  Interstellar neutral atoms provide a remote diagnostic of plasma in the outer
heliosheath and the very local interstellar medium via charge exchange
collisions that convert ions into atoms and vice versa. So far, most studies of
interstellar atoms assumed that daughter hydrogen atoms directly inherit the
kinetic properties of parent protons. This assumption neglects angular
scattering of the interacting particles. However, for low relative velocities,
as expected for charge exchanges in the outer heliosheath, this scattering is
significant. In this study, we present how the parameters of daughter
populations depend on the relative velocity and temperatures of parent
populations. For this purpose, we numerically compute collision terms with and
without this scattering. We find that the secondary population of interstellar
hydrogen atoms, for the parent populations with the relative bulk velocity of
20 km s$^{-1}$ and equal temperatures of 7500 K, has ~2 km s$^{-1}$ higher bulk
velocity if the scattering is taken into account. Additionally, temperatures
are higher by ~2400 K and ~1200 K in parallel and perpendicular direction to
the relative motion of parent populations, respectively. Moreover, a
significant departure of secondary atoms from the Maxwell-Boltzmann
distribution is expected for high relative velocities of parent populations.
This process affects the distribution and density of interstellar atoms in the
heliosphere and production of pickup ions. Thus, we show that angular
scattering in charge exchange collisions is important to include in analyses of
interstellar neutral atoms and pickup ions observed at 1 au and in the outer
heliosphere.
