Abstract
Energetic neutral atoms (ENAs) observed by the Interstellar Boundary Explorer (IBEX) provide powerful diagnostics about the origin of the progenitor ion populations and the physical mechanisms responsible for their production. Here we survey the fluxes, energy spectra, and energy dependence of the spectral indices of ~0.5-6?keV ENAs measured by IBEX-Hi along the lines of sight of Voyager 1 and 2. We compare the ENA spectra observed at IBEX with predictions of Zank et?al. who modeled the microphysics of the heliospheric termination shock to predict the shape and relative contributions of three distinct heliosheath ion populations. We show that (1) the ENA spectral indices exhibit similar energy dependence along V1 and V2 directions?the spectrum hardens to ? ~ 1 between ~1 and 2?keV and softens to ? ~ 2 below ~1?keV and above ~2?keV, (2) the observed ENA fluxes agree to within ~50% of the Zank et?al. predictions and are unlikely to be produced by core solar wind (SW) ions, and (3) the ENA spectra do not exhibit sharp cutoffs at ~twice the SW speed as is typically observed for shell-like pickup ion (PUI) distributions in the heliosphere. We conclude that ENAs at IBEX are generated by at least two types of ion populations whose relative contributions depend on the ENA energy: transmitted PUIs in the ~0.5-5?keV energy range and reflected PUIs above ~5?keV energy. The ~0.5-5?keV PUI distribution is probably a superposition of Maxwellian or kappa distributions and partially filled shell distributions in velocity space.
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