Abstract

Abstract Between 1000 and 1800 UT on October 10, 2001, when Cluster was approaching the plasma sheet from the northern lobe, Geotail traversed the southern lobe also approaching the plasma sheet during two substorm intensifications identified in the Kakioka magnetogram. In this paper we examine the temporal change of the tail configuration and evolution of the plasma sheet between 11 and 13 UT, when multiple intensifications of the first substorm took place. Changes in the tail configuration toward a dipole-like field were identified during all intensifications, suggesting plasma sheet expansion which were embedded in a gradual northward motion of the tail. We emphasize the importance of simultaneous measurements in both hemispheres in order to identify plasma sheet expansion or dipolarization, because flapping motions or local enhancements of the tail current density may mask and mimic the signatures. During a later intensification, Geotail and Cluster encountered the plasma sheet boundary layer, which is the most direct signature of expansion of the plasma sheet. Using the time difference among the Cluster four spacecraft and Geotail, the possible propagation speed of the dipolarization and spatial scale of the disturbance is discussed. In addition to the usual dawn-to-dusk electric field, significant contribution from a north-south electric field were observed both in the lobe and at the boundary of the plasma sheet associated with the dipolarization. This suggests the importance of the effects from a localized source region for dipolarization and/or reconnection process.

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