Scaling of asymmetric reconnection in compressible plasmas

The scaling of the reconnection rate with external parameters is reconsidered for antiparallel reconnection in a single-fluid magnetohydrodynamic (MHD) model, allowing for compressibility as well as asymmetry between the plasmas and magnetic fields in the two inflow regions. The results show a modest dependence of the reconnection rate on the plasma beta (ratio of plasma to magnetic pressure) in the inflow regions and demonstrate the importance of the conversion of magnetic energy to enthalpy flux (that is, convected thermal energy) in the outflow regions. The conversion of incoming magnetic to outgoing thermal energy flux remains finite even in the limit of incompressibility, while the scaling of the reconnection rate obtained earlier [P. A. Cassak and M. A. Shay, Phys. Plasmas 14, 102114 (2007)] is recovered. The assumptions entering the scaling estimates are critically investigated on the basis of two-dimensional resistive MHD simulations, confirming and even strengthening the importance of the enthalpy flux in the outflow from the reconnection site.