Supersonic and Subsonic Expansion Acceleration Mechanisms in Fast Magnetic Reconnection

Abstract

The thermodynamic expansion acceleration mechanism associated with the fast magnetic reconnection is studied by two-dimensional magnetohydrodynamic (MHD) simulations and the Rankine Hugoniot analysis. In the acceleration mechanism caused in the reconnection jet region, the jet generated by a pair of slow shocks can be further accelerated by the expansion and propagation of the plasmoid formed by the reconnection process. It is remarkable that the resulting jet can exceed the Alfven speed measured in the upstream magnetic field region. This acceleration mechanism is classified into supersonic and subsonic cases. Either of those cases occurs depending on the upstream field conditions of the jet. The former is similar to the Parker's solar wind acceleration mechanism, which includes two-dimensional expansion process, but the latter is basically one-dimensional expansion process. This paper consists of two topics. Firstly, the upstream field conditions required to cause the supersonic case is shown. Secondly, it is shown that the plasma expansion acceleration in the supersonic case is stronger than the subsonic case because the supersonic case has the circulating enhancement mechanism of the expansion process while the subsonic case does not.