Charging of ice grains by low‐energy plasmas: Application to Saturn's E ring

The charging of ice grains in planetary plasmas is studied, including the effects of secondary electron emission and backscattering of the incident electrons. It is shown that existing charging models can not be 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 which more carefully account for electron reflection and the threshold for secondary electron emission. Using plasma parameters from Voyager PLS 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 .