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\begin{document}

\title{What Does Our Heliosphere Look Like in Energetic Neutral Atoms? Some
Recommendations for a Low-Energy ENA Camera Onboard the Interstellar
Probe}



\author[1]{André Galli}%
\author[1]{Peter Wurz}%
\author[2]{Jens Kleimann}%
\author[2]{Horst Fichtner}%
\author[3]{Yoshifumi Futaana}%
\author[3]{Stas Barabash}%
\affil[1]{University of Bern, Switzerland}%
\affil[2]{Ruhr University Bochum, Germany}%
\affil[3]{IRF Swedish Institute of Space Physics, Kiruna, Sweden}%


\vspace{-1em}



  
  \date{June 14, 2024}


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\begin{abstract}
The concept of an Interstellar Probe (ISP) offers an intriguing
combination of scientific break-throughs in several disciplines. These
include a global view of our heliosphere, unperturbed sampling of the
interstellar medium, discoveries of Kuiper Belt Objects, and many
others. The current mission concept of the ISP aims at reaching a
distance of 1000 au away from the Sun within this century, far beyond
the heliopause at roughly 100-200 au. In this presentation, we
investigate basic requirements for an Energetic Neutral Atom (ENA)
instrument onboard the ISP for the energy range between 10 eV and 5 keV.
An ENA is produced when a fast ion exchanges its charge with an ambient
neutral atom. The resulting ENA leaves the source region on a straight
trajectory, no longer influenced by electromagnetic fields. This allows
an ENA camera to image the ion distribution of remote plasma regions. We
calculate the energy spectrum of heliospheric ENAs an observer would see
from a given vantage point inside or outside the heliopause. Since the
global shape of the heliosphere is unknown yet, we use two analytical
models to derive proton flowlines for two different heliospheric shapes:
the Parker model {[}Parker 1961{]} modified with a termination shock and
the analytical representation of a full MHD model {[}R\selectlanguage{ngerman}öken et al. 2015,
Kleimann et al. 2017{]}. The ENA intensity then is the line-of-sight
integral of proton density times the local density of neutral hydrogen
times the charge-exchange cross-section. We disregard any other neutral
species inside the heliosphere and we only consider protons as source
for ENAs. The proton populations included are the supersonic solar wind
and pickup ions inside the termination shock and the shocked solar wind
and pickup ions between termination shock and heliopause. The calculated
ENA intensities are first compared to the globally distributed ENA flux
measured by the Interstellar Boundary Explorer and Cassini in the inner
solar system in the energy range from 10 eV to 55 keV. We then proceed
to calculate the ENA intensity as seen by an observer at other positions
near or beyond the heliopause. These predictions can serve as a rough
guideline for the mission concept of ISP: which trajectory offers the
most interesting view on the heliosphere in ENAs and which technical
requirements should a low-energy ENA imager meet?%
\end{abstract}\selectlanguage{ngerman}%



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