On a systematic spectral variation of energetic ions in the Jovian outer magnetosphere: HI-SCALE/Ulysses observations

An important finding of Ulysses flyby of Jupiter is the discovery of a large-scale energetic particle layer adjacent to the magnetopause. Previous studies discussed the particle flux and the anisotropy characteristics of this layer. This study examines the spectral characteristics of the large-scale magnetopause boundary layer of energetic particles. Examination of Ulysses’ observations in the Jovian outer magnetosphere reveals the following picture of energetic ions: (1) The ion flux increases and the spectrum hardens in the direction from the magnetosheath/magnetopause toward the outer magnetosphere. (2) At middle latitudes, the energy spectrum is continuous and is well described by a power law (dj/dE=kE−γ), with a spectral index γ ranging between ∼2.0 and ∼2.5 over the whole energy range (∼60 to ∼4000 keV). (3) Close to the magnetopause the spectrum is described by a power law, but with a different slope at low (<∼400 keV) and higher (>∼400 keV) energies. (4) The ion spectrum near the magnetopause is in general softer than far from the magnetopause (γ≅∼2.6 to ∼3.4 at energies >∼400 keV). (5) The spectral shape varies with a period of ∼5 or 10 h. The observations are explained in terms of a periodic motion of the s/c within the large-scale boundary layer of energetic ions in the outer magnetosphere. This boundary was more evident in the duskside south magnetosphere and was found to extend in radial distances from ∼49Rj to 83Rj in this area.