Scattering of Interstellar Neutral Atoms in the Outer Heliosheath

Interstellar neutral (ISN) atoms from the very local interstellar medium (VLISM) penetrate the heliosphere and are observed by detectors located near 1 au, e.g., on the Interstellar Boundary Explorer (IBEX), and in the future on the Interstellar Mapping and Acceleration Probe (IMAP). Interpretation of these observations provides insight into the physical conditions in the VLISM but requires modeling of the processes that change distributions of ISN atoms inside the heliosphere and beyond. Here, we focus on the consequences of collisional scattering in the outer heliosheath (OHS), beyond the heliopause. Charge exchange collisions create secondary atoms from the OHS ions, which have a different flow speed and temperature than pristine ISN atoms, especially close to the heliopause. It is widely assumed that these collisions do not change directions of interacting particle velocities. We show that this assumption is not justified for the typical collisions speed in the OHS, and therefore the distribution functions of secondary atoms are different than those calculated without this momentum exchange.  This angular scattering in charge exchange collisions results in secondary atom production terms that show elongated distributions aligned with the relative bulk speed of the parent populations, as well as higher temperatures (up to ~3000 K) and shifted bulk speeds (up to ~2 km s^-1). Distributions of ISN atoms are also affected by elastic collisions that similarly show significant scattering for collision energies typical in the OHS. Eventually, these scattering processes modify distributions of ISN atom observed in the heliosphere directly and as pickup ions. These effects may help explain systematical discrepancies between the IBEX data and current models.