Energetic ion characteristics and neutral gas interactions in Jupiter's magnetosphere

Spectra, integral moments, and composition (H, He, O, S) of energetic ions (50 keV to 50 MeV) are presented for selected Jupiter magnetospheric positions near the equator between radial distances of ∼6 to ∼46 Jupiter radii (R_J), as revealed by analysis of the Galileo Energetic Particle Detector data. These characteristics are then used as the basis of interpreting and modeling reported signatures of energetic ion/neutral gas interactions within Jupiter's inner magnetosphere, particularly energetic neutral atom emissions measured during the Cassini spacecraft flyby of Jupiter. Key findings include the following: (1) sulfur ions significantly dominate the energetic (≥50 keV) ion density and pressure at all radial distances >7 R _j; (2) protons dominate integral number and energy intensity planetward of 20-25 R_j, (3) a distinct signature of local, equatorial acceleration of energetic protons is revealed between Io (5.9 R_j) and Europa (9.4 R_j); (4) significant spectral and compositional signatures of neutral gas interactions are also revealed between the orbits of Io and Europa; (5) a previously reported significant depletion of ring current ion populations between Io and Europa during the early-phase operation of Galileo (∼1995), as compared with observations obtained during the Voyager epoch (1979), has persisted and probably deepened during later Galileo phases (1999); and (6) detailed energetic neutral atom emission modeling, based on the in situ results reported here, further constrains recent estimates of the contents of the neutral gas torus of Europa.