Multipoint observations of a small substorm

Abstract

In this paper we present multipoint observations of a small substorm which occurred just after 0110 UT on April 25, 1985. The observations were made by spacecraft (AMPTE CCE, AMPTE IRM, DMSP F6, and DMSP F7), ground auroral stations (EISCAT magnetometer cross, Syowa, Narssarssuaq, Great Whale River, and Fort Churchill), and mid‐latitude stations (Furstenfeldbruck, Toledo, and Argentine Island). These data provide us with a broad range of observations, including the latitudinal extent of the polar cap, visual identification of substorm aurorae and the magnetic perturbations produced directly beneath them, in situ magnetic field and energetic particle observations of the disruption of the cross‐tail current sheet, and observations concerning the spatial expansion of the current disruption region from two radially aligned spacecraft. The DMSP data indicate that the event took place during a period when the polar cap was relatively contracted, yet the disruption of the current sheet was observed by CCE at 8.56 RE. We have been able to infer a considerable amount of detail concerning the structure and westward expansion of the auroral features associated with the event, and we show that those auroral surges were located more than 10° equatorward of the boundary between open and closed field lines. Moreover, we present evidence that the current sheet disruption observed by CCE in the neutral sheet was located on field lines which mapped to the westward traveling surge observed directly overhead of the ground station at Syowa. Furthermore, the observations strongly imply that disruption of the cross‐tail current began in the near‐Earth region and that it had a component of expansion which was radially antisunward.