Plasma convection in the nightside magnetosphere of Saturn determined from energetic ion anisotropies

The Cassini Ion and Neutral Camera measures intensities of hydrogen and oxygen ions and neutral atoms in the Saturnian magnetosphere and beyond. We use the measured intensity spectrum and anisotropy of energetic hydrogen and oxygen ions to detect, qualify, and quantify plasma convection. We find that the plasma azimuthal convection speed relative to the local rigid corotation speed decreases with radial distance, lagging the planetary rotation rate, and has no significant local time dependences. Plasma in the dusk-midnight quadrant sub-corotates at a large fraction of the rigid corotation speed, with the primary velocity being azimuthal but with a distinct radially outward component. The duskside velocities are similar to those obtained from earlier orbits in the midnight-dawn sector, in contrast to the depressed velocities measured at Jupiter using Energetic Particles Detector measurements on the Galileo spacecraft in the dusk-midnight quadrant. We find significant radial outflow in most of the nightside region. The radial component of the flow decreases with increasing local time in the midnight-dawn sector and reverses as dawn is approached. This and previous results are consistent with a plasma disk undergoing a centrifugally induced expansion as it emerges into the nightside, while maintaining partial rotation with the planet. The magnetodisk expansion continues as plasma rotates across the tail to the dawnside. We do not see evidence in the convection pattern for steady state reconnection in Saturn's magnetotail. The outermost region of the magnetodisk, having undergone expansion upon emerging from the dayside magnetopause confinement, is unlikely to recirculate back into the dayside. We conclude that plasma in the outer magnetodisk [at either planet] rotates from the dayside, expands at the dusk flank, but remains magnetically connected to the respective planet while moving across the tail until it interacts with and is entrained into the dawnside magnetosheath flow. This interaction causes plasma in the outer magnetospheric regions of Jupiter and Saturn to decouple from the planet and exhaust tailward down a dawnside low latitude boundary layer. Magnetospheric plasma will also interact with the dayside magnetosheath plasma, moving across the boundary [enhanced by shear instability] and into the magnetosheath, where it is lost to the magnetosphere with the magnetosheath flow.