Energetic electron butterfly distributions near Io

Pronounced variations in the energetic electron distribution observed by the Energetic Particle Detector during the Galileo flyby of Io are described as a quasi-adiabatic response to the changing electric and magnetic field environment near the satellite. The energetic particle signatures can therefore be used to remotely sense the spatial distribution of electric and magnetic fields in the vicinity of Io. Electron pitch angle distributions evolve from a normal pancake distribution (peaked at 90° pitch angle) in the undisturbed torus to a butterfly distribution in the strong field depression/near Io. The strongest flux depletions at 90° pitch angle result from a reduction in kinetic energy due to conservation of the first adiabatic invariant, as electrons are transported into the vicinity of Io. The magnitude of the flux depletion is related to the spectral index n of the electron energy spectrum (J ∼ E^-n). Since the value of n tends to increase with increasing energy, the largest flux drop occurs at higher energy. In the low-speed wake region downstream of Io, electrons exhibit an abrupt transition to a population which is consistent with trapping on bounce orbits within the magnetic depression near Io. This trapped population, which appears in the same spatial region as intense field-aligned beams, is not a result of adiabatic transport from a source region upstream of Io. The phase space density of the "trapped" electron population is reduced, compared to the background torus, and particle tracing calculations in a realistic model environment near Io suggest that such electrons must be scattered into the region sampled by Galileo. Torus electrons with energies well above an MeV are excluded from a broad spatial region surrounding Io. This leads to a pronounced drop in the flux of penetrating particles near Io which allows the modest "trapped" electron population to be detected above the background level for energies up to 200 keV.