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Direct sampling of interstellar neutral (ISN) atoms close to the Sun enablesstudies of the very local interstellar medium (VLISM) around the heliosphere.The primary population of ISN helium atoms has, until now, been assumed toreflect the pristine VLISM conditions at the heliopause. Consequently, theatoms observed at 1 au by the Interstellar Boundary Explorer (IBEX) were usedto determine the VLISM temperature and velocity relative to the Sun, withoutaccounting for elastic collisions with other species outside the heliopause.Here, we evaluate the effect of these collisions on the primary ISN heliumpopulation. We follow trajectories of helium atoms and track their collisionswith slowed plasma and interstellar hydrogen atoms ahead of the heliopause.Atoms typically collide a few times in the outer heliosheath, and only ~1.5% ofthe atoms are not scattered at all. We use calculated differential crosssections to randomly choose scattering angles in these collisions. We estimatethat the resulting primary ISN helium atoms at the heliopause are slowed downby ~0.45 km/s and heated by ~1100 K compared to the pristine VLISM. Theresulting velocity distribution is asymmetric and shows an extended tail in theantisunward direction. Accounting for this change in the parameters derivedfrom IBEX observations gives the Sun's relative speed of 25.85 km/s andtemperature of 6400 K in the pristine VLISM. Finally, this paper serves as asource of the differential cross sections for elastic collisions with heliumatoms.

Slowdown and Heating of Interstellar Neutral Helium by Elastic Collisions Beyond the Heliopause