ELECTRIC FIELD EVIDENCE FOR TAILWARD FLOW AT SUBSTORM ONSET.

Electric field observations made near the neutral sheet of the magnetotail provide additional support for the view that reconnection occurs in the near-earth region of the tail. Southward turnings of the magnetic field that start at, or shortly after, substorm onsets are accompanied by enhancements in the dawn-to-dusk electric field, resulting in a tailward E multiplied by B drift velocity. Both the magnetic and the electric fields in the tailward-flowing plasma are nonuniform and vary with inferred spatial scales of several R//E in the events examined in this paper. These nonuniformities may be the consequence of the tearing-mode process. The E multiplied by B flow was also toward the neutral sheet and away from midnight in the events studied.