The plasma environment, charge state, and currents of Saturn's C and D rings

Abstract

The plasma environment of the Saturnian C and D rings is investigated by modeling the flow of ionospheric plasma from the mid‐ to low‐latitude ionosphere to the vicinity of the rings. The model used is time‐dependent and kinetic and incorporates the gravitational, centripetal, magnetic mirror and ambipolar electric forces. It was found that the plasma density near the C and D rings, at a given radial location, will experience a one to two order of magnitude diurnal variation. With a knowledge of the plasma density and temperature near these rings their charge state is investigated by use of a dust cloud charging model (Havnes et al., 1984, 1987, 1990). The associated azimuthal currents are also found. Results show that the surface charge density of the C and D rings can show significant radial and azimuthal variations, due mainly to variations in the plasma density. In addition to its plasma density and temperature dependence the surface charge density will also depend on structural features of the rings such as the ring thickness and the nature of the particle size distribution. Its magnitude may vary over seven decades. The associated azimuthal currents carried by these rings will also show large diurnal variations resulting in field‐aligned currents which close in the ionosphere as shown by Ip and Mendis [1983]. However, the resulting ionospheric electric fields will probably not produce a significant amount of plasma convection in the topside ionosphere and inner plasmasphere as proposed by these authors, due in part to the level of the currents as well as the height‐integrated Pedersen conductivities at the local times where the currents close.