Acceleration of Oxygen ions by Magnetic Reconnection

Abstract:

Spacecraft observations in the magnetotail show that O+ ions can dominate the pressure and density during storm-time substorms. In this study, we use a 2D version of an implicit Particle-in-Cell simulation (iPIC3D) in the presence of H+ and O+ ions to investigate O+ heating and bulk acceleration processes during collisionless reconnection. In the simulations, a 2D Harris current sheet without a guide field is used for the initial condition. Open boundary conditions are used. Considering O+/H+ = 1:1, we compare the diffusion regions for H+ and O+ and analyze the energy gains of O+ at different locations with respect to the inflow and outflow boundaries, the X-point, the separatrices and dipolarization fronts (DFs). We also calculate the field energy variation and O+ velocity distribution functions at these locations to investigate the heating and acceleration process. The results show that the diffusion regions are not limited near the X-point, but also extend along the separatrices up to 10s of d_H (H+ inertial length) away from the X-point. The proton diffusion region along a separatrix is about ~24 d_H and that of oxygen is about ~40 d_H. Strongly positive J·E is shown near the DF for both species, while there is a weakly negative J·E at the slightly upstream of DF for protons, which does not appear for oxygen. Proton heating is mainly near the X-point and downstream of the separatrices, while oxygen heating extends into the inflow and outflow regions near the X-point and concentrates in the region slightly upstream of the DF. The study of the ion acceleration by reconnection is one of the scientific objectives of the Energetic Particle Detector (EPD) onboard the upcoming Magnetospheric Multiscale (MMS) mission and these results are directly related to the expected observations by MMS.