Kanani, SJ; (2012) Saturn's magnetosphere: influences, interactions and dynamics. Doctoral thesis (PhD), UCL (University College London).

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Abstract

In this thesis we use data from the Cassini spacecraft in order to investigate the influences on Saturn’s magnetosphere, and the dynamics and interactions that occur within it. The primary instrument for this study is the Cassini electron spectrometer (CAPS-ELS), analysing low energy electron data to examine three areas of research, which are discussed and developed in this thesis. The first study concerns the magnetopause of Saturn, the boundary between the region of space dominated by the planet’s magnetic field and the interplanetary magnetic field. We develop a new pressure balance model using a multi-instrument data analysis, building on past models and including new features. It has been shown that the model has improved on previous models due to the inclusion of the suprathermal plasma and variable static pressures in the pressure balance equation providing more realistic results. It is currently the most up to date model of Saturn’s magnetopause. The second study concerns flux tube interchange and injection events in Saturn’s inner magnetosphere. A new survey of these features was carried out across the entire dataset in order to learn more about plasma circulation, and to answer questions such as whether injection events with increases in magnetic field strength are always found at the equator. By examining the electron and ion data, we test previous interpretations of some events, analyse other such events in detail, and demonstrate a method through which the radial plasma flow direction of interchanging flux tubes can be determined. Pitch angle distributions, magnetic field data and plasma flow models were considered in order to help establish plasma circulation patterns in the kronian magnetosphere. We were able to determine the flow direction of 17 events and use magnetic field data from 32 events to conclude that generally increases in magnetic field strength are indeed found at the equator. We believe our work has shown that Saturn exhibits general and isolated plasma flow, with the isolated plasma flow occurring as “bubbles” in the plasma. The third study concerns low energy electron enhancements in the inner magnetosphere, specifically those associated with Saturn’s moon Enceladus. During an investigation of close flybys of Enceladus, discrete, short duration enhancements were found in the low energy electron plasma. We present the new features seen in the Enceladus L shell region of the kronian magnetosphere. The data from these new features were surveyed and some possible creation mechanisms are discussed. In total, over 600 spikes were found between L = 3.5 and 4.5 from data between Saturn Orbit Insertion and June 2010, showing a preference about the equatorial region but no obvious patterns in local time. Some spikes appear to extend to dispersive higher energy signatures, which are generally associated with injection events and hence these types of spikes were investigated accordingly. Although one formation process was not found to account for all the events, we rule out several processes, such as spacecraft charging and thruster engine firing as causes for the spikes. We believe that our results show that different creation mechanisms can be responsible for different types of low energy electron enhancements.

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