Variability in the Energetic Electron Bombardment of Ganymede

Abstract

AbstractThis study examines the bombardment of energetic magnetospheric electrons onto Ganymede as a function of Jovian magnetic latitude. We use the output from a three‐dimensional, hybrid model to constrain features of the electromagnetic environment during the G1, G8, and G28 Galileo encounters when Ganymede was located far above, within, or far below Jupiter's magnetospheric current sheet, respectively. To quantify electron fluxes, we use a test‐particle model and trace relativistic electrons at discrete energies between 4.5 keV ≤E ≤ 100 MeV while exposed to these fields. For each location with respect to Jupiter's current sheet, electrons of all energies bombard Ganymede's poles with average number and energy fluxes of 1 · 108 cm−2 s−1 and 3 · 109 keV cm−2 s−1, respectively. However, bombardment is locally inhomogeneous: poleward of the open‐closed field line boundary, fluxes are enhanced in the trailing hemisphere but reduced in the leading hemisphere. When embedded within the Jovian current sheet, closed field lines of Ganymede's minimagnetosphere shield electrons below 40 MeV from accessing the equator. Above these energies, equatorial fluxes are longitudinally inhomogeneous between the sub‐Jovian and anti‐Jovian hemispheres, but the averaged number flux (4 · 103 cm−2 s−1) is comparable to the flux deposited here by each of the dominant energetic ion species near Ganymede. When located outside of the Jovian current sheet, electrons below 100 keV enter Ganymede's minimagnetosphere via the downstream reconnection region and bombard the leading apex, while electrons of all energies are shielded from the trailing apex. Averaged over a full synodic rotation period of Jupiter, the energetic electron flux pattern agrees well with brightness features observed across Ganymede's polar and equatorial surface.