How unmagnetized oxygen leads to a reduced reconnection rate

We investigate the role of a background oxygen population in magnetic reconnection, using Particle-In-Cell (PIC) simulations with time-dependent inflow. The initially reconnecting current sheet consists of H+ only, with the background oxygen captured by the reconnection process as time evolves. We run a series of simulations, with different temperatures and oxygen densities to understand how the reconnection rate is affected. The simulations show that the background oxygen has little effect on the dissipation region structure. However, the reconnection is significantly reduced if compared to a run without oxygen background. This reduction is not due to simple mass-loading of the system, but rather ballistic acceleration of oxygen by the overall Hall electric fields. This transfers energy to the heavy unmagnetized ions, that would otherwise be converted to mechanical energy of the protons. We show the energy partitioning between the H+ run, and the various runs including O+ with different temperatures and densities. We describe how energy is transferred to the heavier species through acceleration by the Hall electric fields, and other mechanisms that can reduce the reconnection rate.