Energetic ion observations in the middle Jovian magnetosphere

Abstract

The middle magnetosphere of Jupiter has not been well studied prior to the Galileo mission. This region (10–30 RJ) has been traversed by Galileo numerous times allowing spatial and temporal features to be differentiated. We present observations of energetic ions (E ∼4.5–18.4 MeV nucleon−1) in the middle Jovian magnetosphere obtained with the Heavy Ion Counter (HIC) on Galileo. The radial dependence of the intensity, spectrum, and composition of these particles indicates that solar material dominates the 15 to 30 RJ region and diffuses without loss toward Jupiter. Near 10 RJ the composition is more Iogenic and shows evidence of strong losses. These observations can be understood in terms of a common energy per charge spectrum for all heavy ions. Due to the substantially different charge to mass ratios of the Iogenic and solar ions, the O/C and S/C abundance ratios vary with magnetic moment and therefore radial distance when observed over the fixed energy interval of the HIC instrument. Near 25 RJ there is evidence of acceleration of ions to MeV nucleon−1 energies. The location of this acceleration has been suggested previously but not directly observed before the Galileo mission.