UNIVERSITY COLLEGE LONDON Cluster Investigations of Magnetotail Processes Jason Peter Dewhurst 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 October 2005 2 Abstract Physical processes in the Earth’s magnetotail are investigated using data principally from the Plasma Electron And Current Experiment onboard the European Space Agency’s 4 spacecraft Cluster mission. Three topics are presented: the expansion and thinning of the plasma sheet during the course of a substorm; field-aligned current generation during intervals of flow shears in the plasma sheet; and observations of flux-ropes and travelling compression regions. Multipoint observations of the boundary between the plasma sheet and the lobe have allowed the determination of the speed and direction of propagation of the boundary. Using the direction of the boundary motion we looked for a trend of Earthward or tailward directed motion that might be indicative of whether near-Earth neutral line formation or current disruption occurred first at substorm onset. Given the time resolution of the observations, we did not find compelling evidence to confirm either initiator of the substorm process. The occurrence of flow shears in the plasma sheet, associated with phenomena such as bursty bulk flows, may result in the twisting of magnetic field lines. The twisted field should support a field-aligned current and this mechanism has been proposed to be significant at the flow-braking region, where the substorm current wedge is formed. We examine flow shears in the magnetotail and check for the presence of field-aligned currents during these intervals. The variations observed in the field-aligned current can be explained by changes in the position of the greatest flow shear with respect to the spacecraft tetrahedron. 3 Finally we examine the observations of travelling compression regions in the lobe and flux-ropes within the plasma sheet in order to demonstrate their association with each other. The plasma environments of flux-ropes are examined. Successful identification of flux-ropes fitting a force-free model which are co-incident with travelling compression regions is made. 4 5 Dedicated to Margaret Dewhurst, 1944-2002 6 7 Acknowledgements I would like to thank Dr. Christopher Owen for his supervision throughout my research project. As well as his scientific guidance and advice, he has provided support when it was needed and that is much appreciated. I wish to thank Dr. Andrew Fazakerley (PEACE PI) and the PEACE science and operations teams at MSSL for their support, advice and encouragement throughout my research. I also wish to thank the other staff and students at MSSL for making my time there enjoyable and memorable. In addition, thanks go to all the other Cluster science and operations teams who have provided data, interpretation and discussion for the material presented in this thesis. I would like to thank the Particle Physics and Astronomy Research Council for providing me with postgraduate funding. Finally, I wish to thank my family and friends for their encouragement and motivation. 8 Table of Contents 1 Basic Space Plasma Physics......................................................................................... 17 1.1 Introduction........................................................................................................... 17 1.1.1 Definition of a Plasma.................................................................................. 17 1.1.2 Debye shielding and Plasma Frequency ..................................................... 18 1.2 Single Particle Motion.......................................................................................... 19 1.2.1 Electric and Magnetic Fields ....................................................................... 19 1.2.2 Gyration ........................................................................................................ 20 1.2.3 Drifts ............................................................................................................. 21 1.2.4 Magnetic Mirrors.......................................................................................... 22 1.3 Many-Particle Motion .......................................................................................... 23 1.3.1 Kinetic Theory.............................................................................................. 24 1.3.2 Magnetohydrodynamics............................................................................... 24 1.3.2.1 Diffusion and the Frozen-in Theorem..................................................... 25 1.3.2.2 Magnetic Tension and Plasma Beta........................................................ 26 1.3.3 Magnetic Reconnection................................................................................ 27 1.4 Geophysical Plasmas............................................................................................ 28 1.4.1 Solar wind..................................................................................................... 29 1.4.2 Bow shock..................................................................................................... 30 1.4.3 Magnetosheath.............................................................................................. 30 1.4.4 Magnetopause and Magnetosphere ............................................................. 31 1.4.5 Ionosphere, Plasmasphere, and Radiation Belts......................................... 31 1.4.6 Magnetotail................................................................................................... 32 1.5 Convection and Substorms................................................................................... 34 1.5.1 Steady State Convection .............................................................................. 34 1.5.2 Substorm Processes...................................................................................... 37 1.5.2.1 Plasma sheet dynamics ............................................................................ 40 1.5.2.2 Plasma Flows............................................................................................ 42 1.5.2.3 Plasmoid / Flux Rope generation ............................................................ 43 2 The Cluster Mission and PEACE................................................................................. 46 2.1 The Cluster Mission.............................................................................................. 46 9 2.1.1 A Multi-Spacecraft Mission ........................................................................ 47 2.1.2 Experiments .................................................................................................. 48 2.2 The Plasma Electron and Current Experiment (PEACE) .................................. 49 2.2.1 Modes, Commanding and Data Products.................................................... 53 2.2.2 Micro-Channel Plate (MCP) Degradation .................................................. 58 2.2.3 ASPOC Operations and PEACE................................................................. 58 2.2.4 Data Quality.................................................................................................. 60 2.3 Description of Selected Cluster Instruments....................................................... 61 2.3.1 ASPOC – Active Spacecraft POtential Control ......................................... 61 2.3.2 CIS – Cluster Ion Spectrometry Experiment.............................................. 63 2.3.3 EFW – Electric Field and Waves Experiment............................................ 64 2.3.4 FGM – Flux Gate Magnetometer ................................................................ 64 3 Thinning and Expansion of the Substorm Plasma Sheet: Cluster PEACE Timing Analysis.................................................................................................................................. 66 3.1 Introduction........................................................................................................... 66 3.2 Methodology......................................................................................................... 72 3.3 Results ................................................................................................................... 77 3.3.1 Case Study – Single Thinning and Expansion............................................ 77 3.3.2 Case Study – Multiple Thinning and Expansion........................................ 83 3.3.3 Statistical Survey.......................................................................................... 87 3.3.4 Paired and Multiple Events........................................................................ 104 3.4 Summary and Discussion................................................................................... 105 3.5 Conclusions......................................................................................................... 110 4 Flow Shears and Field-Aligned Current Generation ................................................ 112 4.1 Introduction......................................................................................................... 112 4.2 Methodology....................................................................................................... 119 4.3 Application of Methodology.............................................................................