Generation Mechanisms for Low-energy Interstellar Pickup Ions

We present a test-particle simulation describing the interstellar pickup ion (PUI) velocity distribution in the turbulent solar wind (SW). The classical Vasyliunas and Siscoe (V&S) model assumes instantaneous pitch angle scattering that leads to an isotropic distribution in the SW frame, and considers only convection and adiabatic cooling as PUIs propagate in the expanding SW. In this paper, the nearly isotropic PUI transport equation, including the effect of spatial diffusion due to the fluctuating magnetic field, is solved at different heliospheric distances. The creation of PUIs due to the ionization of interstellar neutral hydrogen (H) and charge exchange between SW protons and neutral H are considered separately. The varying SW velocity, density, and temperature with heliocentric distance from a comprehensive fluid model have been incorporated into our simulations. Specifically, we find (1) the spatial diffusion augments adiabatic cooling effects by extending the transport time and distance, which leads to an enhanced production of low-energy PUIs, especially at small heliospheric distances; (2) spatial diffusion is unimportant at large distances (≥15 au), because the particles have had a sufficiently long time to undergo adiabatic cooling; (3) moments of the simulated velocity distribution function are consistent with PUI hydrogen properties measured by the New Horizons’ SW Around Pluto instrument; and (4) the simulated PUI distribution is of potential importance for the PUI measurements to be carried out by IMAP at 1 au.