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\begin{document}

\title{In Situ Measurements of Thermal Ion Temperature in the Martian
Ionosphere}



\author[1]{Kathleen Gwen Hanley}%
\author[2]{James P. Mcfadden}%
\author[2]{David L. Mitchell}%
\author[3]{Christopher M Fowler}%
\author[4]{Shane Wesley Stone}%
\author[4]{Roger Yelle}%
\author[5]{Majd Mayyasi}%
\author[6]{Robert E Ergun}%
\author[7]{Laila Andersson}%
\author[8]{Mehdi Benna}%
\author[8]{Meredith K Elrod}%
\author[9]{Bruce M. Jakosky}%
\affil[1]{Space Science Laboratory, UC Berkerley}%
\affil[2]{University of California, Berkeley}%
\affil[3]{Space Sciences Laboratory, UC Berkeley}%
\affil[4]{University of Arizona}%
\affil[5]{Boston University}%
\affil[6]{Univeristy of Colorado}%
\affil[7]{Laboratory for Atmospheric and Space Physics (LASP)}%
\affil[8]{NASA Goddard Space Flight Center}%
\affil[9]{University of Colorado Boulder}%


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  \date{May 25, 2024}


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\begin{abstract}
In situ measurements of ionospheric and thermospheric temperatures are
experimentally challenging because orbiting spacecraft typically travel
supersonically with respect to the cold gas and plasma. We present O2+
temperatures in Mars' ionosphere derived from data measured by the
SupraThermal And Thermal Ion Composition (STATIC) instrument onboard the
Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft. We focus on
data obtained during nine special orbit maneuvers known as Deep Dips,
during which MAVEN lowered its periapsis altitude from the nominal 150
km to 120 km for one week in order to sample the ionospheric main peak
and approach the homopause. We use two independent techniques to
calculate ion temperatures from the measured energy and angular widths
of the supersonic ram ion beam. After correcting for background and
instrument response, we are able to measure ion temperatures as low as
100 K with associated uncertainties as low as 10\%. It is theoretically
expected that ion and electron temperatures will converge to the neutral
temperature at altitudes below the exobase region
(\textasciitilde{}180-200 km) due to strong collisional coupling;
however, no evidence of the expected thermalization is observed. We have
eliminated several possible explanations for the observed temperature
difference between ions and neutrals, including Coulomb collisions with
electrons, Joule heating, and heating caused by interactions with the
spacecraft. Our current study leaves one plausible heating mechanism,
the release of internal energy from O2+ that becomes vibrationally
excited as a result of atmospheric chemistry, but future work is needed
to assess its validity.%
\end{abstract}%



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