Articles | Volume 17, issue 12
Ann. Geophys., 17, 1602–1610, 1999

Special issue: Equator S

Ann. Geophys., 17, 1602–1610, 1999

  31 Dec 1999

31 Dec 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. Georgescu2,1, J. Birn3, L. M. Kistler4, T. Mukai5, S. Kokubun6, P. Eglitis8,7, L. A. Frank9, and J. B. Sigwarth9 R. Nakamura et al.
  • 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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