Collisionless magnetic reconnection in an asymmetric current sheet

Abstract

The process of collisionless magnetic reconnection in an ion‐scale current sheet containing strong gradients in the density and magnetic field strength across the layer is investigated using two‐dimensional particle‐in‐cell simulations. Such a current sheet configuration contains a strong normal polarization electric field on the high field/low density (magnetospheric) side of the layer. In initial‐value simulations for such an asymmetric sheet, the reconnection rate and saturation level are found to be smaller by factors of 2–3 compared with a similar‐scale symmetric current sheet. These rates are probably too small to explain observations at the dayside magnetopause. The addition of an external‐driving electric field increases the reconnection rate substantially. This driven reconnection configuration is characterized by a nearly parallel inflow of electrons along the magnetosheath separatrices as the electrons attempt to flow from the high density side to the low density side of the layer, a strong outward flow of Poynting flux along the magnetospheric separatrices associated with the normal electric field and out‐of‐plane magnetic field, and a strong ion outflow jet. The outflow region on the magnetospheric side also exhibits a patchy parallel electric field structure and parallel electron velocity distributions with a counterstreaming feature. The addition of a moderate uniform magnetic guide field component (shear angle ≳110°) has no appreciable effect on the reconnection rate but does produce a drift of the X line in the direction of the electron diamagnetic drift at a small fraction of the magnetosheath Alfvén speed.