The inner heliosheath source for keV-ENAs observed with IBEX: Shock-processed downstream pick-up ions

Context. The "IBEX ribbon" feature provides significant challenges to theoretical modeling attempts of the outer heliosphere. Classical ENA production by shock-processed nonthermal tails results in no relevant ENA signatures, mainly due to the downstream solar wind proton temperature observed by the Voyagers being one order of magnitude smaller than expected from the classical monofluid shock model. Aims. Here, we therefore study shock-processed pick-up ions (PUIs) in the inner heliosheath resulting from a multifluid shock model as a source of keV-energetic ENAs as have been detected by the IBEX mission. Unlike previous studies, we apply a semikinetic multifluid shock model, which did prove useful in explaining the apparent temperature discrepancy in the past. Methods. To convert upstream into downstream pick-up ions, we use both kinetic and multifluid theories describing the solar wind termination shock (TS) transition. This allows us to obtain the downstream PUI distribution function as a function of classical shock properties, such as the local magnetic field tilt angle and the compression ratio. In addition, this kinetic model also allows to derive a formulation of latitude- and longitude-dependent spectral intensities between 1 and 100 keV, a region which is not covered by missions such as the Voyagers and needs to be covered by theoretical considerations. Results. After converting shock-processed PUIs to ENAs by charge exchange with cold H-atoms, we find keV ENA fluxes of the same order of magnitude as those observed by IBEX. These fluxes also exhibit a pronounced ring-type feature, with about the correct ratio of ring-to-nose intensities, producing encouraging overall agreement with IBEX data, potential of further improvement.