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 plasma sheet observed during a substorm with multiple intensi®cations 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 ®eld con®guration. During the subsequent poleward expansion in a wider local time sector (20±04 MLT), on the other hand, the magnetic ®eld con®guration at both satellites changed into a more dipolar con®guration and both satellites encountered again the hot plasma sheet. High-speed plasma ̄ows 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 ̄ows included signi®cant dawn±dusk ̄ows and had a shear structure. They may have been produced by an induced electric ®eld at the local dipolarization region and/or by an enhanced pressure gradient associated with the injection in the midnight plasma sheet.