Trapped electrons in Ganymede's magnetic field

The NASA Galileo satellite encountered the Jovian moon Ganymede on 7 May 1997. The Energetic Particles Detector (EPD) measured energetic electron pitch angle distributions characteristic of closed magnetic field lines. Significantly different from distributions observed during other encounters with Ganymede, they displayed loss cone features near both 0 degrees and 180 degrees pitch angles and an additional minimum near 90 degrees pitch angle. These double loss cane, butterfly distributions are characteristic signatures of particles drifting in a distorted magnetic field configuration. Distortions due to the flow of Jovian plasma past Ganymede qualitatively could account for the observed distributions. Electron injection could then occur either through the equatorial downstream hemisphere or at low Ganymede altitudes on high-latitude Ganymede-Jovian field lines adjacent to the closed field line region. The data indicate maximum injection efficiences of 1-10%.