In-situ observations of a neutral gas torus at Europa

[1] A persistent pattern in the pitch angle distributions of energetic protons near the orbit of Europa has been observed with the Energetic Particles Detector (EPD) on board the Galileo spacecraft during each of the Europa orbit crossings in the last 7 years. The proton fluxes at energies larger than 220 keV peak at equatorial pitch angles of 90degrees whereas fluxes of lower energy protons (80-220 keV) at this pitch angle are depleted. We propose that a Jupiter-surrounding neutral gas torus in the vicinity of the orbit of Europa is responsible for the depletion of energetic particle fluxes by charge exchange collisions. In order to reproduce the observed depletion signature an average neutral number density of 20 to 50 cm(-3) is required.