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Europa, one of the Galilean satellites, has a tenuous, oxygen-dominated atmosphere that is usually referred to as a collision-less exosphere. Coupled to the neutral atmosphere and in contact with Europa's surface, a tenuous ionosphere exists on Europa, whose presence has been revealed by multiple observations. Such an ionosphere is thought to be produced by solar photoionization and electron-impact ionization of the oxygen in the atmosphere. However, a recent study showed that the maximum ionosphere coincides with intermittent water plume on Europa, suggesting that water plays an important role in the formation of the ionosphere. Based on the assumption of horizontal uniformity in the middle of the plume, we use the Caltech/Jet Propulsion Laboratory one-dimensional KINETICS model to construct profiles of neutral and ionized species near the plume region. The simulation results, which show that the ionization reactions are initiated by water electron-impact ionization and photoionization and continued by charge transfer between water and oxygen molecules, have successfully reproduced the observations. We find that H2O+ is the dominant species only above the ionopause. Below the ionopause, the density of H2O+ is orders of magnitude lower than the density of O2+, which is the major composition below the ionopause. Our model has also been used to study the dissociation processes of water molecules from the plumes, which can be regarded as an alternative source for the oxygen in the atmosphere.
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