Global Energetics of Solar Flares: X. Petschek Reconnection Rate and Alfven Mach Number of Magnetic Reconnection Outflows

Abstract

We investigate physical scaling laws for magnetic energy dissipation in solar flares, in the framework of the Sweet-Parker model and the Petschek model. We find that the total dissipated magnetic energy $E_{diss}$ in a flare depends on the mean magnetic field component $B_f$ associated with the free energy $E_f$, the length scale $L$ of the magnetic area, the hydrostatic density scale height $\lambda$ of the solar corona, the Alfv\'en Mach number $M_A=v_1/v_A$ (the ratio of the inflow speed $v_1$ to the Alfv\'enic outflow speed $v_A$), and the flare duration $\tau_f$, i.e., $E_{diss} = (1/4\pi) B_f^2\ L\ \lambda\ v_A\ M_A\ \tau_f$, where the Alfv\'en speed depends on the nonpotential field strength $B_{np}$ and the mean electron density $n_e$ in the reconnection outflow. Using MDI/SDO and AIA/SDO observations and 3-D magnetic field solutions obtained with the vertical-current approximation nonlinear force-free field code (VCA-NLFFF) we measure all physical parameters necessary to test scaling laws, which represents a new method to measure Alfv\'en Mach numbers $M_A$, the reconnection rate, and the total free energy dissipated in solar flares.