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Ann. Geophys., 17, 1602-1610, 1999
www.ann-geophys.net/17/1602/1999/
© European Geosciences Union 1999
Substorm observations in the early morning sector with Equator-S and Geotail
R. Nakamura1, G. Haerendel1, W. Baumjohann1, A. Vaivads1, H. Kucharek1, B. Klecker1, E. Georgescu1,2, J. Birn3, L. M. Kistler4, T. Mukai5, S. Kokubun6, P. Eglitis7,8, L. A. Frank9, and J. B. Sigwarth9
1Max-Planck-Institut für extraterrestrische Physik, Garching, Germany
2Institut for Space Sciences, Bucharest, Romania
3Los Alamos National Laboratory, USA
4Space Science Center, University of New Hampshire, USA
5Institut for Space and Astronautical Science, Sagamihara, Japan
6Solar-Terrestrial-Environment Laboratory, Nagoya University, Japan
7Swedish Institute of Space Physics, Uppsala, Sweden
8Finnish Meteorological Institute, Helsinki, Finland
9Department of Physics and Astronomy, University of Iowa, USA
Abstract. Data from Equator-S and Geotail are used to
study the dynamics of the plasma sheet observed during a substorm with multiple
intensifications on 25 April 1998, when both spacecraft were located in the
early morning sector (03–04 MLT) at a radial distance of 10–11 RE.
In association with the onset of a poleward expansion of the aurora and the
westward electrojet in the premidnight and midnight sector, both satellites in
the morning sector observed plasma sheet thinning and changes toward a more
tail-like field configuration. During the subsequent poleward expansion in a
wider local time sector (20–04 MLT), on the other hand, the magnetic field
configuration at both satellites changed into a more dipolar configuration and
both satellites encountered again the hot plasma sheet. High-speed plasma flows
with velocities of up to 600 km/s and lasting 2–5 min were observed in the
plasma sheet and near its boundary during this plasma sheet expansion. These
high-speed flows included significant dawn-dusk flows and had a shear structure.
They may have been produced by an induced electric field at the local
dipolarization region and/or by an enhanced pressure gradient associated with
the injection in the midnight plasma sheet.
Key words. Magnetospheric physics (magnetospheric
configuration and dynamics; plasma sheet; storms and substorms)
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