Kinetic structure of the sharp injection/dipolarization front in the flow‐braking region

Abstract

Observations of three closely‐spaced THEMIS spacecraft at 9–11 Re near midnight and close to the neutral sheet are used to investigate a sharp injection/dipolarization front (SDF) propagating inward in the flow‐braking region. This SDF was a very thin current sheet along the North‐South direction embedded within an Earthward‐propagating flow burst. A short‐lived depression of the total magnetic field (down to 1 nT), devoid of wave activity and intense particle fluxes, stays ahead of the SDF. Clear finite proton gyroradius effects, which help visualize the geometry and sub‐gyroscale of the SDF, are seen centered at the thin current sheet. The SDF nearly coincides with the narrow interface between plasmas of different densities and temperatures. At that interface, we observed strong (40–60 mV/m peak) E‐field bursts of the lower‐hybrid time scale that are confined to a localized region of density depletions. This sharp dipolarization/injection front propagating in the flow‐braking region appears to be a complicated kinetic‐scale plasma structure that combines a number of small‐scale elements (Bz drops, thin current sheets, LH cavities, injection fronts) previously discussed as separate objects.