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Magnetic reconnection, a fundamentally important process in many aspects ofastrophysics, is believed to be initiated by the tearing instability of anelectric current sheet, a region where magnetic field abruptly changesdirection and electric currents build up. Recent studies have suggested thatthe amount of magnetic shear in these structures is a critical parameter forthe switch-on nature of magnetic reconnection in the solar atmosphere, at fluidspatial scales much larger than kinetic scales. We present results ofsimulations of reconnection in 3D current sheets with conditions appropriate tothe solar corona. Using high-fidelity simulations, we follow the evolution ofthe linear and non-linear 3D tearing instability, leading to reconnection. Wefind that, depending on the parameter space, magnetic shear can play a vitalrole in the onset of significant energy release and heating via non-lineartearing. Two regimes in our study exist, dependent on whether the current sheetis longer or shorter than the wavelength of the fastest growing parallel mode(in the corresponding infinite system), thus determining whether sub-harmonicsare present in the actual system. In one regime, where the fastest growingparallel mode has sub-harmonics, the non-linear interaction of thesesub-harmonics and the coalescence of 3D plasmoids dominates the non-linearevolution, with magnetic shear playing only a weak role in the amount of energyreleased. In the second regime, where the fastest growing parallel mode hasno-sub-harmonics, then only strongly sheared current sheets, where oblique modeare strong enough to compete with the dominant parallel mode, show anysignificant energy release. We expect both regimes to exist on the Sun, and soour results have important consequences for the the question of reconnectiononset in different solar physics applications.

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