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Two-dimensional full particle-in-cell simulations of pickup ion mediated oblique shocks were conducted with unprecedentedly long simulation times (over 100 times the inverse ion gyro frequency) and large system sizes (2000 times the ion inertial length) along the shock normal direction. An oblique shock refers to a shock where the angle between the shock normal direction and the upstream magnetic field vector, known as the shock angle, is between 50 and 70 degrees in this case.
We tracked the long-term evolution of oblique shocks, including solar wind electrons, ions, and pickup ions. When the shock angle is 50 degrees, pickup ions reflected at the shock and backstreaming toward the upstream excite large-amplitude waves through resonant instability. These excited waves are convected and impact the shock front, leading to shock reformation and altering the downstream electromagnetic structure. Some pickup ions were accelerated to nonthermal energies over a timescale of about 100 times the inverse ion gyrofrequency. Orbit analysis of the accelerated particles revealed that the shock surfing acceleration mechanism operated during the initial stages, followed by the shock drift acceleration mechanism. The resultant downstream energy distribution function of pickup ions indicates a more efficient generation of nonthermal pickup ions compared to previous hybrid simulations.
While shock surfing acceleration has been considered ineffective at a perpendicular termination shock of the heliosphere, our findings indicate that the electrostatic potential associated with large-amplitude upstream waves contributes to the manifestation of this acceleration mechanism in oblique termination shocks.
