Io encounters past and present: A heavy ion comparison

The heavy ion counter (HIC) on the Galileo spacecraft consists of two low-e nergy telescopes that were proof-test models of the telescopes used for the cosmic ray subsystem (CRS) on Voyager. The telescopes on HIC were improved to better withstand the intense radiation environment of the Jovian system and to measure higher-energy particles with greater sensitivity. The simil arity of these two instruments makes it natural to compare the data in an e ffort to glean insight on the behavior of heavy ion fluxes at higher energi es and how the inner Jovian magnetosphere has changed in the last 18 years. Inside L = 6, the oxygen spectrum suggests a spectral break at similar to 7500 MeV/Gauss, corresponding to ions with gyroradii of similar to 1.5 R-Io at L = 6. A comparison of the phase space densities show remarkable agreem ent between the two time periods outside of the Io orbit, with differential spectra consistent with gamma similar to -8 above similar to 6 MeV/nucleon . Near Io Galileo observed a large density gradient not apparent in the Voy ager data that may be due to local time asymmetries or a decrease in the ra dial diffusion rate.