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UNIVERSITY OF CALIFORNIA Los Angeles Charged Particle Energization and Transport in the Magnetotail during Substorms A dissertation submitted in partial satisfaction of the requirements for the degree Doctor of Philosophy in Physics by Qingjiang Pan 2015 ABSTRACT OF THE DISSERTATION Charged Particle Energization and Transport in the Magnetotail during Substorms by Qingjiang Pan Doctor of Philosophy in Physics University of California, Los Angeles, 2015 Professor Maha Ashour-Abdalla, Chair This dissertation addresses the problem of energization of particles (both electrons and ions) to tens and hundreds of keV and the associated transport process in the magnetotail during substorms. Particles energized in the magnetotail are further accelerated to even higher energies (hundreds of keV to MeV) in the radiation belts, causing space weather hazards to human activities in space and on ground. We develop an analytical model to quantitatively estimate flux changes caused by betatron and Fermi acceleration when particles are transported along narrow high-speed flow channels from the magnetotail to the inner magnetosphere. The model shows that energetic particle flux can be significantly enhanced by a modest compression of the magnetic field and/or ii shrinking of the distance between the magnetic mirror points. We use coordinated spacecraft measurements, global magnetohydrodynamic (MHD) simulations driven by measured upstream solar wind conditions, and large-scale kinetic (LSK) simulations to quantify electron local acceleration in the near-Earth reconnection region and nonlocal acceleration during plasma earthward transport. Compared to the analytical model, application of the LSK simulations is much less restrictive because trajectories of millions of test particles are calculated in the realistically determined global MHD fields and the results are statistical. The simulation results validated by the observations show that electrons following a power law distribution at high energies are generated earthward of the reconnection site, and that the majority of the energetic electrons observed in the inner magnetosphere are caused by adiabatic acceleration in association with magnetic dipolarizations and fast flows during earthward transport. We extend the global MHD+LSK simulations to examine ion energization and compare it with electron energization. The simulations demonstrate that ions in the magnetotail are first nonadiabatically accelerated in the weak field region close to the reconnection site, and then adiabatically accelerated in the high- speed flow channels as they catch up with and ride on the earthward propagating dipolarization structures. The nonlocal adiabatic acceleration mechanism for ions is very similar to that for electrons. However, the motion of energetic electrons is adiabatic except in very limited regions near the reconnection site while the motion of energetic ions is marginally adiabatic in the dipolarization regions. The simulations also show that the earthward transport of both species is controlled by the high-speed flows via the dominant ExB drift in the magnetotail. To understand how the power law electrons are initially produced in the magnetotail, we use an implicit particle- in-cell (PIC) code to model the processes in the near-Earth reconnection region. We find that the power law electrons are produced not in the reconnection diffusion region, but in the immediate iii downstream of the reconnection outflow in the course of dipolarization formation and intensification. Our study illustrates that during substorms, particles are accelerated via a multi- step process, including local acceleration in the reconnection region and nonlocal acceleration during the earthward transport, and the multi-step acceleration occurs on multiple spatial scales ranging from a few kilometers (the scale of electron diffusion region) to more than ten Earth radii (the transport scale). iv The dissertation of Qingjiang Pan is approved. George Morales Christopher T. Russell Raymond J. Walker Maha Ashour-Abdalla, Committee Chair University of California, Los Angeles 2015 v To my mother vi TABLE OF CONTENTS ABSTRACT ii DEDICATION vi LIST OF FIGURES xi LIST OF SYMBOLS xiii ACKNOWLEDGEMENTS xiv VITA xvii 1. Background and Purpose of this Study 1 1.1. Introduction .....................................................................................................................1 1.2. Acceleration by Magnetic Reconnection ........................................................................7 1.3. Acceleration during Plasma Earthward Transport ........................................................13 1.4. Purpose of this Study ....................................................................................................24 1.5. Structure of the Dissertation .........................................................................................26 2. Theory of Adiabatic Acceleration of Charged Particles 28 2.1. Introduction ...................................................................................................................28 2.2. Characteristics of Adiabatic Particle Orbits ..................................................................29 vii 2.3. An Analytical Model of Adiabatic Acceleration ..........................................................33 2.3.1. Motivation .........................................................................................................33 2.3.2. The Adiabatic Acceleration Model ...................................................................34 2.3.3. Comparisons with Observations .......................................................................38 2.3.4. Discussions ........................................................................................................49 3. Modeling Electron Energization and Transport in the Magnetotail during a Substorm 52 3.1. Introduction ...................................................................................................................52 3.2. Observations of the March 11, 2008 Substorm Event ..................................................52 3.2.1. Geotail Observations of the Solar Wind ...........................................................52 3.2.2. THEMIS Observations in the Magnetotail .......................................................54 3.3. Simulation Methodology ...............................................................................................60 3.4. MHD and Electron LSK Simulations of the March 11, 2008 Substorm Event ............62 3.4.1. MHD Simulation Results ..................................................................................62 3.4.2. LSK Simulation Results and Comparisons with Observations .........................67 3.5. Discussions ....................................................................................................................75 3.6. Conclusions ...................................................................................................................79 4. Modeling Ion Energization and Transport Associated with Magnetic Dipolarizations during a Substorm 81 viii 4.1. Introduction ...................................................................................................................81 4.2. Observations of the February 07, 2009 Substorm Event ..............................................81 4.3. MHD Simulation of the February 07, 2009 Substorm Event .......................................88 4.4. Ion LSK Simulation of the February 07, 2009 Substorm Event ...................................92 4.4.1. LSK Simulation Set-up .....................................................................................92 4.4.2. LSK Simulation Results and Comparisons with Observations .........................93 4.5. Conclusions and Discussions ......................................................................................100 5. A Comparison Study of Ion and Electron Energization and Transport Mechanisms during a Substorm 103 5.1. Introduction .................................................................................................................103 5.2. Comparisons of the Electron and Ion LSK Simulation Set-up for the February 07, 2009 Substorm Event ............................................................................................104 5.3. Simulation Results ......................................................................................................107 5.3.1. Comparisons of Simulation Results with Observations ..................................107 5.3.2. Comparisons of Ion and Electron Acceleration Mechanisms .........................111 5.4. Conclusions and Discussions ......................................................................................121 6. Particle-in-cell (PIC) Simulation of Electron Acceleration by Magnetic Reconnection 125 6.1. Introduction .................................................................................................................125 ix 6.2. Simulation Methodology .............................................................................................126 6.3. Simulation Results and Comparisons with Observations ...........................................130 6.3.1. Reconnection Structure ...................................................................................130 6.3.2. Electron Acceleration ......................................................................................134 6.4. Conclusions .................................................................................................................147