Comparison of supersonic and subsonic expansion accelerations associated with fast magnetic reconnection

Abstract

Supersonic and subsonic expansion acceleration mechanisms associated with spontaneous fast magnetic reconnection process are compared by two-dimensional magnetohydrodynamic (MHD) simulations and test fluid (non-charged) particle simulations. When the Petschek reconnection process is steadily established, the reconnection jet generated by a pair of slow shocks becomes either supersonic (Case 1) or subsonic (Case 2), depending on the upstream plasma condition. For Case 1, the jet generated by the slow shocks can be further accelerated by the adiabatic supersonic expansion process. Finally, the jet encounters a fast shock in front of the plasmoid. For Case 2, the jet generated by slow shocks can be further accelerated by the adiabatic subsonic expansion process. The acceleration of Case 1 is stronger than that of Case 2. In Case 2, a fast shock is not formed. In both cases, it is important that the propagation of the plasmoid is driven by slow shocks formed around the plasmoid itself, rather than the reconnection jet.