Kinetic Simulations of the Behavior of Pickup Ions in the Vicinity of a Collisionless Shock

Charge exchange between outflowing solar wind and neutrals of interstellar origin becomes increasingly important at large heliospheric distances. Consequently, the plasma upstream of the heliospheric termination shock (TS) is characterized by the presence of a distinct population of superthermal pickup ions with shell-like distributions (PUIs). It is well-appreciated that PUIs significantly affect the TS structure and gain the majority of the kinetic energy lost by the solar wind upon the shock transition. Quantitative understanding of the behavior of PUIs at the TS is important for many applications, notably to be able to model PUIs in global heliospheric models that rely on MHD framework and thus cannot directly describe the kinetic behavior of PUIs at the TS. In this contribution, we discuss results from fully kinetic and hybrid (kinetic ions + fluid electrons) simulations targeting the question of how PUIs are energized upon transition through the TS. In particular: i) fully kinetic and hybrid simulations of the immediate vicinity of the shock transition are cross-validated and their results are compared to predictions of a particle-tracing model, demonstrating good agreement; validated hybrid simulations are used to study ii) the relaxation of anisotropic distributions generated upon shock transition and iii)  particle reflection from the shock and the generation of upstream perturbations by back-streaming PUIs.