Study of a substorm on May 4, 1986

Abstract

A substorm on May 4, 1986, midway through the PROMIS campaign of coordinated data acquisition, was uniquely well documented, both in its aspects at earth and in its magnetotail aspects. The expansive phase onset was imaged by the Viking satellite at 20-second time resolution. Most of the expansive phase was also imaged by DE 1 at 6-minute time resolution. ISEE 1 and 2 were near the tail's axis 18.5 R E from earth operating at high data rate and data were recorded by several geosynchronous satellites. This multi-satellite study provides evidence that the active substorm aurora occurs at the feet of field lines that map to a magnetic X-line in the near tail. The longitudinal lengthening of the aurora during a substorm is associated with cross-tail lengthening of the near-earth neutral line. The concept of the “poleward leap” of the auroral electrojet (and the auroras) as the culminating feature of the expansive phase finds further support in these data.