An empirical model of Energetic Neutral Atom imaging of the heliosphere and its implications for future heliospheric missions at great heliocentric distances

TL;DR

This paper develops an empirical model of Energetic Neutral Atom imaging for future heliospheric missions at large distances, outlining instrument requirements and potential scientific insights from outside the heliosphere.

Contribution

It introduces a simple empirical ENA model for distant heliospheric missions and specifies instrumentation needs across the full energy spectrum for comprehensive imaging.

Findings

ENA imaging can provide insights into heliosphere shape and IBEX Ribbon from a few au.

Multiple ENA instruments are needed to cover the 10 eV to 100 keV range.

Outside-the-heliosphere measurements offer the best view of the global heliospheric structure.

Abstract

Several concepts for heliospheric missions operating at heliocentric distances far beyond Earth orbit are currently investigated by the scientific community. The mission concept of the Interstellar Probe (McNutt et al. 2018), e.g., aims at reaching a distance of 1000 au away from the Sun within this century. This would allow the coming generation to obtain a global view of our heliosphere from an outside vantage point by measuring the Energetic Neutral Atoms (ENAs) originating from the various plasma regions. It would also allow for direct sampling of unperturbed interstellar medium, and for many observation opportunities beyond heliospheric science, such as visits to Kuiper Belt Objects, a comprehensive view on the interplanetary dust populations, and infrared astronomy free from the foreground emission of the Zodiacal cloud. In this study, we present a simple empirical model of ENAs from the heliosphere and derive basic requirements for ENA instrumentation onboard a spacecraft at great heliocentric distances. We consider the full energy range of heliospheric ENAs from 10 eV to 100 keV because each part of the energy spectrum has its own merits for heliospheric science. To cover the full ENA energy range, two or three different ENA instruments are needed. Thanks to parallax observations, some insights about the nature of the IBEX Ribbon and the dimensions of the heliosphere can already be gained by ENA imaging from a few au heliocentric distance. To directly reveal the global shape of the heliosphere, measurements from outside the heliosphere are, of course, the best option.