Large-scale current systems in the Jovian magnetosphere

The studies contained within this thesis focus on the large-scale azimuthal and radial current systems of Jupiter's middle magnetosphere, i.e. currents with radial ranges of 20-50 RJ. In the first study using magnetometer data from Pioneer-10 and -11, Voyager-1 and -2, and Ulysses, it is discovered that the azimuthal current in the middle magnetosphere is not axi-symmetric as had been assumed for the last twenty-five years, but that it is stronger on the nightside than on the dayside at a given radial distance. A simple empirical model is formulated, which reasonably describes the data in the domain of interest both in radial distance and local time, and allows direct calculation of the current divergence associated with the asymmetry. In a similar way, in the following chapter the radial currents have been computed for the dawn sector of the jovian magnetosphere along various fly-by trajectories. Combination of these radial current estimations with the azimuthal current model allows the total divergence of the equatorial current to be calculated. These current densities mapped to the ionosphere are surprisingly large at ~1A m-2. In order to carry the current, the magnetosphere electrons must be strongly accelerated along the field lines into the ionosphere by voltages of the order of 100 kV. The resulting energy flux is enough to produce deep, bright (Mega Rayleigh) aurora and thus provides the first natural explanation of the main jovian auroral oval. In the final study, newly-available data from the Galileo orbiter mission are combined with the fly-by data in order to compare them to the model derived in the first study. The model is then re-derived for the entire data set, which significantly improves the associated fractional errors.