Trapped Electrons at Ganymede Revisited

The Galileo spacecraft encountered closed field lines at Ganymede on two separate occasions, on orbits G8 in 1997 and G28 in 2000. The first encounter was at high latitude and altitude and the latter at low latitude and significantly closer to the surface of Ganymede. The trapped pitch angle distributions seen at Ganymede 8 were of the butterfly type, whereas on the Ganymede 28 flyby the EPD detector as it flew inwards, first saw butterfly distributions but then encountered distributions of the pancake type with strong maxima near pitch angle values near 90^° . The spectra show a much stronger energy-dependent depletion, primarily at the lowest energies measured, on G8 than at G28. We present the observations and discuss possible physical mechanisms for the observed phenomena. We attribute the differences to differences in the trajectories of the two encounters, to the different source regions and to transport and injection mechanisms responsible for the presence of the electrons in the trapped particle environment of Ganymede. The butterfly distributions may reflect magnetopause shadowing as known from similar effects in the terrestrial magnetosphere, and the pancake distributions, which are located closer to the surface of the satellite, may be a product of pitch angle scattering by electrostatic cyclotron harmonic waves, observed during the encounter. magnetosphere, and the pancake distributions, which are located closer to the surface of the satellite, may be a product of pitch angle scattering by electrostatic cyclotron harmonic waves, observed during the encounter.