Fast, strong ion heating in collisionless magnetic reconnection via an intrinsic stochastic mechanism*

A long-standing problem in collisionless magnetic reconnection is its accompaniment by fast, strong ion heating that cannot be explained by classical mechanisms. We explain this anomaly by showing that stochastic ion heating is inherent to collisionless reconnection and is sufficiently fast and strong to account for the observed anomalous ion heating. In a typical collisionless reconnection geometry, i.e., a perturbation to a Harris equilibrium with a shear length smaller than the ion skin depth, magnetic field lines reconnect while canonical vorticity field lines remain connected. This fact is exploited to approximate the electric and magnetic fields in the inflow and outflow directions and then demonstrate that they meet the stochastic ion heating criteria. The analytical calculation is verified by an electron fluid simulation. Test-ion simulations and comparisons to experiments confirm the existence of this mechanism.