Electron Energization in Guide Field Reconnection Outflows with Kinetic Riemann Simulations

How electrons are heated during magnetic reconnection in the coronahas been a basic puzzle for a long time. Here we carry out PIC Riemannsimulations to explore electron energization including its dependenceon parameters. Riemann simulations, with its simple magnetic geometry,facilitate the study of the reconnection outflow far downstream ofx-line in much more detail than is possible with conventionalreconnection simulations. We find that the electron temperature in theexhaust increases and approaches a constant, suggesting that electronheating in the exhausts can extend to macroscopic scales in thecorona. Such heating only weakly depends on the proton-to-electronmass ratio used in the simulation. We develop a model demonstratingthat it's mostly the ion dynamics that controls the magnitude ofelectron heating: the ions get accelerated at both rotationaldiscontinuities to counter stream and give rise to two slow shocks.Ions get decelerated and partly reflected at the slow shocks byparallel electric potentials, which energize electrons and produceelectron heating. The prediction of electron heating from this modelapproximately agrees with simulations.