Electron structure of the magnetopause boundary layer: Cluster/Double Star observations

Abstract

We present a comparison of two events, monitored by the Double Star and Cluster spacecraft at separate locations on the dayside magnetopause, which exhibit distinct properties at high and low latitudes in the magnetopause boundary layer during the occurrence of low‐latitude reconnection. On 6 April 2004, the four Cluster and TC‐1 spacecraft were on near‐coincident, outbound transits of the dawnside magnetosphere at north and south midlatitudes, respectively. The observations show a series of oppositely directed flux transfer events (FTEs), fed by a low‐latitude reconnection line located between the spacecraft. Although both spacecraft locations were nearly equidistant from the active reconnection region, the associated magnetopause boundary layer was maintained at TC‐1 but not at Cluster. We suggest an asymmetric north and south extent of the LLBL so as to be more extensive at TC‐1, where the local magnetic shear across the magnetopause is small. On 4 January 2005, the Cluster and TC‐1 spacecraft all repeatedly traverse the northern, duskside magnetopause almost simultaneously, before and after a strong reversal in the IMF from northward to southward during a period of turbulent solar wind. Open flux tubes are observed within minutes of the southward turning, arising from the sudden formation of a nearby subsolar reconnection line. Before the IMF change, a complex and energized boundary layer, largely absent at the lower latitudes of TC‐1, and containing an energetic (>40 keV) electron population of locally trapped and field‐aligned distributions, is present at the high‐latitude Cluster locations. Following reconnection onset after the IMF reversal, the boundary layer is seen to extend to TC‐1, and the electron distribution, which depends on position through the boundary layer, develops as an energetic, field‐aligned (bistreaming) distribution. The analysis is utilizes an extended electron distribution for energies ranging from a few to 400 keV and by reordering the transition through the magnetopause to the electron distribution.