  We constrain Europa's tenuous atmosphere on the subsolar hemisphere by
combining two sets of observations: oxygen emissions at 1304 {\AA} and 1356
{\AA} from Hubble Space Telescope (HST) spectral images, and Galileo magnetic
field measurements from its closest encounter, the E12 flyby. We describe
Europa's atmosphere with three neutral gas species: global molecular
($\mathrm{O_2}$) and atomic oxygen (O), and localized water ($\mathrm{H_2O}$)
present as a near-equatorial plume and as a stable distribution concentrated
around the subsolar point on the moon's trailing hemisphere. Our combined
modelling based on the ratio of OI 1356 {\AA} to OI 1304 {\AA} emissions from
Roth (2021) and on magnetic field data allows us to derive constraints on the
density and location of $\mathrm{O_2}$ and $\mathrm{H_2O}$ in Europa's
atmosphere. We demonstrate that $50\%$ of the $\mathrm{O_2}$ and between $50\%$
and $75\%$ of the $\mathrm{H_2O}$ abundances from Roth (2021) are required to
jointly explain the HST and Galileo measurements. These values are conditioned
on a column density of $\mathrm{O}$ close to the upper limit of $6
\times10^{16}~\mathrm{m}^{-2}$ derived by Roth (2021), and on a strongly
confined stable $\mathrm{H_2O}$ atmosphere around the subsolar point. Our
analysis yields column densities of $1.2 \times10^{18}~\mathrm{m}^{-2}$ for
$\mathrm{O_2}$, and $1.5 \times10^{19}~\mathrm{m}^{-2}$ to $2.2
\times10^{19}~\mathrm{m}^{-2}$ at the subsolar point for $\mathrm{H_2O}$. Both
column densities however still lie within the uncertainties of Roth (2021). Our
results provide additional evidence for the existence of a stable
$\mathrm{H_2O}$ atmosphere at Europa.

Space Physics;Earth and Planetary Astrophysics