Flow shear near the boundary of the plasma sheet observed by Cluster and Geotail

Abstract

We report on the transient strong flow shear (large northward/southward electric field) events accompanied by energetic ion beams and localized field‐aligned currents observed at the boundary of the premidnight plasma sheet by Cluster in the Northern Hemisphere and Geotail in the Southern Hemisphere. The events took place associated with plasma sheet expansion during a substorm interval, with the main positive bay onset at 1155 UT on 10 October 2001. Typical timescales of these events were 1–5 minutes. Cluster multipoint analysis showed that the field‐aligned currents consist of upward and downward current layers, the latter located at the outermost edge of the plasma sheet and concentrated in a region with a thickness of 1600 km. Low‐energy proton flow suggested that the electric field was southward at the outer part and northward at the inner part, with a magnitude exceeding 10 mV/m. The electric field reversal region also corresponds to the boundary between beam‐like electrons and more isotropic electron distributions. Geotail observed corresponding plasma and field disturbances simultaneously inside the plasma sheet. We suggest that the strong bipolar electric fields could be related to the Hall effect of the transient reconnection process tailward of Cluster and Geotail and to the leading edge of the plasma flow jetting Earthward from the reconnection region.