Local Time Variations of Radial Plasma Flow in Saturn's Inner Magnetosphere

Extracting plasma parameters from a non-spinning spacecraft such as Cassini is very hard, however recent advances in forward model techniques and error analysis have led to refined values. Prior analysis on Saturnian inner magnetosphere ion data had provided values with uncertainties that were acceptable for large valued parameters such as V_Φ. However for parameters of smaller magnitude, such as V_r, their uncertainties were often greater than 100% - such that setting V_r = 0 was a possibility and no correlations with any co-ordinate system could be carried out. With our improved techniques and error analysis the uncertainties can be reduced allowing us to explore trends in V_r for the first time. While one might expect an average slow outflow at all local times in the inner magnetosphere, the data actually shows an asymmetry: near dusk the V_r component tends to be negative, while near dawn it's positive. Such asymmetries have been identified before in various datasets, discussed recently by Thomsen et al. (2012), including moon absorption micro-signatures observed offset from the moon's orbit, high energy electrons and isotropic thermal electron and plasma temperatures. They suggest that a noon-to-midnight electric field would explain the data. By assuming a uniform electric field they infer the resultant drift velocity of the plasma due to the field. However they did not have V_r data to compare with. The new V_r data presented here adds to their work, while allowing their drift velocity calculation to be tested. Our V_r data is supportive, however implies a velocity drift a factor of 2-3 times greater. The V_r data also suggests that rather than the induced flow being aligned strictly dusk-to-dawn (towards 06 LT), it is slightly offset and directed towards ~08 LT.