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\begin{document}

\title{Vertical Structure of the Callisto Ionosphere from Galileo Radio
Occultation Data and Its Implication on the Moon's Interior.}



\author[1]{Ashok Verma}%
\author[2]{Krishan Khurana}%
\author[1]{Raymond Walker}%
\author[3]{Jean-Luc Margot}%
\affil[1]{University of California Los Angeles}%
\affil[2]{University of California at Los Angeles}%
\affil[3]{UCLA}%


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  \date{May 19, 2024}


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\begin{abstract}
Using the magnetic field observed by Galileo during two flybys of
Callisto, Khurana et al. (1998) demonstrated that Callisto generates a
strong induction response to the time-varying primary field, indicative
of the presence of a subsurface ocean. In contrast, Hartkorn and Saur
(2017) modeled the atmosphere and ionosphere of Callisto and suggested
that the ionosphere could be responsible for a significant part of the
observed magnetic fields. Thus, they concluded that the water ocean
might be located much deeper than previously thought or might not exist
at all. While Khurana et al. (1998) did not account for the induction
within a conductive ionosphere, Hartkorn and Saur (2017) overestimated
the conductivity of the ionosphere by using Cowling conductivity which
is not applicable for the situation at Callisto. In this paper, we
re-analyzed the S-band open-loop one-way Doppler data of the Galileo
spacecraft with the aim to derive the electron density (ED) and neutral
density (ND) profiles of Callisto and address its implication in terms
of moon's conductivities and interiors. Using modern orbit determination
software, MONTE, and the most up-to-date information on the Jovian
system, we reconstructed the Galileo orbit with a full dynamical
approach. The estimated rms values of the Doppler residuals for baseline
measurement vary from 0.01-0.08 Hz, well within the expected noises of
the radio signals. We used these residuals to derive the ED profiles
using the technique discussed in Verma et al., (2019). We found an
appreciable ionosphere for C22 and C23 Ingress occultations with peak
densities of 15600\selectlanguage{ngerman}±900 cm-3 and 17700±600 cm-3, respectively. For other
cases, the detections do not exceed the 3-\selectlanguage{greek}σ \selectlanguage{english}level. While the general
features of the EDs are consistent with Kliore et al. (2002), our
estimated 1-\selectlanguage{greek}σ \selectlanguage{english}formal uncertainties are 2-3 times better presumably
because of the constrained Galileo's orbit. Assuming O2 as the major
component of the Callisto's atmosphere, the estimated ND (weighted mean)
at the surface is 2.0\selectlanguage{ngerman}±0.33 x10-10 cm-3 which corresponds to a column
density of 3.9±0.35 x10-16 cm-2 (see Figure). Finally, we will use these
density profiles to constrain the ionospheric conductivities and address
their implications in terms of the presence of a subsurface ocean.%
\end{abstract}\selectlanguage{ngerman}%



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