The distribution function of ISN He outside and inside the heliopause

The Warm Breeze (WB) is a new population of neutral interstellar (ISN) helium, discovered by IBEX-Lo. It was postulated to be the secondary population of ISN He, originating from charge exchange between the unperturbed ISN He and the interstellar He⁺ population flowing past the heliopause (HP), slowed, compressed and heated in the outer heliosheath (OHS). Original analyses of the ISN He and WB were based on a simple phenomenological model. The signal observed by IBEX-Lo was simulated assuming that the distribution function of neutral He at 150 au from the Sun, i.e., relatively close to the HP, is a superposition of two Maxwell-Boltzmann (MB) populations with different densities, temperatures, flow directions, and flow speeds. Subsequently, it was shown that the ISN He signal can be qualitatively reproduced in the simulations where it is assumed that there is one MB population of ISN He ahead of the OHS and that charge exchange collisions between He atoms and He⁺ ions in the OHS are responsible for additional populating of atom orbits that ballistically connect the OHS and the IBEX-Lo detector. Here, we investigate in detail the distribution function of neutral He within the OHS, at the HP and inside the termination shock calculated using that latter model. We show that this distribution function has a complex non-Maxwellian form in any singular location within the OHS. However, when one takes collectively the distribution function at the HP of those He atoms that reach 1 AU, one obtains forms much more similar to the MB function. Inside the HP the densities of neutral He calculated from a full kinetic simulation closely match the results obtained from the approximation of two independent MB distributions. This suggests that firstly, the simple two-Maxwellian approximation indeed may provide a realistic information on the geometry of the heliosphere due to deflection of the inflow direction of the secondary population from the direction of Sun's motion through the LISM. Secondly, this approximation provides realistic estimates of the macroscopic parameters of ISN He inside the heliosphere, in particular the density and the related production rate of pickup ions.