Field‐aligned plasma flow in MHD simulations of magnetotail reconnection and the formation of boundary layers

Abstract

We have used our two‐dimensional resistive MHD code to study details of the plasma flow in two classes of magnetotail reconnection models: (1) near‐earth reconnection as initiated by the sudden occurrence of anomalous dissipation, which leads to plasmoid formation and ejection consistent with phenomenological magnetospheric substorm models, and (2) distant reconnection which is forced to occur near a given location by a nonuniform inflow of plasma, energy, and magnetic flux through the high‐latitude boundaries modeling the possible effects of plasma entry through the plasma mantle, which expands until it reaches the plasma sheet. Near‐earth reconnection produces predominantly tailward flow, which occupies a major part of the plasma sheet cross section in z. Except for narrow regions close to the neutral sheet, this flow is largely field aligned. Due to the fact that the onset of this fast flow closely coincides with plasma sheet thinning, the observation at a fixed satellite location off the central neutral sheet looks like the crossing of a flow boundary layer when the satellite exits from the plasma sheet into the lobe. The distant reconnection process showed several features consistent with observations in the near and distant tail: the formation of a distant neutral sheet, rather than a single neutral line with the possibility of the occurrence of multiple neutral lines and quasi‐stagnant magnetic islands, and the splitting of the earthward flow into two boundary layers, or “separatrix layers,” inside the magnetic separatrix that limits the region of closed magnetic flux tubes.