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

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.

An Empirical Model of Energetic Neutral Atom Imaging of the Heliosphere and Its Implications for Future Heliospheric Missions at Great Heliocentric Distances

“…We have created a simple empirical model to predict the differential intensity j ENA (in units of cm −2 sr −1 s −1 keV −1 ) of heliospheric ENAs an ENA camera would observe for any given location and viewing direction. The fundamental ENA equation, (Swaczyna et al 2017). This He signal is two orders of magnitude weaker than typical H ENA intensities at 1 au and has not been observed yet, but it should be detectable with the future IMAP-Hi instrument (Swaczyna et al 2017;McComas et al 2018).…”

“…The fundamental ENA equation, (Swaczyna et al 2017). This He signal is two orders of magnitude weaker than typical H ENA intensities at 1 au and has not been observed yet, but it should be detectable with the future IMAP-Hi instrument (Swaczyna et al 2017;McComas et al 2018). The long cooling length of keV helium ions in the heliosheath would make He ENAs ideally suited to probe the far heliosheath (Swaczyna et al 2017).…”

Constraining the Evolution of the Proton Distribution Function in the Heliotail

“…Parameters for the Initial Proton Distribution Functions Downstream of the TS PUIs reflected and then transmitted across the TS. This composite distribution is commonly used to model multicomponent ion species in the heliosheath (e.g.,Zank et al 2010;Zirnstein et al 2014Zirnstein et al , 2017Swaczyna & Bzowski 2017a;Swaczyna et al 2017b). Unlike the case where we process the filled-shell distribution across the TS, here we simply assume that the downstream distribution is represented by three Maxwell-Boltzmann distributions with total density n p,0 and effective temperature T p,0 .…”

Modeling Emission of Heavy Energetic Neutral Atoms from the Heliosphere

“…In a previous paper (Swaczyna et al 2017, Paper I), we modeled helium ENA emission using both components: the globally distributed flux from the inner heliosheath and the IBEX ribbon formed in the outer heliosheath through the secondary ENA mechanism. The present paper complements the estimations of heavy ENA emission from the inner heliosheath with calculations made for three additional chemical elements: oxygen, nitrogen, and neon.…”

“…Swaczyna et al (2014) made assessments of helium ENA emission in two alternative mechanisms of the IBEX ribbon. They showed that helium ENA intensities from the ribbon are typically smaller than the inner heliosheath emission in the hypothesis of the secondary ENA emission from the outer heliosheath (Schwadron et al 2009;Heerikhuisen et al 2010).…”