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\begin{document}

\title{Space-based Remote Sensing Strategies for Tomographic Estimation of
Exospheric Hydrogen Density}



\author[1]{Gonzalo Cucho-Padin}%
\author[1]{Lara Waldrop}%
\affil[1]{University of Illinois at Urbana-Champaign}%


\vspace{-1em}



  
  \date{May 24, 2024}


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\begin{abstract}
It has been four decades since Apollo 16 returned the first wide-field
UV imagery of the Earth and revealed the vast extent of exospheric
hydrogen (H) atoms around the planet. Since that time, appreciation has
grown regarding the significance of this outermost atmospheric layer,
whose charge exchange interaction with ambient ions dissipates
magnetospheric energy, generates the energetic neutral atoms (ENAs)
widely used for remote sensing of the ring current dynamics during
geomagnetic storms, and accelerates gravitational escape and thus
permanent atmospheric evolution. Despite the importance of Earth's H
exosphere to the solar-terrestrial system, however, current
understanding of its global structure and dynamical evolution is
insufficient, such that the origin of persistent discrepancies between
measurements and models remains unresolved. Remote sensing of UV
emission from geocoronal H atoms, generated through resonant scattering
of solar radiation at 121.6 nm (Lyman-alpha) is the only empirical means
available to investigate the terrestrial exosphere. In this work, we
present robust tomographic-based techniques we have developed in recent
years to estimate the 3-D, global and time-dependent H-density
distributions during quiet and storm-time from observations of its
optically thin emission at Lyman-\selectlanguage{greek}α \selectlanguage{english}acquired from the Lyman-alpha
detectors onboard the NASA TWINS satellites. Several examples of recent
2D and 3D data analyses will be used to demonstrate the current
state-of-the-art, reveal surprising exospheric phenomenon, and motivate
future work.%
\end{abstract}%



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