INTERACTION OF THE JOVIAN MAGNETOSPHERE WITH EUROPA - CONSTRAINTS ON THE NEUTRAL ATMOSPHERE

A three-dimensional plasma model was developed to understand the sourc es and sinks that maintain Europa's neutral atmosphere and to study th e 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 O-2 from the water ice surface, and thermal torus ions remove the O-2 atmosphere by sputtering. For an oxygen column density of 5 x 10(18) m (-2) the calculated intensities of the oxygen lines OI 130.4 nm and 13 5.6 nm produced by electron impact dissociation agree with observation s by the Hubble Space Telescope [Hall et al., 1995]. Mass balance is a lso consistent with this column density, with a net atmospheric mass l oss 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 held in the vi cinity of Europa, with the assumption of a constant homogeneous Jovian magnetic field. Europa's ionosphere is created by electron impact ion ization where the coupling of the ionosphere with the energy reservoir of the plasma torus by electron heat conduction supplies the energy t o maintain ionization. The calculated distribution of electron densiti es with a maximum value of nearly 10(4) cm(-3) is in general agreement with densities derived by Kliore et al. [1997] from the Galileo space craft radio occultations. The Alfvenic current system closed by the io nospheric Hall and Pedersen conductivities carries a total current of 7 x 10(5) A in each Alfven wing.