Generation and Decay of Reconnecting Current Structures Downstream of the Bow Shock: 3D Hybrid Simulations

Observations of Earth’s bow shock and magnetosheath have shown that magnetic reconnection occurs within these regions at thin current sheets, typically arising from turbulence and plasma instabilities in the shock transition layer. Broad observational surveys of these regions have shown that, somewhat surprisingly, the prevalence of reconnecting current structures may not be strongly dependent on the shock Mach number or the angle between the upstream magnetic field and shock normal (θ_Bn), despite quasi-parallel shocks typically exhibiting more disordered and non-stationary structure. To investigate how shock reconnection manifests across different parameters, we perform a series of two- and three-dimensional hybrid (fluid electron, kinetic ion) particle-in-cell simulations across a broad range of Mach numbers and orientations. These simulations isolate ion-scale mechanisms for reconnection in the shock, principally those driven by ion-ion beam instabilities in the foot and foreshock. For 2D simulations, we show that reconnection via these ion-driven mechanisms is strongly constrained to quasi-parallel shocks. However, downstream of quasi-parallel shocks, we find that the decay rate of closed-field regions, and hence thin current sheets, is not strongly dependent on upstream shock parameters. We also explore the differences that arise in shock structure, the generation of reconnecting current structures, and their decay rates for three-dimensional simulations.