Enceladus plume variability and the neutral gas densities in Saturn's magnetosphere



[1] Neutral particle dominance over charged particles in Saturn's magnetosphere was evident prior to Cassini's arrival at Saturn in 2004. The observation of active plumes emanating from the south pole of Enceladus suggests that this small moon is likely to be the principal source of neutrals in Saturn's magnetosphere. Cassini has flown through the plumes on several occasions, and the resulting data imply the source rate is variable (∼1027 to 1028 water molecules/s). Here we use Cassini plasma spectrometer and Cassini magnetospheric imaging instrument observations to update neutral particle lifetimes and then use the most recent processed versions of Cassini ion neutral mass spectrometer observations made during encounters E2, E3, and E5 to constrain a 3-D multispecies neutral particle model. This procedure improves constraints on the plume source rate, ejection velocity, and plume divergence. We find that the plume source rate varies by at least a factor of 4 over the 7 month period considered. Additionally, we find that previous estimates of the plume source rates based on E2 observations are most likely overestimated because the background neutral torus has not been adequately account for. On the basis of these results, we discuss the implications of this variability on global neutral particle distributions.