Oxygen Impacts on Dipolarization Fronts and Reconnection Rate

Abstract:

Spacecraft observations near a magnetotail X-line show that oxygen (O⁺) ions are a minor species during the non-storm substorms, but they can become a major species during some storm-time substorms. In this study, we investigated the effects of O⁺ on DFs and the reconnection rate during magnetotail reconnection. We used a 2.5D implicit Particle-in-Cell simulation (iPIC3D) in a 2D Harris current sheet in the presence of H⁺ and O⁺ ions. A simulation run with equal number densities of O⁺ and H⁺ (O⁺ Run), and one with only H⁺ ions (H⁺ Run) were performed. The O⁺ ions in the O⁺ Run were included in both the initial current sheet component and the background component. Comparing the two different runs, we found that (1) both the reconnection rate and the DF speed in the O⁺ Run are much less than those in the H⁺ Run; (2) the outflow magnetic flux and DF propagation are encumbered by the current sheet O⁺ inertia, which reduces the DF speed and delays the reconnection rate in the O⁺ Run; (3) an ambipolar electric field in the O⁺ Run is formed due to the different inflow and outflow speeds of O⁺ and electrons in the O⁺ diffusion region. This ambipolar electric field caused the O⁺ drag on the convective magnetic field in the O⁺ diffusion region. The small reconnection rate found in the O⁺ Run can be attributed to the current sheet inertia and the O⁺ drag on the convective magnetic flux.