The onset of magnetic reconnection in the magnetotail

Abstract

This paper addresses the onset of collisionless magnetic reconnection in the tail of the Earth’s magnetosphere. The two-and-a-half-dimensional version of a fully electromagnetic particle-in-cell code is used to describe the pre- and post-onset dynamics of reconnection in thin current sheets in the magnetotail. The ion/electron mass ratio is set to 100. The simulation starts out from an apparently stable equilibrium configuration. Applying an external electric field, meant to be caused by magnetic flux transfer to the tail, leads to the formation of a thin current sheet in the center of the plasma sheet. This confirms earlier results obtains with fluid and hybrid-methods. In the thin sheet quasi-static force-balance leads to a substantial decrease of the north-south component of the magnetic field in the center of the sheet. This in turn causes the electrons to become significantly nongyrotropic, such that a tearing mode starts growing. Regarding the nonideal process that supports the electric field in the diffusion region, the simulation results are shown to be consistent with the notion that electron pressure anisotropies associated with the nongyrotropy generate the required diffusive electric fields. The destabilizing role of electron nongyrotropy is confirmed by a simplified analysis of the energy principle for two-dimensional collisionless plasmas.