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

TL;DR

This paper analyzes IBEX observations of interstellar helium, revealing elastic scattering by solar wind protons affects helium distribution measurements, leading to revised density estimates of interstellar helium near Earth.

Contribution

It introduces a model accounting for elastic scattering effects, improving the accuracy of interstellar helium density estimations from IBEX data.

Findings

Elastic scattering creates a halo in helium flux distribution.

Correcting for scattering increases helium density estimates by ~10%.

Discrepancies during solar cycle 24 are explained by scattering effects.

Abstract

The IBEX-Lo instrument on the Interstellar Boundary Explorer (IBEX) mission observes primary and secondary interstellar helium in its 4 lowest energy steps. Observations of these helium populations have been systematically analyzed and compared to simulations using the analytic full integration of neutrals model (aFINM). 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 flux distribution, provides an explanation of the observed divergence from simulations. We verify that elastic scattering forms a halo in the wings of the primary He distribution in the spin-angle direction. 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. Thus, based on our analysis of IBEX observations and $\chi ^2$ 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 $~\sim$ 10%