Abstract
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, 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 the unperturbed interstellar medium, as well as 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 on board 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.
Highlights
We investigate basic requirements for energetic neutral atom (ENA) instruments on a spacecraft headed for heliocentric distances beyond Mars orbit for the energetic neutral atoms (ENAs) energy range between 10 eV and 100 keV
Our knowledge about the heliosphere owes a lot to in situ plasma measurements and remote ENA imaging performed at 1 au, but some questions about the global heliosphere shape, plasma populations, and pressure balances beyond the termination shock may be impossible to answer with observations restricted to the inner solar system
Let us consider the case of largest deviations, i.e., a viewing direction toward +z for the case of the large ellipsoid heliopause and ENA energies corresponding to lc = lHS (Equation (8)): with Equation (9) and radial streamlines, half of the total ENA intensity from the heliosheath along the polar line of sight (LOS) (Equation (1)) is contributed by plasma between the termination shock and 0.38lHS within the heliosheath
Summary
We investigate basic requirements for energetic neutral atom (ENA) instruments on a spacecraft headed for heliocentric distances beyond Mars orbit for the ENA energy range between 10 eV and 100 keV. The resulting ENA leaves its source region on a ballistic trajectory, no longer influenced by electromagnetic fields This allows an ENA camera to image the ion distribution of remote plasma regions (Wurz 2000; Fahr et al 2007). Our knowledge about the heliosphere owes a lot to in situ plasma measurements and remote ENA imaging performed at 1 au (see Section 2.5), but some questions about the global heliosphere shape, plasma populations, and pressure balances beyond the termination shock may be impossible to answer with observations restricted to the inner solar system. We limit ourselves to the prediction and discussion of heliospheric ENAs: we have integrated the existing measurements of heliospheric ENAs from the heliosheath and beyond into a simple empirical model This allows us to make some predictions for an ENA instrument on board a spacecraft at heliocentric distances beyond Mars orbit. A fast-moving spacecraft ( 25 km s−1) must be heading to a vantage point in the upwind hemisphere to seize this opportunity for ISN sampling
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