Particle Heating and Energy Partition in Low-$β$ Guide Field Reconnection with Kinetic Riemann Simulations

Abstract: Kinetic Riemann simulations have been completed to explore particle heating during guide field reconnection in the low-$\beta$ environment of the inner heliosphere and the solar corona. The reconnection exhaust is bounded by two rotational discontinuities (RD) and two slow shocks (SS) form within the exhaust as in magnetohydrodynamic (MHD) models. At the RDs, ions are accelerated by the magnetic field tension to drive the reconnection outflow as well as flows in the out-of-plane direction. The out-of-plane flows stream toward the midplane and meet to drive the SSs. The SSs differ greatly from those in the MHD model. The turbulence at the shock fronts and both upstream and downstream is weak so the shocks are laminar and produce little dissipation. Downstream of the SSs the counterstreaming ion beams lead to higher density, which leads to a positive potential between the SSs that acts to confine the downstream electrons to maintain charge neutrality. The potential accelerates electrons from upstream of the SSs to downstream region and traps a small fraction but only modestly increases the downstream electron temperature above the upstream value. In the low-$\beta$ limit the released magnetic energy is split between bulk flow and ion heating with little energy going to electrons. That the model does not produce strong electron heating nor an energetic electron component suggests that other mechanisms, such as multiple x-line reconnection, are required to explain energetic electron production in large flares. The model can be tested with the expected data from the Parker Solar Probe.