Positive feedback enhancement of the plasmoid compression associated with the supersonic magnetic reconnection

Fast magnetic reconnection and the related magnetic loop (plasmoid) are studied on the basis of MHD (magnetohydrodynamic) simulations. When the upstream magnetic field region is of sufficiently low beta, and the magnetic diffusion region is strongly localized, the reconnection jet can be supersonic, leading to the formation of a fast shock in front of the downstream magnetic loop (plasmoid), and hence strong plasma heating (compression) in the magnetic loop. According to MHD simulations, this reconnection process is basically established by two steps of plasma acceleration, slow shock acceleration and adiabatic expansion acceleration. On the other hand, when the beta value is higher, subsonic reconnection occurs. In that case, fast shock does not appear and, hence, plasma heating and compression become weak. It is first shown how that is the difference between supersonic and subsonic magnetic reconnections. Second, showing the compression ratio obtained from the MHD simulations, the positive feedback enhancement of the plasma compression (heating) is caused only in the supersonic case.