Slowdown and Heating of Interstellar Neutral Helium by Elastic
  Collisions Beyond the Heliopause
  Direct sampling of interstellar neutral (ISN) atoms close to the Sun enables
studies of the very local interstellar medium (VLISM) around the heliosphere.
The primary population of ISN helium atoms has, until now, been assumed to
reflect the pristine VLISM conditions at the heliopause. Consequently, the
atoms observed at 1 au by the Interstellar Boundary Explorer (IBEX) were used
to determine the VLISM temperature and velocity relative to the Sun, without
accounting for elastic collisions with other species outside the heliopause.
Here, we evaluate the effect of these collisions on the primary ISN helium
population. We follow trajectories of helium atoms and track their collisions
with slowed plasma and interstellar hydrogen atoms ahead of the heliopause.
Atoms typically collide a few times in the outer heliosheath, and only ~1.5% of
the atoms are not scattered at all. We use calculated differential cross
sections to randomly choose scattering angles in these collisions. We estimate
that the resulting primary ISN helium atoms at the heliopause are slowed down
by ~0.45 km/s and heated by ~1100 K compared to the pristine VLISM. The
resulting velocity distribution is asymmetric and shows an extended tail in the
antisunward direction. Accounting for this change in the parameters derived
from IBEX observations gives the Sun's relative speed of 25.85 km/s and
temperature of 6400 K in the pristine VLISM. Finally, this paper serves as a
source of the differential cross sections for elastic collisions with helium
atoms.
