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UNIVERSITY COLLEGE LONDON Cassini Observations of Low Energy Electrons in and Around Saturn's Magnetosphere

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UNIVERSITY COLLEGE LONDON Cassini Observations of Low Energy Electrons in and Around Saturn's Magnetosphere UNIVERSITY COLLEGE LONDON Cassini Observations of Low Energy Electrons in and around Saturn’s Magnetosphere Hazel Joanna McAndrews Mullard Space Science Laboratory Department of Space and Climate Physics University College London A thesis submitted to the University of London for the degree of Doctor of Philosophy January 2007 1 I, Hazel Joanna McAndrews, 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……………………………………… 2 Abstract The work in this thesis utilises data from the Cassini spacecraft in the analysis of the Saturnian system. Data from the electron spectrometer (ELS), the ion spectrometer (IMS) and the magnetometer (MAG) gathered during the first two years of the mission have been used in two main areas of research. The first is the investigation of the magnetopause, which forms the boundary between the region of space dominated by the planetary magnetic field and currents, and the interplanetary magnetic field. The second concerns the high energy electrons fluxes in the inner magnetosphere, and the effects caused when the icy moons carve out cavities in these populations. We find evidence strongly suggestive of reconnection events as observed at Earth at two separate encounters with the magnetopause. The energisation of the plasma, together with the open field configuration provides us with an estimate for the reconnection voltage at Saturn. It is found to, at times, be comparable to the corotation electric field, which is assumed to dominate the dynamics of the system. The large database of ~ 200 crossings of the magnetopause is also used to derive general characteristics of the electron behaviour at this boundary. We find that boundary layer plasma is often present inside the magnetopause of Saturn, although the evidence suggests that internal plasma may play a role in formation of this layer. The ELS is able to utilise the penetration of high energy electrons directly into the detector to study the ‘microsignatures’ of the icy moons in the inner magnetosphere. The absorption of the high energy electrons leaves a cavity in the plasma. The subsequent re-filling over time by the surrounding plasma has allowed us to derive diffusion rates at Tethys, Enceladus and Mimas. In general, outward diffusion is faster than inward diffusion, although this varies at Enceladus. 3 Acknowledgements There are many people that deserve special thanks for supporting me through my three years. Firstly I would like to acknowledge Dr James Green of NASA who I met in 2001 in a Bier Garten while attending the International Space University. We became friends and later, knowing of my interest in space physics, Jim suggested that I should come and work with him, but I’d need a PhD. “Hang on!” I said…and now here I am. I’m not going to work for Jim now, but who knows what the future holds. Since Jim, my main driving force has been my two supervisors, Andrew Coates, for providing supervision in the first year and on all things Cassini after that, and Chris Owen. Chris bravely picked up the baton in my second year and managed to provide solid scientific advice on my areas of research, despite his own lying nearly three billion km away. I would also like to thank the many colleagues and friends at MSSL: Aurélie, Yulia, Nicolas, Jason, Rob W., Yasir, Ilya, ‘office’ Chris and Claire. Rob F. deserves a special mention for increasing my understanding of plasma processes, cricket and the need to continually rile the French – I cannot thank him enough for the many afternoons of hilarity he provided. To the ELS team, Gethyn and Lin for always being there to answer any IDL query or request, and to ‘my’ postdoc Abi, may the wine never stop flowing my friend! My friends have, of course, been particularly understanding, especially throughout the trying time of writing up, “What do you mean you can’t come to the pub?!” was often heard being yelled on a Sunday afternoon. My family have offered wonderful support, or, more accurately, I have offered them wonderful support, with job application letters, GCSE revision and essential big sister guidance, but I wouldn’t be where I am today without them. Thanks to my mum, who never hassled me to complete my homework, knowing full well I’d done it already and who did a fantastic job of raising us. To my gran and grandpa, who kick started this whole space obsession by taking me on the trip of a lifetime to KSC when I was 8 and who had such an influence on my outlook on life. Finally, to my husband-to-be, Fraser, who beat me to being a Dr., but who knows I did it with more finesse. He is my life and without him I would never have had the guts to start this, never mind finish it. There is no-one else that I would rather drag to the desert. I can’t wait for the rest of our lives together, this is for you. 4 Table of Contents 1 Introduction ......................................................................................................... 12 1.1 Introductory concepts in space plasma physics ............................................ 12 1.2 Saturn’s magnetosphere............................................................................... 25 1.3 The magnetopause....................................................................................... 33 1.4 Moon-magnetosphere interactions ............................................................... 50 1.5 This thesis ................................................................................................... 52 2 Instrumentation and Dataset ................................................................................. 53 2.1 The Cassini-Huygens Mission ..................................................................... 53 2.2 The Cassini Orbiter...................................................................................... 56 2.3 The Cassini Plasma Spectrometer (CAPS)................................................... 57 2.4 The Fluxgate Magnetometer (FGM) ............................................................ 73 2.5 Magnetospheric Imaging Instrument (MIMI)............................................... 73 3 Low Energy Plasma Investigations of Saturn’s Magnetopause ............................. 75 3.1 The aim of the chapter................................................................................. 75 3.2 Looking for evidence of reconnection at Saturn’s magnetopause in plasma and magnetic field data ............................................................................... 76 3.3 Data Overview ............................................................................................ 77 3.4 17 April 2005 .............................................................................................. 78 3.5 21 September 2005.................................................................................... 110 3.6 Conclusions............................................................................................... 131 4 Low energy electrons and magnetic field at Saturn’s magnetopause and in the boundary layer ................................................................................................... 133 4.1 Introduction............................................................................................... 133 4.2 Data........................................................................................................... 134 4.3 Event selection .......................................................................................... 137 4.4 Analysis methods ...................................................................................... 139 4.5 Plasma and field changes across the magnetopause.................................... 148 4.6 Survey of Magnetopause crossings ............................................................ 158 4.7 Plasma Depletion Layers (PDL’s).............................................................. 169 4.8 Signatures of plasma energisation.............................................................. 177 4.9 Summary of observations .......................................................................... 179 4.10 Discussion................................................................................................. 182 4.11 Conclusions............................................................................................... 188 5 Saturn’s Icy Satellites: results from the CAPS Electron Spectrometer ................ 190 5.1 This chapter............................................................................................... 190 5.2 Introduction............................................................................................... 190 5.3 Previous observations of high-energy satellite wakes................................. 191 5.4 Energetic particles in the inner magnetosphere .......................................... 197 5.5 Observations.............................................................................................. 205 5.6 Survey of moon wakes at Saturn................................................................ 227 5.7 Review of findings .................................................................................... 236 5.8 Discussion................................................................................................. 237 5 6 Summary and Conclusions................................................................................. 248 6.1 Future work..............................................................................................
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