Energetic electron butterfly distributions near Io

Pronounced variations in the energetic electron distribution observed by th e Energetic Particle Detector during the Galileo flyby of Io are described as a quasi-adiabatic response to the changing electric and magnetic field e nvironment near the satellite. The energetic particle signatures can theref ore be used to remotely sense the spatial distribution of electric and magn etic fields in the vicinity of Io. Electron pitch angle distributions evolv e from a normal pancake distribution (peaked at 90 degrees pitch angle) in the undisturbed torus to a butterfly distribution in the strong field depre ssion near Io. The strongest flux depletions at 90 degrees pitch angle resu lt from a reduction in kinetic energy due to conservation of the first adia batic 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 e lectron energy spectrum (J similar to E-n). Since the value of n tends to i ncrease with increasing energy, the largest flux drop occurs at higher ener gy. In the low-speed wake region downstream of Io, electrons exhibit an abr upt transition to a population which is consistent with trapping on bounce orbits within the magnetic depression near Io. This trapped population, whi ch appears in the same spatial region as intense field-aligned beams, is no t a result of adiabatic transport from a source region upstream of Io. The phase space density of the "trapped" electron population is reduced, compar ed to the background torus, and particle tracing calculations in a realisti c model environment near Io suggest that such electrons must be scattered i nto 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 al lows the modest "trapped" electron population to be detected above the back ground level for energies up to 200 keV.