Calculation of Loss Rate from Saturn's E Ring

Ionization--whether through solar photons, impacting particles, or charge exchange--is a significant loss process in the neutral E ring. When these particles are ionized, they are energized by the electric field associated with the corotating magnetized plasma. If the pick-up energy is great enough, the particles generate ion cyclotron waves with a magnetic field amplitude that is determined by the energy of the pick-up ions. These waves, with frequencies near the local water-group gyrofrequencies, were first seen in the E ring by the Pioneer 11 and Voyager 1 magnetometers, but it is Cassini's large coverage of radial distance, local time, and latitude within the E ring that allows us to use this spacecraft's magnetometer to conduct a comprehensive study of these waves. Using a simple plasma profile and modeling the E ring as a mass source in the equatorial plane, we use the ion cyclotron waves to calculate the ionization rate through the ring. We find that between 3.5 and 7 Rs, the E ring loses a total of 6 x 1026 neutrals, or 18 kilograms of H2O, per second. This total is about one fourth of the amount expected from previous models and the radial erosion rate agrees qualitatively with E ring density profiles, except at 4.6 Rs. Using the ion cyclotron waves, our calculations show a discontinuity in the erosion rate at that equatorial distance and all latitudes. This discontinuity can not be explained with the assumed smoothly varying plasma profiles or production rates.