Interpretation of the IBEX-Lo Observations of Energetic Neutral Atoms along the Voyager Trajectories

This Letter addresses a long-standing discrepancy between simulated energetic neutral atom (ENA) fluxes and Interstellar Boundary Explorer (IBEX) observations in the ∼0.02–0.1 keV energy range. We investigate the origin of low-energy ENA fluxes using simulations of the solar wind (SW) interaction with the local interstellar medium (LISM), with particular emphasis on proton distribution functions both in the inner heliosheath and in the perturbed LISM. Compared with previous studies, the new models reduce the discrepancy with IBEX-Lo observations in the Voyager directions up to 2 orders of magnitude, while also reproducing the IBEX-Hi fluxes. The simulations indicate that IBEX-Lo fluxes are significantly affected by the thermal ion distribution functions at the heliospheric interface. The results further suggest that outer-heliosheath protons may be described by the κ distributions. ENA fluxes are also sensitive to SW variability and global model assumptions.