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

Abstract:

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.