Energetic Neutral Atom Flux from the Inner Heliosheath and Its Connection to Termination Shock Properties

We present statistical comparisons between energetic neutral atom (ENA) fluxes obtained using a global simulation of the heliosphere and data collected by the Interstellar Boundary Explorer (IBEX) spacecraft. The simulation of the inner heliosheath (IHS) ENA flux is based on a 3D steady-state heliosphere, while the data are from the IBEX-Hi instrument over the time period 2009-2015. The statistical comparison is performed by calculating the chi-square value between the simulated ENA fluxes and the data for each line of sight in the sky. A comparison with exposure-averaged data for solar minimum and solar maximum conditions is also performed to see the effect of solar wind (SW) properties on the IHS ENA fluxes. The model matches well with the data in the flanks and parts of the nose of the heliosphere, whereas the match is poor in the downwind tail, ribbon, and polar regions. We interpret these results to mean that (i) heliosheath plasma in the polar region consists of advected fast (or slow) SW during the solar minimum (or maximum) condition, and (ii) heliospheric termination shock parameters are likely different over the poles. A poor match at around 30° north and south of the downwind direction is likely due to the existence of a mixture of plasma that comes from fast and slow SW. While our results are consistent with a single heliotail, the shape of the heliosphere continues to be an area of active research, and more data and further modeling are needed to determine its true structure.