intrusion of fast stream into the
near tail may be the origin of mesoscale field-aligned current
(FAC) system (via the [grad P grad V] mechanism and of
modified precipitation. A more detailed discussion of mecha-
nisms, including mechanisms to form the auroral precipitation,
will be given elsewhere.
There exists a number of direct observations confirming this
picture. The most important element is the association of auroral streamers with the midtail fast flows. In detail this was
recently confirmed for the streamers h and i of our convection
bay event where three spacecraft (Interball/Auroral, Geotail,
and geosynchronous L80) were favorably connected to the
same auroral streamer to follow the propagation of fast stream
over the distance of ⱖ30 R E which took about 8 min [Sergeev
et al., 2000]. General temporal and spatial connection between
the fast plasma sheet flows and auroral activations have been
recently discussed by Nakamura et al. [1998], Lyons et al.
[1999], and Fairfield et al. [1999]. Narrow injections to 6.6 R E
at the stage C of streamer development have been shown by
Henderson et al. [1998] and, in more detail, by Sergeev et al.
[1999].
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SERGEEV ET AL.: SUBSTORM AND CONVECTION BAY COMPARED
A comment is required concerning how narrow the auroral
streamers are and how much transport they can provide. Plate
2 clearly indicates that the streamers during active stage B are
about (or less than) 0.5 hour MLT wide in longitude. The
separation of streamers which develop simultaneously can be
as small as ⬃0.5–1 hour MLT (e.g., compare streamers c and
e). These two characteristics indicate a small cross-tail scale of
the magnetospheric source of auroral streamer, being about
2–4 R E if mapped to the magnetosphere. This nicely agrees
with a ⬃3 R E cross-tail extent of bursty flows obtained in rare
observations with the pair of closely spaced magnetospheric
spacecraft [Sergeev et al., 1996b; Angelopoulos et al., 1997]. The
electric field is so large (a few mV m⫺1) during the BBFs that
with 3 R E cross section of the plasma stream the potential
drop (⬃60 kV for E ⫽ 3 mV m⫺1) is able to provide the total
expected flux transport in the tail in one stream at a time
[Sergeev et al., 2000].
A noteworthy feature is that the streamer-associated earthward bursty flows observed during convection activity invariably show the signatures of the plasma bubbles, for example,
the decrease of plasma pressure and density accompanied by a
sharp increase of both B z component and B inclination if
observed in the high-␤ plasma sheet (see Figure 5 and Kauristie
et al. [2000] for other examples of BBFs observed at ⬃12 R E
during the continuously disturbed period).
In fact, the bubbles have been suggested as another way to
realize the earthward convection in the tail-like configurations
containing some “pressure crisis” [Chen and Wolf, 1993, 1999].
That is why we believe that possibility of steady (balanced)
convection in the tail is favored by the combination of both
specific (less crisis) magnetic configuration plus the action of
plasma bubble mechanism realizing the earthward convection.
5.
Conclusions
We studied the convection bay event in which the magnetotail responded to the enhanced solar wind driving in a directly
driven mode, with very little change of global magnetospheric
configuration, hence, with the balanced flux transport on both
dayside-to-tail and plasma sheet segments of the global circulation pattern. We modeled the magnetospheric configuration
and argued it to be of the same type as during previously
studied steady convection events, with reduced lobe magnetic
flux, increased magnetic flux closure through the midtail
plasma sheet, stretched field lines in the near tail, and a local
B z minimum at ⬃12 R E . The pressure crisis is significantly
reduced in such a configuration, and this seems to be a necessary condition for the magnetosphere to respond in the directly
driven mode.
On the other hand, we found that most distinct morphological feature of the convection bay was the sporadic appearance
of multiple intense auroral streamers which are associated with
bursty fast plasma flows in the plasma sheet. Following previous work, we argue that these streamers are optical manifestations of sporadic narrow fast plasma sheet streams. These
narrow streams transporting plasma from the distant tail to the
inner magnetosphere have the characteristics similar to the
plasma bubbles. They could be the main process supporting
the earthward convection in the plasma sheet, and therefore
this activity can be a one more necessary condition to realize
the driven mode.
Comparison of signatures of the substorm and of convection
bay indicates a difficulty to distinguish between these two, so
different types of magnetospheric response when using the
standard monitoring tools, like AE index, auroral zone magnetograms, and Pi2 pulsations. Both types of events produced
the drops of similar magnitude in the Dst index. Other tools,
including global auroral imaging and tail lobe magnetic field,
are required to do the identification of magnetotail response
more reliable.
Acknowledgments. We thank S. Kokubun (principal investigator of
Geotail MGF instrument), R. Lepping and K. Ogilvie (principal investigators of Wind MFI and SWE instruments), R. Belian and D.
McComas (principal investigators of SOPA and MPA instruments at
LANL spacecrafts), and S. Romanov (principal investigator of Interball MIF instrument) for their data available on Internet through
DARTS, CDAWeb, and Interball-Tail databases. Magnetic field data
from Kakioka and Lovozero were made available through WDC-C
(Kyoto) and Polar Geophysical Institute, and particle data from
NOAA 14 spacecraft (D. Evans and H. Sauer, principal investigators
of TED and MEPED instruments) were provided through WDC-A
(Boulder). V. Sergeev and M. Kubyshkina thank the DFG for support
during their stays in MPE, Garching, as well as partial support by the
RFBR grants 98-05-04114, 00-05-64885, Russian Universities grant N
992709 and INTAS grant N99-000078. The work at APL was supported by the NASA grant NAG5-7724.
Michel Blanc thanks Susumu Kokubun and another referee for their
assistance in evaluating this paper.
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M. V. Kubyshkina and V. A. Sergeev, Institute of Physics, St. Petersburg State University, St. Petersburg, 198904, Russia.
([email protected])
K. Liou and P. T. Newell, Applied Physics Laboratory, The Johns
Hopkins University, Laurel, MD 20723.
T. Mukai, Institute of Space and Astronautical Science, Sagamihara
229, Kanagawa, Japan.
R. Nakamura, Max-Planck-Institut fur extraterrestrische Physik,
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G. Parks, Space Sciences Division, Geophysics Program, University
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(Received February 7, 2000; revised May 31, 2000;
accepted June 2, 2000.)
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