Interstellar neutral atoms entering the heliosphere undergo charge exchange with solar wind ions, producing interstellar pickup ions (PUI) characterized by a filled-shell velocity distribution. Observations from New Horizons indicate that the number density of PUIs gradually increases with heliocentric distance. Studying their acceleration at interplanetary shocks is crucial for understanding the interaction between the outer heliospheric solar wind and the local interstellar medium. In this study, a two-dimensional hybrid simulation is employed to investigate the parameter dependence of quasi-perpendicular interplanetary shocks in the outer heliosphere. The results show that under strong background turbulence, low Alfvén Mach number shocks (MA < 3) can still accelerate a portion of PUIs and form a power-law suprathermal tail through diffusive shock acceleration (DSA). Under the same turbulence intensity, the ion acceleration energy increases with Mach number, and PUIs are more efficiently accelerated by shocks, dominating the high-energy portion of the downstream spectra. Analysis of New Horizons observations reveals that some low-Mach-number shocks in the outer heliosphere can indeed produce power-law suprathermal tails consistent with the simulation results. In certain events, solar wind alpha particles may undergo charge exchange with interstellar hydrogen, enhancing the contribution of He⁺ near the PUI cutoff energy. Other events exhibit a power-law spectrum in the intermediate-energy range (~2.5–4 keV) on both sides of the shock, though the lack of in-situ magnetic-field measurements on New Horizons leaves this phenomenon open to further investigation. Overall, by combining numerical simulations and observations, this study qualitatively identifies the key parameters controlling ion acceleration in the outer heliosphere and provides theoretical insights into the origin of anomalous cosmic rays (ACRs) downstream of the heliospheric termination shock.
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