CHARGING OF ICE GRAINS BY LOW-ENERGY PLASMAS - APPLICATION TO SATURN E-RING

The charging of ice grains in planetary plasmas is studied, including the effects of secondary electron emission and backscattering of the i ncident electrons. It is shown that existing charging models can not b e simply extrapolated to the low-energy electron regime (below 30 eV) common in planetary magnetospheric plasmas. We derive expressions for the electrical potential of a grain immersed in a low-energy plasma wh ich more carefully account for electron reflection and the threshold f or secondary electron emission. Using plasma parameters from Voyager P LS experiment, we calculate the potential of Saturn's E ring grains to vary from -5.5 V at 4 R(s) to 5 V at 10 R(s).