Pitch Angle Distributions of Energetic Particles Near Callisto

The Galileo spacecraft performed close flybys of the moon Callisto between 1996 and 2001. We reanalyzed particle data of the energetic particles detector onboard Galileo and derived pitch angle distributions in the energy range of several keV to MeV during Callisto flybys C3, C9, C10, and C30. We establish that field-aligned beams observed during the flyby periods are more likely to originate from Callisto's magnetospheric interaction rather than by independent magnetospheric processes. These beams are prominent only during flyby C3, they come mainly from the North, and connect the moon and the ionosphere of Jupiter. For short intervals they have also been observed propagating from the South. The beams are regularly unidirectional and typically extend to 300 keV in energy, occasionally reaching above 600 keV. Energetic particle depletions in Callisto's wake during the downstream flybys are not at all obvious, even at low altitudes and in the wake center. The signature of the wake becomes more apparent when energetic particle observations are organized in pitch angle. In that case, pitch angle distribution minima at 90 deg can be discerned in some flybys, however, not always associated with a profound drop in the absolute signal intensity. They instead indicate that field-aligned particle flux within the wake is higher. Outside the wake these minima continue in an energy-dependent disturbed region toward Jupiter which seems to be at least partially collocated with Alfven-wing structures as predicted from magnetohydrodynamic simulation results or simple flyby geometry considerations.

Automated summary

The Galileo spacecraft observed particle beams originating from the moon Callisto during its flybys between 1996 and 2001. These beams, predominantly from the North but occasionally from the South, are more likely to be a result of Callisto's magnetospheric interaction rather than independent magnetospheric processes. Energetic particle depletions in Callisto's wake were not observed, but pitch angle distribution revealed a higher field-aligned particle flux within the wake. Outside the wake, disturbance regions were found, potentially related to Alfven-wing structures.