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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 (03â04 MLT) at a radial distance of 10â11 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 (20â04 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 2â5 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)
Highlights
The dynamics in the near-Earth region is expected to be controlled by dipolarization and formation of the current wedge
In this study we examine the response of the early morning plasma sheet to a substorm, which started initially in the premidnight sector and expanded in longitude during the course of three intensi®cations
We have studied plasmaow and ®eld characteristics in the morningside magnetosphere during a substorm with multiple intensi®cations
Summary
The dynamics in the near-Earth region (inside 15 RE) is expected to be controlled by dipolarization and formation of the current wedge. The magnetic ®eld returns to a more dipolar con®guration and the plasma sheet expands within a longitude range that is spanned by the substorm current wedge (McPherron et al, 1973). The current wedge and associated dipolarization of the ®eld expand in longitude both eastward and westward (Nagai, 1982a, b, 1991; Arnoldy and Moore, 1983). Several studies have reported plasma sheet thinning in the early morning sector during the recovery phase of substorms with multiple intensi®cations which could be. Plasma sheet thinning in the early morning sector was observed near the geosynchronous orbit (Kopanyi and Korth, 1995) during the substorm recovery phase. We discuss the possible deformation of the plasma sheet in the morningside in the course of substorm evolution and how these high-speedows could evolve in the early morning sector
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