RICE UNIVERSITY Understanding the energy balance of Transition Region structures observed by IRIS in non-equilibrium emission by Shah Mohammad Bahauddin A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE Doctor of Philosophy APPROVED, THESIS COMMITTEE Dr. Stephen J Bradshaw, Chair Associate Professor of Physics and Astronomy Dr. David Alexander OBE Director, Rice Space Institute Professor of Physics and Astronomy Dr. Laurence Yeung Assistant Professor, Department of Earth, Environmental and Planetary Sciences HOUSTON, TEXAS March 2019 Copyright Shah Mohammad Bahauddin 2019 ABSTRACT Understanding the energy balance of Transition Region structures observed by IRIS in non-equilibrium emission by Shah Mohammad Bahauddin The corona, the outer atmosphere of the Sun, is a multi-million degree plasma, nearly three orders magnitude hotter than the visible surface. The exact mechanism by which the corona is heated is still the subject of debate, but possibilities include magnetic reconnection and magnetohydrodynamic waves. Studying the thin boundary layer connecting the cooler chromosphere to the hotter corona, named the TR, is an important step toward understanding mass and energy transport from the chromosphere to the corona. Thus, spectral emissions from the cool (< 1 MK) loop- like structures in this region are in need of extensive study and analysis. Because observations lack sufficient spatial resolution, this type of structure was called the “unresolved fine structure”, which is now considered resolved by the Interface Region Imaging Spectrograph (IRIS). In the active TR of the Sun, IRIS has observed loop-like structures with intermittent brightenings which are thought to originate from impulsive heating. In this thesis, the author present evidence of magnetic field line braiding and reconnection mediated brightenings of TR loops using IRIS slit-jaw images and spectral data, complemented by the EUV channels of the Atmospheric Imaging Assembly (AIA) of the Solar Dynamics Observatory (SDO). The set of observables used to characterize the brightenings consists of diagnostics of temperature, density, line broadening, and Doppler-shift on a pixel-by-pixel basis. The characterization scheme is extended by accumulating time dependent differential emission measure (DEM) distributions to define the nature of the spatial heating profile and frequency. A field-aligned hydrodynamic simulation and a forward modeling code, designed to generate synthetic observations from numerical experiments for comparison with real data, are employed. Non-equilibrium ionization is included in the computation of synthetic spectra. In addition, the relatively high-density TR plasma requires the inclusion of density-dependent dielectronic recombination rates to calculate the ion populations and the emission line intensities. We show that the observations and the numerical experiments are consistent with reconnection mediated impulsive heating at the braiding sites of multi-stranded TR loops. The combination of observation and numerical analysis will provide the building blocks of time-dependent 3D models of these loops and their contribution to active region emission which will, in turn, help us to understand the energy balance of these structures and may shed light on the long standing coronal heating problem: “Why is the Sun’s corona so much hotter than the surface?”. Acknowledgments Foremost, I would like to express my sincere gratitude to my advisor Prof. Stephen J. Bradshaw for the continuous support of my graduate study and research, in addition to his patience, motivation, and guidance. Besides my advisor, I would like to thank the rest of my thesis committee: Prof. David Alexander and Prof. Laurence Yeung for their encouragement, insightful comments and questions. I thank my colleague and friend in the Bradshaw Solar Physics group: Will T. Barnes for the stimulating discussions, for the sleepless nights we were working together and for all the fun we had in the last four years. Also, I thank my friends and teachers at the University of Dhaka. In particular, I am grateful to the Late Prof. Zahid Hasan Mahmood for believing in me during my time in college. My sincere thanks also go to Prof Atiqur Rahman Ahad, Mr. Mahbubul Hoq and Prof Biduyt Kumar Bhodro, who were selfless and kindhearted toward me and were sources of inspiration and knowledge. I would also like to take this opportunity to express gratitude to the Quazi family (Fairuz Zaman Chowdhury, Mohiuzzaman Quazi and P Zaman Quazi) without whom I would not be able to take care of my family and study. Last but not the least, I would like to thank my family: my parents Md Quamruz Zaman and Salma Zaman, and my sister Raahima Zannat (with Budim) for their love and care throughout my life. I am also grateful to my wonderful wife Mahbuba Khan (Mouree) for her love and grace, and my son Ehaan M Muhtadeen for making me realize how precious life truly is. Contents Acknowledgments ..................................................................................................... iv Contents .................................................................................................................... v List of Figures ........................................................................................................... vii List of Tables ............................................................................................................. xv Nomenclature ......................................................................................................... xvii Introduction ............................................................................................................... 1 The Sun and Its Atmosphere ....................................................................................... 8 2.1. Origin and Characteristics ........................................................................................ 8 2.2. Composition and Structure .................................................................................... 12 2.3. The Solar Atmosphere ............................................................................................ 15 2.4. Magnetism and Solar Activity ................................................................................ 18 Coronal EUV and X-Ray Radiation ............................................................................. 22 3.1. Spectral Lines in the Coronal Model Approximation ............................................. 22 3.2. Line Broadening Mechanisms ................................................................................ 25 3.2.1. Natural Broadening .......................................................................................... 26 3.2.2. Collisional Broadening ..................................................................................... 26 3.2.3. Doppler Broadening ......................................................................................... 27 3.2.4. Non-thermal Line Broadening ......................................................................... 29 3.2.5. Line Broadening due to the Limitation of Instrument ..................................... 30 3.3. Line Formation by Charge State Transitions .......................................................... 31 3.4. Calculation of Ion Population ................................................................................. 38 3.5. The Optically Thin Radiative Plasma ...................................................................... 41 3.6. Effect of Abundance ............................................................................................... 43 Hydrodynamic Modeling of Solar Loops .................................................................... 46 4.1. The Hydrodynamic Model ...................................................................................... 46 4.2. The Hydrodynamic and Radiative Emission (HYDRAD) Code ................................. 52 4.3. Forward Modeling Methods .................................................................................. 56 IRIS Observations of Transition Region Loops............................................................ 62 vi 5.1. Pre-IRIS Era and Theoretical Models ...................................................................... 62 5.2. The IRIS Mission ..................................................................................................... 66 5.2.1. Instrument Overview ....................................................................................... 69 5.2.2. IRIS Science Results .......................................................................................... 71 5.3. The Dynamic Transition Region of Solar Active Regions ........................................ 77 5.4. Non-equilibrium Ionization .................................................................................... 79 5.5. IRIS observations of TR loops ................................................................................. 83 5.6. Doppler Shift and Line Broadening ........................................................................ 90 5.7. Density Diagnostics ................................................................................................ 91 5.8. Co-aligning the IRIS data with AIA .......................................................................... 95 5.9. Differential Emission Measure Analysis ................................................................. 96 5.10. TR Loops in AR 12396