Spectral properties of keV-energetic ion populations inside the heliopause reflected by IBEX-relevant energetic neutral atoms

Abstract

For more than three years now, the IBEX mission monitors fluxes of energetic neutral atoms (ENAs) in the range between 0.1 to 6 keV from all directions of the sky. During this extended observation period, it could be confirmed that an outstanding ENA emission feature, called the “ribbon”, has been continuously found over the past years. In addition, spectral flux features also have become the target of refined observations. In this study, we thus aim at delivering theoretically calculated results on spectral ENA flux properties. The ENAs we are considering in this paper are due to two types of transcharged pick-up ions present in the heliosheath region: primary pick-up ions that have been energetically processed at their passage over the termination shock, and ions induced to the pick-up ion regime from adiabatically cooled anomalous cosmic ray ions. Both components have different spectra, so we need to consider ENA fluxes generated by charge-exchange processes from the superposed distributions of these two populations. We derive the spectral features of ENA fluxes along a line-of-sight as a function of the celestial direction and compare them with IBEX observations. The power-law characteristics of the resulting spectra exhibit a good match with the average power indices observed by IBEX in large sections of the heliosphere, with the notable exception of the heliotail, for which our adopted heliospheric model does not provide an accurate enough representation.