Tailward progression of magnetotail acceleration centers: Relationship to substorm current wedge

Abstract

During a fortuitous near‐alignment of the IMP 8 and Geotail spacecraft along the magnetotail axis, geomagnetic activity monitors on the ground and at geosynchronous altitude detected the evolution of a magnetospheric substorm. We searched for evidence of the neutral line formation and evolution during this substorm. There exists evidence for multiple, time‐varying, localized (<9 RE) acceleration regions in the magnetotail at the time. Late in the expansion phase of the substorm a reversal of the predominant direction of the energetic particle anisotropy as well as the plasma flow velocity from tailward to Earthward was observed at Geotail at X=−61 RE. The tailward‐to‐Earthward anisotropy reversal is consistent with a tailward progression of the acceleration centers at Geotail. This phenomenon could be identified as the tailward retreat of the neutral line. However, the tailward progression of the activity is in reality the re appearance of new acceleration regions at gradually more tailward sites. Although the large temporal and spatial development is consistent with that of a reconnection geometry, the local Geotail acceleration is bursty and quite possibly localized. An ensuing increase of the northward magnetic field component seen at IMP 8 at X =−32 RE is consistent with a tailward expansion of the substorm current wedge out to that distance. Current wedge formation at IMP 8 was delayed by several minutes relative to the anisotropy reversal at Geotail. We interpret this as evidence that the location of current wedge and the location of magnetotail particle acceleration are separated by at least 30 RE during the late expansion phase of this substorm. Given the ∼9 RE cross‐tail separation of IMP 8 and Geotail, there still exists some uncertainty as to the three dimensional evolution of the acceleration and dipolarization regions in the late expansion phase of this substorm. Given the scarcity of similar IMP 8‐Geotail conjunctions during the distant tail Geotail phase, a full understanding of that evolution has to await a future multiprobe mission.