Pitch Angle Distributions of Energetic Electrons Near Ganymede: Galileo EPD Measurements

Abstract Galileo flew‐by Ganymede six times between 1996 and 2000. The Energetic Particles Detector (EPD) performed energy resolved, directional electron measurements, and together with the magnetometer data, pitch angle distributions (PADs) could be derived. PADs are a very useful diagnostic to investigate particle dynamics and the field geometry in the vicinity of the moon. Here we present PADs from five Ganymede flybys and highlight aspects of their topology and energy dependence in more detail compared to past studies and the Juno flyby in 2021. We find that at about 300 keV, a transition between a single to a double loss cone is observed on open field lines mapping to Ganymede's north pole (flyby G2), indicative of the plasma slowdown within Ganymede's magnetosphere. Butterfly PADs with a deep 90° pitch angle minimum, seen also with Juno, are resolved in all Galileo flybys outside Ganymede's magnetopause, but their energy extent, exact PAD shape, and topology change drastically between different flybys and are only partly reproduced by numerical models. Finally, while previous studies reported field‐aligned electron beams within Ganymede's magnetosphere extending up to about 500 keV, trapped distributions in Ganymede's closed field line regions reach well above 1 MeV. This indicates that even if field aligned beams are significant electron source for the closed field line region, additional energization or particle sources are needed to explain the electron energies within Ganymede's radiation belts.