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Ann. Geophys., 15, 587-595, 1997
www.ann-geophys.net/15/587/1997/
© European Geosciences Union 1997
The INTERBALL-Tail ELECTRON experiment: initial results on the low-latitude boundary layer of the dawn magnetosphere
J.-A. Sauvaud1, P. Koperski1, T. Beutier1, H. Barthe1, C. Aoustin1, J. J. Thocaven1, J. Rouzaud1, E. Penou1, O. Vaisberg2, and N. Borodkova2
1Centre d'Etude Spatiale des Rayonnements, BP4346, F-31029, Toulouse, France
2Space Research Institute, Profsoyuznaya 84/32, Moscow, Russia
Abstract. The Toulouse electron spectrometer flown on
the Russian project INTERBALL-Tail performs electron measurements from 10 to 26
000 eV over a 4<pi> solid angle in a satellite rotation period. The
INTERBALL-Tail probe was launched on 3 August 1995 together with a subsatellite
into a 65° inclination orbit with an apogee of about 30 RE.
The INTERBALL mission also includes a polar spacecraft launched in August 1996
for correlated studies of the outer magnetosphere and of the auroral regions. We
present new observations concerning the low-latitude boundary layers (LLBL) of
the magnetosphere obtained near the dawn magnetic meridian. LLBL are encountered
at the interface between two plasma regimes, the magnetosheath and the dayside
extension of the plasma sheet. Unexpectedly, the radial extent of the region
where LLBL electrons can be sporadically detected as plasma clouds can reach up
to 5 RE inside the magnetopause. The LLBL core electrons have
an average energy of the order of 100 eV and are systematically field-aligned
and counterstreaming. As a trend, the temperature of the LLBL electrons
increases with decreasing distance to Earth. Along the satellite orbit, the
apparent time of occurrence of LLBL electrons can vary from about 5 to 20 min
from one pass to another. An initial first comparison between electron- and
magnetic-field measurements indicates that the LLBL clouds coincide with a
strong increase in the magnetic field (by up to a factor of 2). The resulting
strong magnetic field gradient can explain why the plasma-sheet electron flux in
the keV range is strongly depressed in LLBL occurrence regions (up to a factor
of \sim10). We also show that LLBL electron encounters are related to
field-aligned current structures and that wide LLBL correspond to northward
interplanetary magnetic field. Evidence for LLBL/plasma-sheet electron leakage
into the magnetosheath during southward IMF is also presented.
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