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\begin{document}

\title{Magnetic evidence for an extended hydrogen exosphere at Mercury}



\author[1]{Daniel Schmid}%
\author[2]{Helmut Lammer}%
\author[3]{Ferdinand Plaschke}%
\author[4]{Audrey Vorburger}%
\author[5]{Nikolay Erkaev}%
\author[4]{Peter Wurz}%
\author[6]{Yasuhito Narita}%
\author[7]{Martin Volwerk}%
\author[8]{Wolfgang Baumjohann}%
\author[9]{Brian J. Anderson}%
\affil[1]{Space Research Institute Graz (IWF), Austrian Academy of Sciences (OeAW)}%
\affil[2]{Space Research Inst., Austrian Acad. Sciences}%
\affil[3]{IGEP, Technische Universität Braunschweig}%
\affil[4]{University of Bern}%
\affil[5]{Institute of Computational Modeling}%
\affil[6]{Space Research Institute, Austrian Academy of Sciences}%
\affil[7]{Space Research Institute, OEAW}%
\affil[8]{Space Research Institute}%
\affil[9]{John Hopkins Univ.}%


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  \date{June 10, 2024}


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\begin{abstract}
Remote observations by the Mariner10 and MESSENGER spacecraft have shown
the existence of hydrogen in the exosphere of Mercury. However, to date
the hydrogen number densities could only be estimated indirectly from
exospheric models, based on the remotely observed Lyman-alpha radiances
for atomic hydrogen (H), and the detection threshold of the Mariner10
occultation experiment for molecular hydrogen (H\_2). Here we show the
first on-site determined altitude-density profile of atomic H, derived
from in-situ magnetic field observations by MESSENGER. The results
reveal an extended H exosphere with densities that are
\textasciitilde{}1-2 orders of magnitude larger than previously
predicted. Using an exospheric model that reproduces the H
altitude-density profile, allows us to constrain the so far unknown H\_2
density at the surface which is \textasciitilde{}2-3 orders of magnitude
smaller than previously assumed. These findings demonstrate the
importance (1) of dissociation processes in Mercury's exosphere and (2)
of in-situ measurements giving complementary evidence of processes to
remote observations, that will be realized in the near future by the
BepiColombo mission.%
\end{abstract}%



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