Pitch angle distributions (PADs) near the Galilean moons of Jupiter: Galileo flybys revisited

Pitch angle distribution of energetic particles can be used to understand the global topology of field lines (connected on both ends to the planet, connected on both ends to the moon or connected on one end to the moon and the other end to the planet etc.). The pitch angle distributions on field lines connected on one end to the moon can also be used to gauge the speed at which the field lines are convecting past a moon.

The Galileo spacecraft flew by the Galilean moons multiple times and explored the charged particle distributions in their vicinities. In this paper we will revisit the data of the Energetic Particles Detector EPD onboard Galileo in terms of pitch angle distributions for various energy channels  for different ion species and electrons in the energy range from 15 keV up to several tens of MeV.

We analyzed the Europa flybys E11, E12, E14, E15, E26.; Ganymede flybys G2, G7, G8, G28, G29; Callisto flybys C3, C9, C10.

We will show how different the energy-dependent pitch angle distributions are upstream and downstream of the moons and how different those distributions are between electrons and the various ions.

Electron PADs near Europa are trapped for energies of several 100 keV while for lower energies the PAD shapes are uncertain. Dropout signatures indicate that charged particles are lost in the moons' tenuous exospheres and onto their surface.

PADs near Ganymede showed bi-directional electron distributions for low energies upstream and trapped for higher energies while PADs of protons and heavy ions are more isotropic. Inside Ganymede’s magnetosphere trapped distributions have been observed when the S/C was connected to closed field lines.

The signal-to-noise ratio of energetic electron and ion fluxes near Callisto is a factor of 10 lower than for Europa. PADs near the moon are less clear compared to the other Galilean satellites. Bi-directional ion PADs close to the moon have been observed.