Substorm observations in the early morning sector with Equator-S and Geotail

Data from Equator-S and Geotail are used to study the dynamics of the plasm a sheet observed during a substorm with multiple intensifications on 25 Apr il 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 premid night and midnight sector, both satellites in the morning sector observed p lasma sheet thinning and changes toward a more tail-like field configuratio n. During the subsequent poleward expansion in a wider local time sector (2 0-04 MLT), on the other hand, the magnetic field configuration at both sate llites changed into a more dipolar configuration and both satellites encoun tered again the hot plasma sheet. High-speed plasma flows with velocities o f 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 flow s 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 injecti on in the midnight plasma sheet.