Cluster observations of collisionless Hall reconnection at high-latitude magnetopause

Abstract

The first in situ observation of collisionless Hall reconnection in the high-latitude region when the interplanetary magnetic field (IMF) B_y dominates is reported in this paper. On 23 March 2002, Cluster crossed the magnetopause current sheet when it traveled outbound in the Northern Hemisphere. The interface between the cusp and the magnetosheath is clearly associated with sharp transitions of the magnetic field and plasma moments. These variations suggest that the interface is a clear current sheet with a thickness of 218 km. Energetic electrons, protons, helium, and oxygen ions were observed and their intensity peaked at this current sheet. During that time period, the IMF B_z is around zero and the IMF B_y component dominates. The geometry of the magnetic field preferred B_y reconnection. The bipolar signature in the magnetic field observed by Cluster is consistent with Hall effects in the reconnection process. Characteristics of three B_y reconnection events are presented in this paper.