Injection process of pickup ion acceleration at an oblique heliospheric termination shock

Abstract: Injection process of pickup ion acceleration at a heliospheric termination shock is investigated. Using two-dimensional fully kinetic particle-in-cell simulation, accelerated pickup ions are self-consistently reproduced by tracking long time evolution of shock with unprecedentedly large system size in the shock normal direction. Reflected pickup ions drive upstream large amplitude waves through resonant instabilities. Convection of the large amplitude waves causes shock surface reformation and alters the downstream electromagnetic structure. A part of pickup ions are accelerated to tens of upstream flow energy in the time scale of $\sim 100$ times inverse ion gyro frequency. The initial acceleration occurs through shock surfing acceleration mechanism followed by shock drift acceleration mechanism. Large electrostatic potential accompanied by the upstream waves enables the shock surfing acceleration to occur.