Ion abundance ratios in the Jovian magnetosphere

Galileo, as the first orbiting spacecraft around Jupiter, provides the opportunity to
study globally the composition of the energetic ion population in the equatorial plane of
the Jovian magnetosphere. This enables us to derive the relative importance of the various
sources and sinks of plasma and energetic particles in the largest magnetosphere of our
solar system. In this paper we use data from the Energetic Particles Detector (EPD) on
board Galileo and derive relative ion abundance ratio maps of S/O, S/He, O/He, and p/He.
We extend the previous work in terms of global coverage, species, and energy range. In
addition we compare them with previous results and especially with those derived on
board Voyager 2. We found that the S/O abundance ratio is relatively constant throughout
the magnetosphere decreasing slightly with radial distance. Within the error bars the S/O
ratios could be reproduced. Only a minor energy dependence is observed for this ratio
pointing to a common source for both ions. The S/He-, O/He-, and p/He-ratios decrease
with increasing radial distance, furthering the notion that sulfur, oxygen, and protons
originate mainly from a source within the Jovian system, in contrast with helium, which
originates from the solar wind. A spectral kink observed at energies of several 100 keV/
nuc for all species and most pronounced for helium explains the observed energy
dependence of the ratios relative to helium. Differences in the abundance ratios up to more
than one order of magnitude between sequential orbits are evidence of large temporal
variations taking place in the Jovian magnetosphere. These variations and the strong
energy dependence can explain the differences between the results derived from Galileo
EPD measurements and those from Voyager data.