The charge state of helium in the Jovian magnetosphere: a possible method to determine it

Ion measurements from the EPAC instrument on board the Ulysses spacecraft inside the Jovian dayside magnetosphere in February 1992 are reported. Energy/charge (Eq) spectra for protons (0.5–1.0 MeV) and different ions (helium, oxygen and sulphur) in the energy range 0.4–9.0 MeV N−1 are concentrated on. In agreement with the earlier work of Hamilton et al. (J. geophys. Res. 86, 8301–8318, 1981) it is found that a common power law for protons, oxygen and sulphur can fit the Eq spectra assuming charge states q = 2 for oxygen and q = 3 for sulphur. However, the Eq spectrum for helium ions can be fit to that common curve if different charge states at different radial distances to the planet are allowed: q ≈ 1.75 in the outer part of the magnetosphere and q ≈ 1 in the inner part. This observation is taken as an indication of different sources of helium ions in the Jovian magnetosphere. If such a common energy/charge spectrum is assumed then the adapted charge state can be used as a measure of the percentage of He+ and He2+ in the Jovian magnetosphere. It is found c. 40% in the outer, c. 70% in the middle and c. 95% He+ in the inner Jovian magnetosphere.