Origins of Europa Na cloud and torus

Following the discovery observations by Brown and Hill [Brown, M.E., Hill, R.E., 1996. Discovery of an extended sodium atmosphere around Europa. Nature 380, 229-231], only few new observations of Europa Na cloud and torus have been reported [Brown, M.E., 2001. Potassium in Europa's atmosphere. Icarus 151, 190-195; Brown, M.E., 2004. The structure and variability of Europa's sodium atmosphere. Icarus. Submitted for publication]. Previous works [Johnson, R.E., Leblanc, F., Yakshinskiy, B.V., Madey, T.E., 2002. Energy distributions for desorption of sodium and potassium from ice: The Na/K ratio at Europa. Icarus 156, 136-142; Leblanc, F., Johnson, R.E., Brown, M., 2002. Europa's sodium atmosphere: An ocean source? Icarus 159, 132-144] have reproduced some of these observations and concluded that the origin of Europa's Na cloud is primarily sputtering of Europa's surface by jovian magnetospheric energetic particles. Leblanc, Johnson, and Brown [Leblanc, F., Johnson, R.E., Brown, M., 2002. Europa's sodium atmosphere: An ocean source? Icarus 159, 132-144] suggested there was a correlation of the rapid variation of the total content of Europa's Na cloud with the variation of Europa's jovian centrifugal latitude. The contribution to the observed signal due to Na atoms ejected from Io and energetic Na ejected from Europa by atmospheric sputtering were also considered. In this work, we present improved results using a more accurate description of the variation of the magnetospheric incident flux, electron impact ionization and Io background with respect to Europa's and Io's positions in Jupiter's magnetosphere. We also describe the energetic component due to atmospheric sputtering. This model of the sodium cloud is constrained by new observations of Europa's sodium cloud close to Europa's surface by Potter and co-workers also presented here. Their high spectral resolution provides unprecedented detail of the velocity distribution of the Na atoms at different positions around Europa and for various Europa positions around Jupiter. This analysis confirms a number of earlier conclusions and highlights observed variations of Europa Na cloud with respect to Europa's local time within the jovian magnetosphere. These large observed variations require that the ballistic transport across Europa's surface must be taken into account.