# Helium Energetic Neutral Atoms from the Heliosphere: Perspectives for Future Observations

## TL;DR

This paper explores the potential for future observations of helium energetic neutral atoms (He ENAs) in the heliosphere, highlighting their sources, expected intensities, and detection challenges, which could enhance understanding of distant heliospheric regions.

## Contribution

It provides the first detailed estimates of He ENA intensities and discusses detection strategies, advancing the remote sensing of helium in the heliosphere beyond hydrogen observations.

## Key findings

- He ENAs mainly originate from the inner heliosheath.
- Future detectors can observe He ENAs at energies of a few to tens of keV/nuc.
- Detection requires mass spectrometry due to low helium-to-hydrogen ratios.

## Abstract

Observations of energetic neutral atoms (ENAs) allow for remote sensing of plasma properties in distant regions of the heliosphere. So far, most of the observations have concerned only hydrogen atoms. In this paper, we present perspectives for observations of helium energetic neutral atoms (He ENAs). We calculated the expected intensities of He ENAs created by the neutralization of helium ions in the inner heliosheath and through the secondary ENA mechanism in the outer heliosheath. We found that the dominant source region for He ENAs is the inner heliosheath. The obtained magnitudes of intensity spectra suggest that He ENAs can be observed with future ENA detectors, as those planned on Interstellar Mapping and Acceleration Probe. Observing He ENAs is most likely for energies from a few to a few tens of keV/nuc. Estimates of the expected count rates show that the ratio of helium to hydrogen atoms registered in the detectors can be as low as 1:10^4. Consequently, the detectors need to be equipped with an appropriate mass spectrometer capability, allowing for recognition of chemical elements. Due to the long mean free paths of helium ions in the inner heliosheath, He ENAs are produced also in the distant heliospheric tail. This implies that observations of He ENAs can resolve its structure, which seems challenging from observations of hydrogen ENAs since energetic protons are neutralized before they progress deeper in the heliospheric tail.