Interaction of the Jovian magnetosphere with Europa: Constraints on the neutral atmosphere

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Abstract
A three‐dimensional plasma model was developed to understand the sources and sinks that maintain Europa's neutral atmosphere and to study the interaction of the Jovian magnetosphere with this atmosphere and the formation of an ionosphere. The model includes self‐consistently the feedback of the plasma action on the atmosphere through mass balance. Suprathermal torus ions with a contribution from thermal ions sputter O2 from the water ice surface, and thermal torus ions remove the O2 atmosphere by sputtering. For an oxygen column density of 5×1018 m−2 the calculated intensities of the oxygen lines OI 130.4 nm and 135.6 nm produced by electron impact dissociation agree with observations by the Hubble Space Telescope [Hall et al., 1995]. Mass balance is also consistent with this column density, with a net atmospheric mass loss of 50 kg s−1. For a given neutral atmosphere and magnetospheric conditions, the electrodynamic model computes self‐consistently plasma density, plasma velocity, electron temperature of the thermal and the suprathermal population, electric current and electric field in the vicinity of Europa, with the assumption of a constant homogeneous Jovian magnetic field. Europa's ionosphere is created by electron impact ionization where the coupling of the ionosphere with the energy reservoir of the plasma torus by electron heat conduction supplies the energy to maintain ionization. The calculated distribution of electron densities with a maximum value of nearly 104 cm−3 is in general agreement with densities derived by Kliore et al. [1997] from the Galileo spacecraft radio occultations. The Alfvénic current system closed by the ionospheric Hall and Pedersen conductivities carries a total current of 7×105 A in each Alfvén wing.