Characteristics of Outer Heliospheric Interplanetary Shocks and Ion Acceleration

Interstellar neutral atoms entering the heliosphere can be ionized by solar ultraviolet radiation or charge exchange with solar wind ions, then picked up by the solar wind to form interstellar Pickup Ions (PUIs). New Horizons observations show that PUIs’ number density fraction in the solar wind increases with heliocentric distance, highlighting their importance in the outer heliosphere. In this study, we conduct a parametric study of outer heliospheric interplanetary shocks using two-dimensional hybrid simulations constrained by in-situ data. Our results reveal that, for a shock with fixed Mach number, stronger ambient turbulence enhances ion multiple shock crossings via intensified upstream wave-particle scattering, prolonging particle residence near the shock. Sufficiently strong turbulence enables even low Alfvén Mach number ($ {M}_{\mathrm{A}}<3 $) shocks to efficiently accelerate PUIs into power-law suprathermal tails by diffusive shock acceleration. For given turbulence intensity, acceleration efficiency rises with Mach number due to increased compression ratio, and PUIs are accelerated more efficiently than solar wind ions, forming prominent downstream suprathermal tails. New Horizons observations of outer heliospheric low MA shocks show downstream PUI power-law tails consistent with simulations. Additionally, solar wind alpha particle-related charge exchange may enhance He⁺ near the PUI cutoff energy, more pronounced in high-speed streams due to larger cross-sections. This paper provide outer heliospheric ion acceleration factors and insights into anomalous cosmic ray origins downstream of the heliospheric termination shock.