DMSP F7 observations of a substorm field‐aligned current

Abstract

In this paper we present observations of a substorm field‐aligned current (FAC) system that DMSP F7 traversed just after 0300 UT on April 25, 1985. Ground magnetometer data show that a major substorm was in progress at that time and that DMSP F7 flew through a region of predominantly upward FAC. The DMSP F7 magnetic field data are consistent with this interpretation. The precipitating particle data suggest that there were three distinct large‐scale FAC systems. In ascending latitude these were a downward current, an upward current, and a paired upward/downward current system. We identify the first current, which was coincident with the diffuse aurora, as region 2. The next (upward) FAC was coincident with a spatially unstructured region of energetic (∼12 keV) electron precipitation. This was the substorm‐associated FAC that made up part of the current wedge. The upward/downward current pair was coincident with a region of highly structured precipitation. We suggest that these currents may have been the duskside region 1 and, poleward of that, the extension of the dawnside region 1. The particle data show that the upward substorm current lay well equatorward of the boundary between open and closed field lines. In fact, using a model field, the equatorward boundary of the substorm FAC maps to the neutral sheet at 6.9 RE. While one should be cautious in stressing results obtained by mapping model field lines, our result is consistent with scenarios for substorms which postulate a disruption and diversion of the near‐Earth cross‐tail current.