Analyses of field-aligned currents in Saturn’s nightside magnetosphere

This thesis is concerned with the study of magnetic field perturbations associated with large scale electrical current systems that flow along magnetic field lines. We will consider three such current systems within Saturn’s magnetosphere. The first is associated with the subcorotation of plasma in Saturn’s outer magnetosphere. The remaining two systems are directly associated with the northern and southern near-planetary period oscillations (PPOs) observed in Saturn’s magnetic field, as well in other magnetospheric phenomena such as the Saturn kilometric radio emission and related auroral emissions. Presented within this thesis are three detailed studies of the above current systems in Saturn’s northern and southern nightside hemispheres during the 2008 interval of high-latitude orbits of the Cassini spacecraft. The first of these studies is a statistical analysis of the southern hemisphere field-aligned currents, which reveals for the first time their form, magnitude and position is modulated by the phase of the southern PPO system. By exploiting the symmetry properties for the two main field-aligned current systems, they are approximately separated. Due to the smoothing effects of the statistical study the second study focuses on the structure of the southern hemisphere field-aligned current sheets, in terms of their strength and position. The positional modulation is quantified, and the phasing indicates the origin of the southern PPO system is flows in the polar atmosphere. Finally, the third study concerns the northern hemisphere field-aligned current signatures, this reveals that these are modulated in form and magnitude by not only by the northern PPO system, but also the southern PPO system. This provides the first direct evidence of the proposed interhemispheric current flow. Together, these studies provide new insights into the behaviour of the field-aligned currents and can place constrains on the theoretical discussions of the origin and driving of the Saturn’s aurora and periodicities.