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Investigating the Neutral Sodium Emissions Observed at Comets

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Investigating the Neutral Sodium Emissions Observed at Comets Investigating the neutral sodium emissions observed at comets K. S. Birkett M.Sci. Physics, Imperial College London, UK (2012) Department of Space and Climate Physics University College London Mullard Space Science Laboratory, Holmbury St. Mary, Dorking, Surrey. RH5 6NT. United Kingdom THESIS Submitted for the degree of Doctor of Philosophy, University College London 2017 2 I, Kimberley Si^anBirkett, 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. 3 Abstract Neutral sodium emission is typically very easy to detect in comets, and has been seen to form a distinct neutral sodium tail at some comets. If the source of neutral cometary sodium could be determined, it would shed light on the composition of the comet, therefore allowing deeper understanding of the conditions present in the early solar system. Detection of neutral sodium emission at other solar system objects has also been used to infer chemical and physical processes that are difficult to measure directly. Neutral cometary sodium tails were first studied in depth at comet Hale-Bopp, but to date the source of neutral sodium in comets has not been determined. Many authors considered that orbital motion may be a significant factor in conclusively identifying the source of neutral sodium, so in this work details of the development of the first fully heliocentric distance and velocity dependent orbital model, known as COMPASS, are presented. COMPASS is then applied to a range of neutral sodium observations, includ- ing spectroscopic measurements at comet Hale-Bopp, wide field images of comet Hale- Bopp, and SOHO/LASCO observations of neutral sodium tails at near-Sun comets. The author finds that COMPASS is relatively successful at reproducing the morphology of the neutral sodium tails seen in wide field images, and to a lesser extent the intensity profiles produced by spectroscopic measurements. The success of COMPASS indicates that the current understanding of the physics of the production and evolution of neutral cometary sodium is broadly accurate. Using a simplistic Finson and Probstein[1968] dust tail source the author also finds that a source of neutral sodium within the dust tail is likely to result in a secondary neutral sodium tail feature, that has not yet been observed to the best of their knowledge. 4 Acknowledgments The work in this thesis was supported by STFC grant ST/K50239X/1. Thanks to UCL Graduate school and Federation of Finnish Learned Societies Grant for support to attend ACM in July 2014, and to the Royal Astronomical Society for support to attend AGU in December 2013. I would like to thank my supervisors, Geraint Jones and Andrew Coates, for their support and guidance throughout my PhD. Thanks to the near-Sun comets team at ISSI, particularly Colin Snodgrass. Special thanks to Karl Battams for helping me make sense of Solarsoft and to Lin Gilbert, my IDL guru. Thanks also to the Planetary and Plasma groups at MSSL - particularly Yudish and the guys from 108 - and to cake club and croquet for making long afternoons seem shorter. Finally, thank you to my family: Mum (Margi), Dad (Ian), Luke, Grandma (Terry), Grandad (Jim), Muriel and Anthony for their patience, understanding and support. 5 \I may not have gone where I intended to go, but I think I have ended up where I needed to be." Douglas Adams, The Long Dark Tea-Time of the Soul. Contents Abstract.......................................3 Acknowledgments..................................4 Contents.......................................6 List of figures....................................9 List of tables.................................... 24 1 Introduction 27 1.1 Why Study Comets?............................. 27 1.1.1 What is a comet?.......................... 28 1.1.2 Origin and Formation of Comets.................. 30 1.1.3 Key features of the cometary environment............ 32 1.2 Why Study Cometary Sodium?....................... 38 1.2.1 Sodium in the Solar System..................... 39 1.2.2 Sodium in Comets.......................... 40 1.3 Aims and Approaches of this Thesis.................... 41 2 Review of the Formation of Neutral Sodium Tails at Comets 43 2.1 Introduction.................................. 43 2.2 The Theory of the Formation of a Neutral Cometary Sodium Tail... 43 2.3 General Methods of Releasing Neutral Sodium from Cometary Material 48 2.4 Neutral Sodium Observations before Comet Hale-Bopp......... 50 2.5 The Neutral Sodium Tail of Comet Hale-Bopp.............. 51 2.6 Other Observations of Neutral Sodium Tails and Similar Features at Comets.................................... 68 2.6.1 Sodium Observed in Near-Sun Comets............... 68 2.6.2 Neutral Sodium Observed by the Rosetta Spacecraft....... 74 2.7 Summary................................... 75 3 Instrumentation 81 3.1 Introduction................................. 81 3.2 Ground-based observing platforms relevant to this thesis in the study of comet C/1995 O1 (Hale-Bopp)....................... 82 3.2.1 Imaging................................ 83 CONTENTS 7 3.2.2 Spectroscopy............................. 83 3.3 Space based remote observing platforms relevant to this thesis in the study of near-Sun comets: SOHO/LASCO................ 86 4 Previous Approaches to Modelling Cometary Neutrals 91 4.1 Modelling the Outflow of Neutral Cometary Gas............. 91 4.2 Modelling Neutral Cometary Sodium.................... 92 5 Cometary Orbital Motion At Perihelion: An Adaptable Sodium Sim- ulation (COMPASS) 99 5.1 Introduction.................................. 99 5.2 COMPASS Methodology........................... 100 5.2.1 Steps taken prior to running COMPASS............. 101 5.2.2 The COMPASS Simulation..................... 102 5.2.3 The COMPASS Plotting Suite................... 110 5.2.4 Convergence............................. 113 5.3 Summary................................... 116 6 Application of COMPASS to Spectroscopic Observations of Comet Hale-Bopp (C/1995 O1) 117 6.1 Introduction.................................. 117 6.2 Aims of this study.............................. 117 6.3 Observations................................. 118 6.4 Preparation of the Observations....................... 121 6.5 COMPASS Implementation......................... 122 6.5.1 COMPASS Simulation Parameters................. 122 6.5.2 COMPASS Output Plot Parameters................ 124 6.5.3 Convergence in COMPASS..................... 125 6.6 Comparison between COMPASS results and Observations for Comet Hale-Bopp (C/1995 O1)........................... 127 6.7 Conclusions.................................. 134 6.8 Further Work................................. 139 7 Application of COMPASS to Imaging Observations of Comet Hale- Bopp (C/1995 O1) 140 7.1 Introduction.................................. 140 7.2 Aims of this study.............................. 140 7.3 Observations................................. 141 7.4 Preparation of the Observations....................... 141 7.5 COMPASS Implementation......................... 144 7.5.1 COMPASS Simulation Parameters................. 144 7.5.2 COMPASS Output Plot Parameters................ 145 7.5.3 Convergence in COMPASS..................... 145 CONTENTS 8 7.6 Comparison between COMPASS results and Observations for Comet Hale-Bopp (C/1995 O1)........................... 147 7.7 Conclusions.................................. 151 7.8 Further Work................................. 151 8 Application of the COMPASS true dust tail model to relevant obser- vations 152 8.1 Introduction.................................. 152 8.2 Aims of this study.............................. 152 8.3 The Finson and Probstein[1968] model as an Input to COMPASS... 153 8.4 Preparation of Observations and COMPASS Implementation...... 155 8.5 Comparison between COMPASS results and Observations........ 157 8.6 Conclusions.................................. 158 8.6.1 Further Work............................. 159 9 Application of COMPASS to comets observed by LASCO 162 9.1 Introduction.................................. 162 9.2 Aims of this study.............................. 163 9.3 Observations................................. 164 9.4 Preparation of the Observations....................... 167 9.5 COMPASS Implementation......................... 175 9.5.1 COMPASS Simulation Parameters................. 176 9.5.2 COMPASS Output Plot Parameters................ 177 9.5.3 Convergence in COMPASS..................... 177 9.6 Comparison between COMPASS results and comets observed by LASCO 179 9.6.1 Comet McNaught.......................... 180 9.6.2 Comet NEAT............................ 182 9.6.3 Comet Hyakutake.......................... 184 9.7 Conclusions.................................. 191 9.8 Further Work................................. 192 10 Concluding Remarks 194 10.1 Summary and Conclusions......................... 194 10.2 Further Work................................ 196 A Comet Orbital Elements, Taxonomy and the Origins of Comets 201 References 205 List of figures 1.1 Diagram illustrating the theoretical surface structure of a cometary nu- cleus. Reproduced from Bleeker et al.[2001], which was adapted from Rickman[1991]................................ 29 1.2 Diagram illustrating positions of the comet reservoirs on the outer edges of the Solar System. Reproduced from Stern[2003]............ 32 1.3 Image from the OSIRIS camera taken on 23 September 2014. OSIRIS one of the suite of instruments on board the Rosetta spacecraft, which is
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