Hybrid simulations of the decay of reconnected structures downstream of the bow shock

Observations by Magnetospheric Multiscale have demonstrated that magnetic reconnection occurs at Earth's bow shock, typically at thin current sheets arising from plasma instabilities and turbulence in the shock transition region. Observational surveys of both the shock transition and the magnetosheath downstream suggest that the number of current sheets in these regions may not be strongly dependent on the shock Mach number M-A or the angle between the upstream magnetic field and shock normal (theta(Bn)). This result is somewhat surprising given that quasi-parallel and high Mach number shocks tend to have a more disordered and non-stationary structure. In order to investigate how shock reconnection manifests across different parameters, we perform a series of hybrid (fluid electron, kinetic ion) particle-in-cell simulations across a range of Mach numbers and orientations. Given that hybrid simulations cannot resolve electron-scale current sheets and reconnection, these simulations isolate an ion-scale mechanism for shock reconnection driven by an ion-ion beam instability in the foot. We find that this mechanism is strongly constrained to quasi-parallel shocks across all simulated Mach numbers. By quantifying reconnection using the area occupied by plasma on closed magnetic field lines, we find the number of reconnecting structures and closed field area increase with M-A and decrease with theta(Bn) in the upstream and ramp regions. Downstream of the shock, however, we find a similar result to observational surveys: within the subset of quasi-parallel shocks, the decay rate of the closed field area (and hence thin current sheets) is not strongly dependent on upstream shock parameters. (c) 2023 Author(s).

Automated summary

Observations by Magnetospheric Multiscale have shown that magnetic reconnection occurs at Earth's bow shock, mainly in thin current sheets caused by plasma instabilities and turbulence. The number of current sheets in the shock transition and the magnetosheath downstream seems to be independent of the shock Mach number and the angle between the upstream magnetic field and shock normal. Hybrid particle-in-cell simulations were performed to investigate shock reconnection under different parameters, revealing a strong correlation between a specific ion-scale mechanism and quasi-parallel shocks. The quantification of reconnection through the area occupied by plasma on closed magnetic field lines shows that the number of reconnecting structures and closed field area increase with the shock Mach number and decrease with the angle between the upstream magnetic field and shock normal, but this relationship weakens downstream of the shock.