Astronomy Unit School of Physics and Astronomy Queen Mary University of London Pickup Ion Processes Associated with Spacecraft Thrusters Implications for Solar Probe Plus Adam Clemens Submitted in partial fulfillment of the requirements of the Degree of Doctor of Philosophy 1 Declaration I, Adam Clemens, confirm that the research included within this thesis is my own work or that where it has been carried out in collaboration with, or supported by others, that this is duly acknowledged below and my contribution indicated. Previ- ously published material is also acknowledged below. I attest that I have exercised reasonable care to ensure that the work is original, and does not to the best of my knowledge break any UK law, infringe any third party's copyright or other Intellectual Property Right, or contain any confidential material. I accept that the College has the right to use plagiarism detection software to check the electronic version of the thesis. I confirm that this thesis has not been previously submitted for the award of a degree by this or any other university. The copyright of this thesis rests with the author and no quotation from it or information derived from it may be published without the prior written consent of the author. Signature: Date: 24/09/2015 2 Abstract Chemical thrusters are widely used in spacecraft for attitude control and orbital manoeuvres. They produce a plume of neutral gas which produces ions via pho- toionisation and charge exchange. Measurements of local plasma properties will be affected by perturbations caused by the coupling between the newborn ions and the plasma. A model of neutral expansion has been used in conjunction with a fully three-dimensional hybrid code to study the evolution and ionisation over time of the neutral cloud produced by the firing of a mono-propellant hydrazine thruster as well as the interactions of the resulting ion cloud with the ambient solar wind. A parameter survey was performed for varying angles of injection and injection rates, particle kinetics were also investigated. Results are presented which show that the plasma in the region near to the spacecraft will be perturbed for an extended period of time with the formation of an interaction region around the spacecraft, a moder- ate amplitude density bow wave bounding the interaction region and evidence of an instability at the forefront of the interaction region which causes clumps of ions to be ejected from the main ion cloud quasi periodically and the ways in which these features are modified by the degree of solar wind mass loading and the relative ori- entation of the magnetic field to the angle of injection. This may affect Solar Probe Plus for a significant duration as data taking and delicate sensory equipment may be required to cease operation until local fluctuations return to a more moderate level. The scale of the fluctuations seen are dependent upon the duration of the thruster firing and the specific geometry and therefore effects may vary in-situ. 3 Acknowledgements First and foremost I would like to thank Prof. David Burgess for all of the help and support he has given me over the last four years, without his friendly advice and guidance this thesis would never have been completed. Even when things took a turn for the worse his relentless optimism made me believe that perhaps it wasn't so bad and as it turned out we had the technology and we did rebuild! I would also like to thank Chris Haynes for the many discussions, hints, tips and examples on everything MATLAB related without which I would likely still be smashing my head against today. Of course I have to thank all of my fellow PhD students, both current and past for the company, discussions, help and most importantly for the beer and vodka at the SCR. You all helped keep me (kind of) sane! I thank my mother and family for the continuous moral support and belief. Even when I lost faith in myself you were there to say \You can do it!" Last, but certainly not least, I would like to thank Harriet. You helped, supported and encouraged me throughout this PhD and more in ways I probably never even knew, especially when the proverbial hit the fan. Thank you, I wouldn't be here without you. This work was supported by the Science and Technology Facilities Council (STFC), grant number ST/J500860/1 4 Contents Abstract 3 Acknowledgements 4 1. Introduction 8 2. The Physics of Pickup Ions 14 2.1. Introduction . 14 2.2. Interstellar Pickup Ions . 21 2.2.1. Source of Neutral Particles . 21 2.2.2. Filtration and Other Effects . 23 2.2.3. Anomalous Cosmic Rays . 25 2.2.4. Shock Acceleration of Interstellar Pickup Ions . 27 2.3. Pickup Ions at Comets . 30 2.4. Pickup Ions at Planets and Moons . 39 2.4.1. Jupiter . 40 2.4.2. Mars . 41 2.4.3. Saturn . 42 2.4.4. Mercury . 42 2.4.5. Earth . 43 2.4.6. Venus . 44 2.5. Active Release Experiments . 45 2.5.1. CRRES . 46 2.5.2. AMPTE . 47 2.5.3. CRIT II . 49 3. Modelling Spacecraft Thruster Pickup Ions 52 3.1. Introduction . 52 3.2. Neutral Expansion . 61 3.3. Ionisation . 72 3.4. Implementation . 74 3.5. Hybrid Model . 76 4. Interaction of Spacecraft Thrusters with the Solar Wind - An Overview 80 4.1. Introduction . 80 4.2. Simulation Setup . 81 4.3. Results . 82 4.4. Conclusions . 100 5 Contents 6 5. Interaction of Spacecraft Thrusters with the Solar Wind - A Pa- rameter Survey 102 5.1. A Qualitative Overview . 102 5.1.1. Injection at 45◦ to the Initial Magnetic Field . 102 5.1.2. Injection at 0◦ to the Initial Magnetic Field . 104 5.1.3. Low Injection Rate at 90◦ to the Initial Magnetic Field . 105 5.1.4. High Injection Rate at 90◦ to the Initial Magnetic Field . 107 5.2. Magnetic Pileup . 108 5.2.1. Injection at 45◦ to the Initial Magnetic Field . 108 5.2.2. Injection at 0◦ to the Initial Magnetic Field . 110 5.2.3. Low Injection Rate at 90◦ to the Initial Magnetic Field . 112 5.2.4. High Injection Rate at 90◦ to the Initial Magnetic Field . 114 5.3. Magnetic Field Draping . 114 5.3.1. Injection at 45◦ to the Initial Magnetic Field . 114 5.3.2. Injection at 0◦ to the Initial Magnetic Field . 118 5.3.3. Low Injection Rate at 90◦ to the Initial Magnetic Field . 119 5.3.4. High Injection Rate at 90◦ to the Initial Magnetic Field . 119 5.4. Bow Waves . 122 5.4.1. Injection at 45◦ to the Initial Magnetic Field . 122 5.4.2. Injection at 0◦ to the Initial Magnetic Field . 125 5.4.3. Low Injection Rate at 90◦ to the Initial Magnetic Field . 126 5.4.4. High Injection Rate at 90◦ to the Initial Magnetic Field . 126 5.5. Interaction Region . 129 5.5.1. Injection at 45◦ to the Initial Magnetic Field . 129 5.5.2. Injection at 0◦ to the Initial Magnetic Field . 131 5.5.3. Low Injection Rate at 90◦ to the Initial Magnetic Field . 134 5.5.4. High Injection Rate at 90◦ to the Initial Magnetic Field . 134 5.6. Mass Loading . 137 5.6.1. Injection at 45◦ to the Initial Magnetic Field . 137 5.6.2. Injection at 0◦ to the Initial Magnetic Field . 137 5.6.3. Low Injection Rate at 90◦ to the Initial Magnetic Field . 140 5.6.4. High Injection Rate at 90◦ to the Initial Magnetic Field . 140 6. Interaction of Spacecraft Thrusters with the Solar Wind - Particle Trajectories 143 6.1. Reference Simulation . 143 6.2. Low Injection Rate Simulation . 151 6.3. High Injection Rate Simulation . 157 6.4. 45◦ Injection Simulation . 163 6.5. 0◦ Injection Simulation . 169 6.6. Conclusions . 175 7. Conclusions and Further Work 177 Appendix A. Ionisation Calculations for H2 182 Appendix A.0.1. Rate of decay of H2 .................. 183 Contents 7 + Appendix A.0.2. Rate of production and decay of H2 ......... 183 Appendix A.0.3. Rate of production and decay of H . 183 Appendix A.0.4. Rate of production of H+ ............... 184 Appendix A.0.5. Rate of production of electrons . 184 Appendix B. Ionisation Calculations for N2 186 Appendix B.0.6. Rate of Decay of N2 .................. 186 + Appendix B.0.7. Rate of production and decay of N2 ......... 186 Appendix B.0.8. Rate of production and ionisation of N . 187 Appendix B.0.9. Rate of production of N+ ............... 188 References 189 1. Introduction The solar wind is a continuous stream of ions and electrons which are emitted from the Sun's outer corona. These ions are accelerated as they travel outward until it becomes a supersonic flow of plasma. The interplanetary magnetic field (IMF) is embedded in the solar wind due to its very high conductivity and is carried outwards along with it, in the limit of infinite conductivity the magnetic field lines move with the fluid flow in the so-called `frozen-in' approximation. The solar wind with its frozen-in magnetic field fills the solar system and defines the boundaries of the heliosphere and interacts with many different types of celestial object along the way. If the interaction is with a magnetised body then the solar wind will be deflected due to the inability of the separate magnetic fields to mix and, if the body's magnetic field is sufficiently powerful, can form a magnetosphere which shields any neutral atmosphere/exosphere it may have from the effects of the solar wind.