A multisatellite case study of the expansion of a substorm current wedge in the near‐Earth magnetotail

Abstract

This study presents observations made by four spacecraft (AMPTE CCE, AMPTE IRM, GOES 5, and GOES 6) and two ground stations (San Juan and Tucson) during a substorm that occurred at ∼0830 UT on April 19,1985. The spacecraft were arrayed in a configuration that allows for the examination of the spatial evolution of the substorm current wedge. CCE was located between the GOES spacecraft in longitude, but at a radial distance of 8.0 RE. IRM was located west of the other three spacecraft in the same sector as Tucson, but at a radial distance of 11.6 RE. The relative times at which the signature of the substorm current wedge was first observed at the GOES spacecraft and the ground stations are consistent with a simple longitudinally expanding current wedge. However, the times at which IRM and CCE observed the current wedge are not consistent with a current wedge that expanded only longitudinally. IRM first observed the signature of the current wedge at about the same time the signature was observed by GOES 6 and Tucson, and CCE observed the current wedge only after both GOES satellites and the ground stations had done so. Moreover, both GOES spacecraft observed signatures consistent with entry into the central plasma sheet before CCE and IRM did, even though we estimate that CCE was slightly closer to the neutral sheet than the geosynchronous spacecraft. The sequence of events suggests that during this substorm the disruption of the cross‐tail current sheet, the formation of the substorm current wedge, and the expansion of the plasma sheet began in the near‐Earth region, and subsequently spread tailward as well as longitudinally.