On the formation of Ganymede's surface brightness asymmetries: Kinetic simulations of Ganymede's magnetosphere

Ganymede possesses strong surface brightness asymmetries both between its polar cap and equatorial regions and between its leading and trailing hemispheres. Here we test the hypothesis that these asymmetries are due to differential Jovian plasma and energetic particle precipitation to the surface with the combination of a hybrid plasma model (kinetic ions and fluid electrons) and a particle tracing model. We describe the hybrid model, the first of its kind applied to Ganymede, and compare the results to both Galileo observations and previous MHD and MHD-EPIC models of Ganymede. We calculate spatially resolved precipitating Jovian ion fluxes to the surface of Ganymede for energies 1< E < 10(4) keV and find (1) precipitating fluxes peak near 100 keV and (2) excellent correlation between the precipitating flux and Ganymede's surface brightness variations. Thus, we conclude that precipitating energetic particle fluxes are the primary driver for altering the surface brightness of Ganymede.