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1 SATURN’S MAGNETOSPHERE: INFLUENCES, INTERACTIONS AND DYNAMICS Sheila Jagdish Kanani Mullard Space Science Laboratory Dept. of Space and Climate Physics University College London Gower Street. London, WC1E 6BT, U.K. THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY OF THE UNIVERSITY COLLEGE LONDON -2012- I, Sheila Kanani, confirm that the work presented in this thesis is my own. Where information has been derived from other sources, I confirm that this has been indicated in the thesis. Signed .............................................................. “Ishimasi! What are you doing?” “I’m writing a book about Saturn, Ashton.” “I know what Saturn is. It’s a far far away planet with lots of rings round it made of stuff and planet things.” Ashton Jayanti Ferdinand, aged 3.5 years. He later decided his baby sister Naia was from Saturn or Jupiter and we’d “need a rocket ship to take her back”. 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. TABLE OF CONTENTS Abstract .................................................................................................................................................................. 4 Table of Contents ............................................................................................................................................... 6 List of Figures .................................................................................................................................................... 11 List of Tables ...................................................................................................................................................... 29 CHAPTER 1 Introduction ............................................................................................................................ 30 1.1 Space Plasma Physics ............................................................................................................ 31 1.1.1 What is a plasma? .......................................................................................................... 31 1.1.2 Single particle motion ................................................................................................. 36 1.1.3 Kinetic theory and the many particle motion approach ............................... 51 1.1.4 Magnetohydrodynamics ............................................................................................. 51 1.2 Planetary magnetospheres ................................................................................................. 59 1.2.1 The solar wind ................................................................................................................ 59 1.2.2 Planetary magnetic fields .......................................................................................... 61 1.2.3 Planetary magnetospheres ....................................................................................... 62 1.3 Dynamics of the magnetosphere ...................................................................................... 70 1.3.1 Currents ............................................................................................................................ 70 1.3.2 Reconnection .................................................................................................................. 71 1.3.3 The Dungey Cycle and the Vasyliunas Cycle ...................................................... 74 1.4 Summary .................................................................................................................................... 77 CHAPTER 2 Saturn and its magnetosphere ........................................................................................ 78 2.1 History of Saturn observations ......................................................................................... 81 2.2 Saturn’s magnetic field and magnetosphere ............................................................... 84 2.2.1 magnetic field ................................................................................................................. 84 2.2.2 Solar wind conditions at Saturn .............................................................................. 85 2.2.3 Saturn’s magnetosphere............................................................................................. 86 2.2.4 Electron distributions in the magnetosphere ................................................... 91 2.3 Magnetospheric dynamics .................................................................................................. 92 2.3.1 Influences on Saturn’s magnetosphere, SKR and rotation period ............ 93 2.3.2 Sources and sinks of plasma ..................................................................................... 95 2.3.3 Plasma transport in the magnetosphere ............................................................. 96 2.4 Other features of Saturn’s magnetosphere .................................................................. 97 2.4.1 Aurora ................................................................................................................................ 97 2.4.2 Oscillations ....................................................................................................................... 99 2.4.3 Radiation belts ................................................................................................................ 99 2.4.4 Ring current ................................................................................................................... 101 2.5 Saturn’s icy moons ............................................................................................................... 103 2.6 Saturn-magnetosphere-icy moon interactions......................................................... 105 2.7 Thesis overview ..................................................................................................................... 108 CHAPTER 3 Instrumentation and an introduction to the dataset ......................................... 111 3.1 Cassini orbiter and mission .............................................................................................. 111 3.2 Cassini Plasma Spectrometer (CAPS) ........................................................................... 117 3.2.1 Electron Spectrometer
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