Changes of the energetic particles characteristics in the inner part of the Jovian magnetosphere : A topological study

Based on 33 Galileo passages through the inner part of the Jovian magnetosphere, the properties of the energetic charged particle population are investigated. Pronounced changes in the particle intensities, and in their energy and pitch angle distributions (PADs) are generally observed. The good coverage in local time and radial distance provided by Galileo allowed to trace the location of the most distinct changes within the equatorial plane. Independent of local time they occur at distances between 10 and 30R _J. Within this region particle intensities drop by several orders of magnitude and the energy spectra softens considerably. However, most prominent is a change of the electron PAD from a pancake or trapped (maximum at 90°) to a distribution which maximises at different pitch angle values (bi-directional or butterfly distributions). The change occurs rapidly, usually within the time resolution of the measurements, giving rise to a sharp and distinct boundary. This boundary is located dependent on the orbit between 10 and 17R_J. It does not exhibit striking local time dependencies, specifically a pronounced dawn-dusk asymmetry, with the exception of the boundary being slightly closer to the planet near noon compared to the other local times. The change in the electron pitch angle distribution occurs in a region where the magnetic field topology is still close to a dipolar field configuration. The softening of the particle energy spectra is collocated with the transition of the magnetic field from a dipolar to a current sheet topology. Possible connections to and implications for the Jovian high-latitude aurora are addressed.