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\begin{document}

\title{The Earth's outer exospheric density distributions derived from
PROCYON/LAICA UV observations}



\author[1]{Gonzalo CUCHO-PADIN}%
\author[2]{Shingo Kameda}%
\author[3]{David Gary Sibeck}%
\affil[1]{NASA Goddard Space Flight Center}%
\affil[2]{Rikkyo University}%
\affil[3]{GSFC}%


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  \date{June 14, 2024}


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\begin{abstract}
Current three-dimensional, data-based models for the terrestrial
exosphere have been derived from measurements of optically thin
Lyman-alpha (Ly-\selectlanguage{greek}α\selectlanguage{english}) emissions scattered by neutral hydrogen atoms. Such
models are only valid for the middle exospheric region (3-8 Earth radii
geocentric distances) since the orbital paths of the space-based
platforms used to acquire Ly-\selectlanguage{greek}α \selectlanguage{english}radiance were located within the
exosphere, thus precluding the proper detection of the faint outer
exospheric emission. Notwithstanding, accurate specifications of density
distributions beyond 8 RE are needed to support comprehensive studies of
the solar-terrestrial interactions. Two upcoming missions, the Solar
wind Magnetosphere-Ionosphere Link Explorer (SMILE) and the Lunar
Environment Heliospheric X-ray Imager (LEXI), will image the Earth's
magnetosheath in soft X-rays, and neutral densities are crucial to
extract ion distributions through inversion of the acquired images. This
work develops a technique to estimate the Earth's outer exospheric
density distributions using far-ultraviolet wide-field data acquired by
the Lyman-Alpha Imaging Camera (LAICA) onboard the Proximate Object
Close Flyby with Optical Navigation mission. Our approach formulates an
inverse problem based on the linearity between measurements of scattered
Ly-\selectlanguage{greek}α \selectlanguage{english}flux and the local atomic hydrogen density, which is solved using
the Bayesian approach known as Maximum a posteriori estimation. We use
the LAICA image to derive global, 3-D hydrogen density distributions at
6-35 RE geocentric distances. We find that the spatial structure of the
outer exosphere agrees well with the predictions of radiation pressure
theory. Further, we find that the mean hydrogen density at 10 RE
subsolar point is 26.51 atoms/cm\textsuperscript{3}.%
\end{abstract}%



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\verb`supporting information .docx`  available at \url{https://authorea.com/users/521385/articles/602117-the-earth-s-outer-exospheric-density-distributions-derived-from-procyon-laica-uv-observations}

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