It is also interesting to compare the substorm signature in the polar
magnetosphere with that in the magnetotail.
The GEOTAIL spacecraft was located around
(-6.4,-8.8,-2.4) () in GSM coordinates
at 0500 UT. Figure 2
shows the data of POLAR and GEOTAIL in the same time frame.
The bottom panel shows
data from the low-energy plasma detector (LEP) on board GEOTAIL
[ Mukai et al., 1994].
The GEOTAIL spacecraft was located within the plasmasheet throughout the
interval of the figure: Before
0500 UT, the ion density was rather
high and the ions were not moving much (as shown in the ion bulk velocity
data), which are features of the plasmasheet ions. The gradual temperature
decrease from
0300 UT to
0500 UT can be explained in terms of
the plasmasheet thinning during the loading phase of the substorm: Because
of the thinning, the relative distance of the spacecraft from the center
of the plasmasheet increased, which caused the decrease in temperature.
The ion density gradually increased from
0300 UT to
0430 UT
and then decreased until
0500 UT. The decrease is consistent with
the above-explained increase in the spacecraft distance from the center of
the plasmasheet during the loading phase. The increase until
0430 UT may have been caused by the compression of the plasma sheet
during the loading phase, which overcame the effect of the relative motion
of the spacecraft away from the plasmasheet center. Another possibility
is the dawnward motion of the spacecraft: As a spatial structure of the
plasmasheet, its density increases with deceasing distance from the flank
magnetopause [e.g., Lennartsson and Shelley, 1986].
After
0500 UT,
the ion temperature jumped up, and there was a burst of earthward and
dawnward ion flow with the duration of
3 min.
Thus, at first sight, GEOTAIL data appear to
suggest that the substorm onset was
0500 UT. There is a 52 min lag
from 0408 UT, the initial onset time on the ground (line B).
A possible way to explain this difference is the Y position of GEOTAIL.
That is, because GEOTAIL was located at
, or at 3.6 hour MLT,
dawnward propagation of
substorm signal, from the onset region, might have taken
several tens of minutes to reach the GEOTAIL position.
Nagai [1982, Figure 12] reports that the east-west propagation
speed of the substorm onset region, or so called the current wedge,
is
[min/MLT hour].
We note in Figure 2 that the multiple
onsets are more apparent at the 327
geomagnetic longitude
chain than in any
other longitudes, thus the onsets are likely to have
happened in the premidnight
sector, around 22 hour MLT. Then, the propagation time from 22 hour to 3.6
hour MLT is estimated to have been
min. The observed time
lag (52 min) is larger than 40 min, but at least some of the lag
may be explained in terms of the east-west propagation
of the substorm heating of the plasmasheet.
If the event B did not reach the position of GEOTAIL but the event C
(0435 UT) did, then the lag is 25 min, which is well within the range of
17-40 min.
In relation to this, we note that POLAR was similarly distant
from the expansion onset
region (presumably in the near-Earth tail) to that of GEOTAIL.
Thus the onset signal would have spent similar time to propagate to POLAR,
but the time delay from the initial ground onset to the
maximum at
POLAR (from
0408 to
0430 UT) was smaller than that at GEOTAIL
(from
0408 to
0500 UT). Thus the delay for POLAR may not be
explained in terms of the propagation of the onset signal.
Still another possibility is that neither event B nor C reached the
position of GEOTAIL, but the event D did.
However we do not prefer this possibility because event B is expected to
have been large, judging from its observability at many local times (top
panel of Figure 2) and from the fact that the
at POLAR decreased after the event B (middle panel).