Superfast expansion acceleration mechanism in the spontaneous fast magnetic reconnection Original Research Article

Thermodynamic supersonic (superfast) plasma expansion acceleration generated in the spontaneous fast magnetic reconnection process is studied in 2-dimensional magnetohydrodynamic (MHD) simulations. In contrast to the Petschek reconnection model, the reconnection outflow jet is found to exceed steadily the Alfven velocity measured in the upstream magnetic field region. In our MHD simulation for the symmetric anti-parallel magnetic field model, the final velocity of the plasma jet is observed to reach 1.4 times of the Alfven velocity, which is maintained until the jet encounters a fast shock generated in front of the magnetic loop (plasmoid). Also in asymmetric magnetic field models, in which the current sheet is put between two straight magnetic field regions with different intensities, the supersonic plasma acceleration mechanism is detected. Especially, in the asymmetric model in which uniform plasma density is initially assumed, the supersonic acceleration region tends to shift to the side of the higher intensity magnetic field region. In addition, the plasma jet region consists of two jet layers which have different Mach numbers and almost the same jet velocity. It means that the reconnection jet almost have reached a steady state. Hence, the generation of the superfast jet can be predicted by the Rankine Hugoniot relation for the slow shock. Once the superfast jet and magnetic loop are generated, the thermodynamic supersonic expansion acceleration can occur due to the change of the pressure balance around the magnetic loop.