Cluster electron observations of the separatrix layer during traveling compression regions

Abstract

We present Cluster 4-point observations of electrons during traveling compression regions (TCRs) on 19 September 2001. The electron and |B| signatures vary with distance from the plasma sheet, confirming that transient plasma sheet bulges propagate past Cluster. TCRs with |B| increases have either no electron signature, or unidirectional ~1 keV electrons at the plasma sheet edge. However, spacecraft initially near the plasma sheet edge are engulfed within the bulge and observe a diamagnetic reduction in |B|. In cases where the underlying plasma sheet bulge moves earthward, electrons at the plasma sheet edge stream tailward. We suggest this represents either a remote observation of electrons closing the Hall current system in an ion diffusion region located farther tailward, or the outflow jets along the separatrix formed by a second neutral line located farther earthward of the spacecraft. The latter case implies the simultaneous action of multiple X-lines in the near-Earth tail.