LARGE-SCALE HYBRID SIMULATIONS OF THE MAGNETOTAIL DURING RECONNECTION

Large-scale, 2-D hybrid simulations are used to investigate the ion ki netic physics associated with quasi steady-state reconnection in the m agnetotail. The simulations encompass a significant portion (20 x 120 R(E)) of the tail. After formation of transient plasmoids, the results show the features of fast Petschek-type reconnection. There are two p airs of thin transition layers attached to the x-point which divert an d accelerate the flow within a few ion inertial lengths. These transit ion layers do not quite conform to the properties of the expected slow shocks. The reason for this appears to be the fact that the ion dissi pation scale is comparable to the thickness of the developing plasma s heet. As a result, we find signatures of only partially thermalized, c ounterstreaming ions in what resembles the plasma sheet boundary layer . A fast ion beam forms immediately upstream of the boundary layer. Th e results are consistent with the notion that slow shocks or similar t ransition layers are responsible for the heating and formation of the central plasma sheet and for the ion beams observed in the plasma shee t boundary layer.