The Onset of 3D Magnetic Reconnection and Heating in the Solar Corona

Magnetic reconnection, a fundamentally important process in astrophysics, is believed to be initiated by the tearing instability of an electric current sheet, a region where magnetic field abruptly changes direction. Recent studies have suggested that the amount of magnetic shear in these structures is a critical parameter for the switch-on nature of magnetic reconnection in the solar atmosphere, at large spatial scales. We present results of visco-resistive magnetohydrodynamic simulations of magnetic reconnection in 3D current sheets with conditions appropriate to the solar corona. We follow the evolution of the linear and nonlinear 3D tearing instability. We find that, depending on the parameter space, magnetic shear can play a vital role in the onset of significant energy release and plasma heating. Two regimes in our study exist, dependent on whether the current sheet is longer or shorter than the wavelength of the fastest growing mode, thus determining whether subharmonics are present in the actual system. In one parameter regime, where the fastest growing parallel mode has subharmonics, the subsequent coalescence of 3D plasmoids dominates the nonlinear evolution, with magnetic shear playing only a weak role in the amount of energy released. In the second parameter regime, where the fastest growing parallel mode has no subharmonics, only strongly sheared current sheets, where 3D effects are strong enough, show any significant energy release. We expect both regimes to exist on the Sun, and so our results have important consequences for the question of reconnection onset in various solar physics applications.