Distant plasma sheet ion distributions during reconnection

Abstract

Previous models of the plasma sheet following reconnection and current sheet acceleration predict ‧lima‐bean‧ ion distributions. These are inconsistent with observational constraints. We postulate that following initial interaction with the current sheet, a fraction of outflow ions are backscattered and re‐encounter the current sheet. Fermi acceleration processes then generate an additional high‐energy outflow population. In the backscatter region these ions form a complete shell in velocity space, providing sufficient pressure to support weak fields. Further from the current sheet, where backscatter is less efficient, a hemispherical shell of ions moves away from the current sheet, and field strengths are nearer the external lobe value. The fastest ions stream ahead of the bulk population to form the plasma sheet boundary layer. This model predicts a multi‐layered plasma sheet structure, consistent with recent GEOTAIL observations.