IBEX Observations of Elastic Scattering of Interstellar Helium by Solar Wind Particles

Abstract The IBEX-Lo instrument on the Interstellar Boundary Explorer (IBEX) mission observes primary and secondary interstellar helium in its four lowest energy steps. Observations of these helium populations have been analyzed and compared to simulations using the analytic full integration of neutrals model. A systematic difference is observed between the simulations and observations of secondary helium during solar cycle (SC) 24. We show that elastic scattering of primary helium by solar wind protons, which redistributes atoms from the core of the primary helium flux distribution, provides an explanation of the observed divergence from simulations. We test the validity of this explanation by showing that the elastic scattering also forms a halo in the wings of the primary helium distribution in the spin-angle plane. Correcting the simulation for the effects of elastic scattering requires an increase of the estimated density of primary helium, compared to previous estimates by Ulysses/GAS. The combined density of interstellar helium, derived from the previous studies, has been recalculated to be 0.0153 ± 0.0011. Based on our analysis of IBEX observations and χ 2 minimization of simulation data that include the effects of elastic scattering, any estimation of neutral interstellar helium density at 1 au by direct detection of the peak flux of neutral helium needs to be adjusted by ∼12%.