Trapped energetic ions in Jupiter's inner magnetosphere

Ion measurements made with the high‐flux telescope at ∼1 MeV/nucleon during the passage of the Ulysses spacecraft through Jupiter's inner magnetosphere are presented. Stable pancake‐shaped pitch angle distributions were observed for protons inside ∼17 R J . They are fitted by a time‐independent model in which an assumed directional flux at the magnetic equator is transformed to the spacecraft's position using Liouville's equation and the O6CS magnetic field model. Nongyrotropic features are fitted with a flow in the corotation direction. The derived equatorial pitch angle distribution is approximately independent of radial distance and has broad shoulders. However, it is not as isotropic as that predicted for strong diffusion. From a flux‐composition fitting analysis, we find (1) the equatorial proton omnidirectional flux decreases approximately exponentially with magnetic equatorial distance R M and is nearly longitudinally symmetric, (2) the equal energy per nucleon abundance ratios for O/H, S/H, and S/O decrease with R M , (3) the ion spectral index softens linearly with R M , (4) the phase space densities of protons and oxygen at a constant magnetic moment increase rapidly with R M , and (5) the sulfur phase space density turns up inside 10 R J . The data are best fitted with similar spectral indices for oxygen and sulfur. Thus there is likely a source of energetic sulfur in the Io torus. Particle losses throughout the region are required to fit the radial variation of the phase space densities with a physically reasonable radial diffusion coefficient. Further work is needed to determine whether these losses are consistent with weak equatorial pitch angle diffusion.