Energetic Ions and Magnetic Fields Upstream From the Kronian Magnetosphere

The existence of energetic particle events to ~200 Rs upstream and ~1300 Rs downstream of Saturn was established during the Voyager 1, 2 flybys in 1980 and 1981, respectively. The origin of the events could not be determined with certainty because of lack of particle charge state and species measurements at lower (<300 keV) energies, which dominate the spectra. High sensitivity observations of energetic ion directional intensities, energy spectra, and ion composition were obtained by the Ion and Neutral Camera (INCA) of the MIMI instrument complement with a geometry factor of ~2.5 cm2 sr and some capability of separating light (H, He) and heavier (C, N, O) ion groups (henceforth referred to as "hydrogen" and "oxygen" respectively). Charge state information was provided where possible by the Charge-Energy-Mass-Spectrometer (CHEMS) over the range ~3 to 220 keV per charge, and magnetic field (IMF) data by the MAG instrument on Cassini. The observations revealed the presence of distinct upstream bursts of energetic hydrogen and oxygen ions whenever the IMF connected the spacecraft to the planetary bow shock, up to distances of 135 RS. The events exhibited the following characteristics: (1) Hydrogen ion bursts are observed in the energy range 3 to 220 keV (and occasionally to E > 220 keV) and oxygen ion bursts in the energy range 32 to -300 keV. (2) Particle onsets are nearly field-aligned, but the distribution tends to isotropize as the event progresses in time. (3) The duration of the ion bursts is several minutes up to 4 hrs. (4) The events are of varying composition, with some exhibiting significant fluxes of oxygen. (5) The bursts have a filamentary structure with some exhibiting distinct signatures of "velocity- filtering effects" at the edges of convecting IMF filaments. (6) Some ion bursts are accompanied by distinct diamagnetic field depressions and exhibit wave structures consistent with ion cyclotron waves for H+, and O+. Given the repeated magnetic field configuration during the detection of the events and that energetic ions trapped within the magnetosphere of Saturn are mostly H+ and O+ we conclude that O+-rich upstream events must be particles leaking from Saturn's magnetosphere under favorable IMF conditions. The spectral evolution of the upstream events and their anisotropy characteristics will be presented and discussed in the context of current models.