Numerical simulations of magnetic reconnection in an asymmetric current sheet

Abstract： Magnetic reconnection in a symmetric current sheet has been thoroughly investigated in previous studies. The out-of-plane magnetic field is found to have a quadrupole structure in the diffusion region. The perpendicular electric field and the parallel electric field with a bipolar structure exist around the four separatrices. In this paper, by performing two-dimensional (2-D) particle-in-cell (PIC) simulations, we investigate the electric and magnetic structures of magnetic reconnection in an asymmetric current sheet. Compared with magnetic reconnection in a symmetric current sheet, the quadrupole structure of the out-of-plane magnetic field now disappears in the diffusion region, and both the parallel and perpendicular electric field only exist in the upper separatrices. The parallel electric field with a bipolar structure is considered to be the results of the nonlinear evolution of the Buneman instability, which is excited by the high-speed electron flow formed after their acceleration around the X line. The disappearance of the parallel electric field around the lower separatrices may be due to the transverse instability, which is unstable in a weak magnetized plasma.