Heavy coronal ions in the heliosphere. II. Expected fluxes of energetic neutral He atoms from the heliosheath

TL;DR

This paper models the fluxes of energetic neutral helium atoms originating from the heliosheath, aiming to use these fluxes to infer properties of the heliosheath plasma and structure.

Contribution

It develops a detailed model of helium ion neutralization and predicts ENA fluxes, providing a basis for interpreting IBEX observations to understand heliosheath conditions.

Findings

He ENA fluxes are predicted to be detectable by IBEX.

Energy spectra of He ENA peak between 0.2 and 3 keV.

Tail-to-apex intensity ratios vary significantly with model assumptions.

Abstract

Aims. A model of heliosheath density and energy spectra of alpha-particles and He+ ions carried by the solar wind is developed. Neutralization of heliosheath He+ ions, mainly by charge exchange (CX) with neutral interstellar H and He atoms, gives rise to ~0.2 - ~100 keV fluxes of energetic neutral He atoms (He ENA). Such fluxes, if observed, would give information about plasmas in the heliosheath and heliospheric tail. Methods. Helium ions crossing the termination shock (TS) constitute suprathermal (test) particles convected by (locally also diffusing through) hydrodynamically calculated background plasma flows (three versions of flows are employed). The He ions proceed from the TS towards heliopause (HP) and finally to the heliospheric tail (HT). Calculations of the evolution of alpha- and He+ particle densities and energy spectra include binary interactions with background plasma and interstellar atoms, adiabatic heating (cooling) resulting from flow compression (rarefaction), and Coulomb scattering on background plasma. Results. Neutralization of suprathermal He ions leads to the emergence of He ENA fluxes with energy spectra modified by the Compton-Getting effect at emission and ENA loss during flight to the Sun. Energy-integrated He ENA intensities are in the range ~0.05 - ~50 cm^-2 s^-1 sr^-1 depending on spectra at the TS (assumed kappa-distributions), background plasma model, and look direction. The tail/apex intensity ratio varies between ~1.8 and ~800 depending on model assumptions. Energy spectra are broad with maxima in the ~0.2 - ~3 keV range depending on the look direction and model. Conclusions. Expected heliosheath He ENA fluxes may be measurable based on the capabilities of the IBEX spacecraft. Data could offer insight into the heliosheath structure and improve understanding of the post-TS solar wind plasmas. HT direction and extent could be assessed.