Evidence from radial velocity measurements of a global electric field in Saturn's inner magnetosphere

Abstract

The presence of a convection pattern in the Saturnian inner magnetosphere was first observed in moon signatures that were displaced from their expected locations. A noon to midnight directed electric field can account for these positional offsets, and also can explain asymmetries found in plasma temperatures between the day and night sides. By assuming magnetic field models and drift times, estimates of the electric field strength have been made. However the origin of the electric field remains a mystery. This paper, which presents new evidence for the convection pattern, uses measured in situ velocities with measured magnetic field values, at a range of radial distances without the restriction of requiring the presence of a moon. Advances in extracting plasma parameters with reduced uncertainties clearly show the convection pattern that was previously hidden when uncertainties were large. We confirm the earlier findings that there is a distinct drift velocity superposed on the corotating plasma that pushes the plasma toward ∼dawn that dominates over the general plasma outflow. We find that an electric field directed toward 01-02 h Saturnian local time best describes this drift, and that the value of the electric field strength is not constant with radial distance. Key PointsUses in-situ velocity and magnetic field data to investigate convection patternExB drift dominates radial flow below 10 Rs, E ranges from 0.1-0.4 mV/mElectric Field is offset towards 01-02 LT rather than directed towards midnight ©2013. American Geophysical Union. All Rights Reserved.