Heliospheric Structure as Revealed by the 3–88 keV H ENA Spectra

Energetic neutral atoms (ENAs) are an important tool for investigating the structure of the heliosphere. Recently, it was observed that fluxes of ENAs (with energy ≤55 keV) coming from the upwind and downwind regions of the heliosphere are similar in strength. This led the authors of these observations to hypothesize that the heliosphere is bubble-like rather than comet-like, meaning that it has no extended tail. We investigate the directional distribution of the ENA flux for a wide energy range (3–88 keV) including observations from IBEX (Interstellar Boundary Explorer), INCA (Ion and Neutral Camera, on board Cassini), and HSTOF (High-energy Suprathermal Time Of Flight sensor, on board the Solar and Heliospheric Observatory). An essential element is the model of pickup ion (PUI) acceleration at the termination shock proposed by Zank. We use state-of-the-art models of the global heliosphere, interstellar neutral gas density, and PUI distributions. The results, based on the "comet-like" model of the heliosphere, are close in flux magnitude to ENA observations by IBEX, HSTOF, and partly those by INCA (except for the 5.2–13.5 keV energy channel). We find that the ENA flux from the tail dominates at high energy (in agreement with HSTOF, but not INCA). At low energy, our comet-like model produces ENA fluxes of similar strength from the upwind and downwind directions—which, therefore, removes this as a compelling argument for a bubble-like heliosphere.