PUI Acceleration at an Oblique Heliospheric Termination Shock: 2D Hybrid Kinetic Simulation

Abstract We investigate the acceleration of pickup ions (PUIs) at an oblique heliospheric termination shock (HTS) using 2D kinetic hybrid simulations. The background magnetic field, B 0 , is set in-plane and makes an angle of Θ Bn = 50° with respect to the shock normal. The shock compression ratio is r = 3.24. PUIs are reflected at the shock front and backstream upstream, while the thermal ion component is only heated across the shock wave. The backstreaming PUI component excites large-amplitude waves upstream of the shock ( δB ∼ 0.2–1.0 B 0 ), propagating parallel to the background magnetic field, for which the wavenumber satisfies the resonant condition with the backstreaming PUI component. The large-amplitude wave further initiates shock rippling. PUIs form a power-law energy spectrum downstream of the shock, and the power-law index is fitted to −2.2, which is similar to the value estimated from diffusive shock acceleration (DSA) theory, Γ = − r + 2 r − 1 ∼ − 2.33 . PUIs primarily gain energy by specular reflection and the shock-drift-type acceleration mechanism, and when the gyroradius becomes much larger than the shock transition scale, such an energy gain formally becomes equivalent to the energy gain in the DSA theory, thereby explaining the consistency of the power-law distribution with the DSA power-law index. This study can help understand energetic neutral atom (ENA) distributions inferred from the polar regions, where the HTS tends to be oblique due to the configuration of the solar magnetic field. Large-scale shock obliquity is also possible in lower-latitude oblique regions of the HTS. We expect, from this study, that the ENA spectrum from the polar regions should resemble that expected for a power-law index in the DSA theory.