Electron microdiffusion in the Saturnian radiation belts: Cassini MIMI/LEMMS observations of energetic electron absorption by the icy moons

Abstract

Since Saturn orbit insertion (SOI), Cassini has performed numerous crossings of Saturn's inner moons' L shells. The Low-Energy Magnetospheric Measurement System (LEMMS) has detected a large number of microsignatures in its lowest-energy electron channels (20-100 keV) as well as in the MeV energy range. We have catalogued and analyzed more than 70 microsignatures in the LEMMS data from the first 22 Cassini orbits and have correlated their evolution with electron diffusive processes. Our results on the L-dependence of the radial diffusion coefficients, DLL, show that radial microdiffusion driven by magnetic field impulses is the dominant mechanism to account for their refilling. The dependency of DLL from equatorial pitch angles also points toward this mechanism. The large scattering of the DLL values at Tethys and Dione suggest that these field impulses might be related to injections. Our analysis also supports the, inferred from pre-Cassini studies, filtering effect by the icy moons on radially diffusing electrons, which starts at the orbit of Dione, at 6.28 Saturn radii, Rs. This is suggested primarily by the very low radial diffusion speeds and by the characteristics of four microsignatures attributed to the moons Mimas and Epimetheus that all seem to have been formed in energies between 1.6 and 3.5 MeV. Despite the low D LL, diffusing electrons can escape absorption and be transported in the inner magnetosphere due to nonaxisymmetric drift shells, which can be detected even along the orbit of Enceladus. We estimate that a significant contribution to the filtering comes from the core of the E ring. Our results also show that L displacements due to the nonaxisymmetric drift shells are orders of magnitude higher than the icy moon L shell variability due to the nonzero eccentricities and inclinations, and total plasma losses on the moon surfaces should be reevaluated. We also examine the energy dependence of D LL and we present a series of possible explanations for the faster depletion of microsignatures at MeV energies. Using the high-energy resolution PHA channels we assess that this faster depletion could result partly from an increase of DLL with energy. The larger passbands of the high-energy electron detectors could amplify the erosion of the microsignature signal. Copyright 2007 by the American Geophysical Union.