Solar wind He pickup ions as source of tens-of-keV/n neutral He atoms observed by the HSTOF/SOHO detector

Context. Since 1996, during periods of low solar activity, the HSTOF instrument on board the SOHO satellite has been measuring weak fluxes of He atoms of 28–58 keV/n (helium energetic neutral atoms, He ENAs). The probable source region is the inner heliosheath. Aims. We aim to understand the emission mechanism of He ENAs based on knowledge of the heliosheath spatial extent and plasma content resulting from Voyager 1 and 2 measurements in the period after termination shock crossings. Methods. He ENAs are generated by charge-exchange neutralization of energetic helium ions on interstellar neutral H and He. Energy spectra of helium ions in the heliosheath are calculated by following the evolution of their velocity distribution functions when carried by and undergoing binary interactions with plasma constituents of a background flow whose particle populations are modeled to approximately render post-termination-shock Voyager data. Results. The observed HSTOF He ENAs form a higher energy part of general heliospheric He ENA fluxes and can be explained by the proposed mechanism to within 2σ error. The main factor determining the level of emission (and its uncertainty) is the energy spectrum of He + pickup ions in post-termination-shock plasmas.