Detailed Measurements of Ion Anisotropies by the Cassini INCA Experiment and Calculated Convection Velocities in Saturn's Magnetosphere

The Ion and Neutral Camera (INCA), part of a cluster of instruments on the Cassini spacecraft, measures intensities of hydrogen and oxygen ions and neutral atoms in the Saturnian magnetosphere. We use the measured intensity spectrum and anisotropy of hot hydrogen and oxygen ions to calculate the plasma bulk velocity. We find that, throughout a wide region of this magnetosphere, the bulk plasma is capable of nearly rigid co-rotation within Titan's orbit, beyond which the bulk flow falls increasingly further behind the rigid rate. The anisotropies are sometimes characterized well by convecting isotropic populations, but at other times are not. In some cases time aliasing due to a dynamic or inhomogeneous ion population convecting past the spacecraft prevents such a calculation. However, we have performed calculations in a wide region of the magnetosphere, where some ion populations are nearly isotropic in their convection frame. We find that, in the dawn and dusk regions, convection approaching rigid co-rotation speeds can be maintained out to the vicinity of the Titan orbit. We have examined equatorial orbits as well as data at low to moderate latitudes in the dusk side. Thus far, we find no evidence for a dusk side depression similar to that found within Jupiter's magnetosphere. We have performed calculations at low to moderate latitudes within orbits highly inclined to Saturn's equatorial plane on the night side and find evidence of convection at substantial fractions of the rigid co-rotation as determined by the given L-shell equatorial distance, consistent with the pattern found within equatorial orbits. We find evidence of similar behavior in the day side region.