Simulations of pickup ion mediated quasi‐perpendicular shocks: Implications for the heliospheric termination shock

Abstract

The microstructure of the heliospheric termination shock and the accompanied local acceleration processes of both ions and electrons are investigated by utilizing one‐dimensional full particle‐in‐cell simulations for a variety of parameters. The relative pickup ion density is assumed to be 20–30%. The magnetic field and the shock potential profiles exhibit significant differences, since the former mostly reflects the dynamics of solar wind ions, whereas the latter is mainly sustained by the bulk motion of the reflected pickup ions in the extended foot. The discrepancy between the magnetic field profile and the potential profile increases with Alfvén Mach number. Most of the downstream thermal energy is gained by the pickup ions, while some heating of the solar wind ions and electrons occurs through the modified two‐stream instability excited in the extended foot. Self‐reformation can occur when the relative pickup ion density is 20% but is blurred when it becomes as large as 30%. Reformation is also suppressed if the local solar wind ion temperature in the extended foot is high, which can either be due to heating by the modified two‐stream instability or is already determined by the solar wind temperature far upstream. In all runs presented in this study no evidence for shock surfing acceleration of pickup ions could be found. Nonthermal particle acceleration occurs for oblique shocks. Electron (pickup ion) shock drift acceleration is evidenced when the shock angle is below 80° (60°).