Distribution Function of Interstellar Neutral He As Will Be Observed From Interstellar Probe

Interstellar neutral gas interacts by charge exchange with interstellar matter within the outer heliosheath, producing the so-called secondary interstellar neutral atoms, which inherit the kinematic parameters of the parent ions. The secondary population of interstellar He was observationally detected by IBEX-Lo at 1 au. While the secondary interstellar atoms carry information on the physical state of the parent plasma in the outer heliosheath, this information is largely wiped out between the source region and 1 au due to very strong kinematic selection effects. Consequently, observations from 1 au mostly inform on the global-scale deformation of the heliosphere from axial symmetry due to the action of interstellar magnetic field (Bzowski et al. 2017, Kubiak et al. 2019) and the ionization state of interstellar medium (Bzowski et al. 2019). Here, we show that performing observations of interstellar neutral He at distances from the Sun beyond 10 au in the upwind and downwind directions, and particularly performed within the inner and outer heliosheath provide a much more comprehensive view of the plasma state in the outer heliosheath. The upwind and potentially downwind region will be penetrated by the postulated Interstellar Probe spacecraft. We show how the evolution of the flux of interstellar He atoms observed from such spacecraft can be used to understand the plasma distribution in the heliosheath and possibly also in the heliosphere tail region.