Plasma Circulation and Escape at the Magnetospheres of Jupiter and Saturn

The Ion and Neutral Camera (INCA) 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 detect and quantify plasma convection. Within Titan's orbit, we find that the plasma azimuthal convection speed increases with radial distance but lags the SKR rotation rate, with insignificant local time anomalies. We find evidence of some radial outflow in the nightside region. In the outer regions of the magnetosphere, equatorial plasma azimuthal speeds plateau. This plasma is increasingly less likely to recirculate into the dayside at greater radial distance. We find evidence that this plasma interacts with the magnetopause and escapes along the dawn flank of the magnetotail. Given that nightside reconnection is apparently sporadic and that simulation suggests that a small fraction of plasma produced in the inner magnetosphere actually escapes via reconnection, we conclude that energetic ion convection and composition sensed by the INCA detector are consistent with a significant escape of plasma near the dawn magnetopause. Given the detection of Kelvin-Helmholtz waves at the dawn magnetopause, reconnection may be readily initiated there and contribute to the escape of plasma from the system. We also consider observations by energetic particle detectors on the Voyager and Galileo spacecraft in Jupiter's magnetosphere, and find that azimuthal convection measurements and observations of plasma deep down the dawn flank of the magnetotail suggest a similar pattern of plasma motion and escape as that operating in Saturn's magnetosphere. Thus we conclude that there is evidence consistent with plasma production and transport from inner subcorotating regions to extended nightside regions where plasma under centrifugal stress becomes increasingly less likely to recirculate to the dayside, but largely remains magnetically connected to the respective planet until it interacts with the dawnside magnetosheath flow, causing plasma to exhaust down a dawnside boundary layer.