Impact of Heavy Ions on Reconnection Rate and Dipolarization Fronts during Magnetotail Reconnection

Spacecraft observations show that near a magnetotail X-line, the concentration of oxygen (O+) ions varies greatly between storm-time and non-storm substorms. While O+ is a minor ion species during the non-storm substorms, it can become a major ion species during some storm-time substorms. It is important to understand how such a significant amount of O+ influences the onset of reconnection, the reconnection rate and the subsequent energy transfer at propagating dipolarization fronts (or reconnection jet fronts). In this work, we have studied the effects of O+ on the reconnection rate and DFs during magnetotail reconnection. We used a 2.5D implicit Particle-in-Cell simulation in a 2D Harris current sheet in the presence of H+ and O+ ions. We carried out a simulation with equal number densities of O+ and H+ (O+ Run) and compared the results with a simulation run using only H+ ions (H+ Run). We found that the reconnection rate in the O+ Run is much less than that in the H+ Run and identified two factors that contribute to this difference: (1) the O+ drag on the convective magnetic flux via an ambipolar electric field in O+ diffusion region; (2) the current sheet O+ inertia, which reduces the DF speed and delays the fast reconnection phase in the O+ Run. For factor (2) the O+ ions provide the main force contributions at the DFs and thereby determine the thickness of DFs provided the concentration of O+ is large enough. The velocity distribution functions of O+ have several peaks that result from ion reflection and acceleration near the DFs. These results illustrate some of the differences between the storm-time and non-storm substorms due to a significant concentration of heavy ions. They also are directly related to the expected observations by the Magnetospheric Multiscale (MMS) mission.