Faint Coronal Hard X-rays From Accelerated Electrons in Solar Flares by Lindsay Erin Glesener A dissertation submitted in partial satisfaction of the requirements for the degree of Doctor of Philosophy in Physics in the Graduate Division of the University of California, Berkeley Committee in charge: Professor Robert P. Lin, Co-chair Doctor S¨am Krucker, Co-chair Professor Stuart D. Bale, Co-chair Professor Gibor Basri Professor Steven E. Boggs Fall 2012 Faint Coronal Hard X-rays From Accelerated Electrons in Solar Flares Copyright 2012 by Lindsay Erin Glesener 1 Abstract Faint Coronal Hard X-rays From Accelerated Electrons in Solar Flares by Lindsay Erin Glesener Doctor of Philosophy in Physics University of California, Berkeley Professor Robert P. Lin, Co-chair Doctor S¨am Krucker, Co-chair Professor Stuart D. Bale, Co-chair Solar flares are huge explosions on the Sun that release a tremendous amount of energy from the coronal magnetic field, up to 1033 ergs, in a short time (100–1000 seconds), with much of the energy going into accelerated electrons and ions. An efficient acceleration mech- anism is needed, but the details of this mechanism remain relatively unknown. A fraction of this explosive energy reaches the Earth in the form of energetic particles, producing geomag- netic storms and posing dangers to spaceborne instruments, astronauts, and Earthbound power grids. There are thus practical reasons, as well as intellectual ones, for wishing to understand this extraordinary form of energy release. Through imaging spectroscopy of the hard X-ray (HXR) emission from solar flares, the behavior of flare-accelerated electrons can be studied. The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) spacecraft launched in 2002 with the goal of better understanding flare particle acceleration. Using rotation modulation collimators, RHESSI is able to cover a wide energy range (3 keV–17 MeV) with fine angular and energy resolutions. RHESSI’s success in the last 10 years in investigating the relationship between energetic electrons and ions, the nature of faint sources in the corona, the energy distribution of flares, and several other topics have significantly advanced the understanding of flares. But along with the wealth of information revealed by RHESSI come some clear ob- servational challenges. Very few, if any, RHESSI observations have come close to imaging the electron acceleration region itself. This is undoubtedly due to a lack of both sensitiv- ity (HXRs from electron beams in the tenuous corona are faint) and dynamic range (HXR sources at chromospheric flare footpoints are much brighter and tend to obscure faint coronal sources). Greater sensitivity is also required to investigate the role that small flares in the quiet Sun could play in heating the corona. The Focusing Optics X-ray Solar Imager (FOXSI) is a developing project to address these observational difficulties. FOXSI is a sounding rocket payload developed under NASA’s Low Cost Access to Space program. The project spearheads a shift to using direct imaging via focusing grazing-incidence HXR optics rather than the indirect Fourier techniques used 2 by RHESSI and its predecessors. Such optics can attain higher sensitivity since photons are focused onto a small detector volume and have significantly better dynamic range than Fourier methods do. On November 2, 2012 the FOXSI rocket payload was flown for a 6- minute observation and successfully imaged a solar flare, providing the first focused HXR spectroscopic images of the Sun above 5 keV. The motivation, construction, testing, and flight of FOXSI will be described in this text, along with case studies on the use of RHESSI to analyze unique coronal HXR sources from two solar flares. i Dedication These pages have been finished with many tears, for we have lost a great scientist and an inspiring man with the passing of my advisor Bob Lin. Bob was dear to everyone who knew him and I am very lucky to have known and have been taught by this extraordinary man. In a draft of thesis acknowledgements written before his sudden stroke, I had this to say about him: “My advisor, Bob Lin, is an inspiration to me with the enthusiasm and cre- ativity with which he approaches his work. Bob is someone you want to be like: he has an encyclopedic knowledge gained from a lifetime of hard work; he treats everyone he meets with kindness and respect, and he enjoys his life.” Looking back at these words, I could not agree more. Bob was distinctive because of his intelligence, his kindness, and his love of his life and his work. He created a healthy and encouraging environment for his students and employees. He will always have my unending gratitude, and in the course of the career ahead of me I will make every effort to make him proud. While Bob was with us he taught me much about the field of space physics. In death he continues to teach me much about life. ii Contents List of Figures vi List of Tables xviii Acknowledgments xx 1 Introduction 1 1.1 Welcome...................................... 1 1.2 Solarstructure .................................. 2 1.3 Solarflares..................................... 5 1.4 What’sahead ................................... 8 2 Solar flare processes 9 2.1 Introduction.................................... 9 2.2 Thestandardflaremodel............................. 9 2.2.1 Reconnection ............................... 10 2.2.2 Standardflaregeometry . .. .. 13 2.2.3 Interchangereconnection . 14 2.3 Flare acceleration mechanisms . 16 2.4 Thecoronalheatingproblem. 18 2.4.1 Flareheating ............................... 20 2.4.2 Nanoflareheating............................. 20 2.5 Methodsofobservation.............................. 23 2.5.1 X-rayemissionfromtheSun. 23 2.5.2 Radio emission . 27 2.5.3 Timeevolutionofflares ......................... 28 3 The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) 30 3.1 Introduction.................................... 30 3.2 Imagingsystem .................................. 31 3.3 Spectrometer ................................... 36 3.4 Software...................................... 38 Contents iii 3.4.1 Imagereconstructionmethods . 39 3.4.2 Spectroscopy ............................... 42 4 Hard X-ray Observations of a Jet and Accelerated Electrons in the Corona 44 4.1 Introduction.................................... 44 4.2 Observations.................................... 48 4.2.1 TRACE EUVobservations........................ 48 4.2.2 RHESSI HXRobservations ....................... 50 4.2.3 Radioobservations ............................ 53 4.3 Discussion..................................... 54 4.3.1 Evidence for interchange reconnection . 54 4.3.2 HXRemissionfromthejet........................ 56 4.4 Summary ..................................... 60 5 Energetics and Heating in a Solar Plasma Ejection Observed By RHESSI and AIA 62 5.1 Introduction.................................... 63 5.2 Observations.................................... 65 5.2.1 AIAobservationsofejectedplasma . 65 5.2.2 RHESSI HXRsfromtheejecta ..................... 71 5.3 Discussion..................................... 73 5.3.1 Thermalenergy.............................. 74 5.3.2 Collisional energy loss . 74 5.3.3 Electron injection profile . 76 5.4 Summary ..................................... 76 6 Introduction to the Focusing Optics X-ray Solar Imager (FOXSI) 78 6.1 ScientificMotivation ............................... 78 6.1.1 Coronalsources.............................. 79 6.1.2 Nanoflaresandcoronalheating . 81 6.2 Advantagesoffocusingoptics . 82 6.3 The FOXSI soundingrocket ........................... 83 6.3.1 FOXSI optics ............................... 84 6.3.2 FOXSI detectors ............................. 85 6.4 FOXSI’ssciencegoals............................... 85 6.4.1 Activeregiontemperatures. 86 6.4.2 Nonthermal emission from quiet Sun nanoflares . 88 6.5 Chaptersummary................................. 89 7 Focusing Hard X-ray Optics for FOXSI 91 7.1 Introduction to X-ray Focusing Telescopes . .... 91 7.1.1 X-rayreflectivity ............................. 91 Contents iv 7.1.2 Focusingopticsgeometries . 98 7.2 Heritage of the FOXSI optics .......................... 101 7.2.1 The HERO balloonprogram....................... 101 7.3 Description of the FOXSI optics......................... 104 7.4 FOXSI opticstestingandresults . 106 7.4.1 Calibrationsetup ............................. 106 7.4.2 Pointspreadfunction. .. .. 107 7.4.3 Halfpowerdiameter ........................... 107 7.4.4 Effectivearea ............................... 109 7.4.5 X-ray alignment with FOXSI detectors ................. 111 7.5 Conclusion..................................... 115 8 Double-sided Silicon Strip Detectors for FOXSI 116 8.1 Introduction to semiconductor radiation detectors . ....... 116 8.1.1 Signal formation in a semiconductor . 117 8.1.2 Theeffectofimpurities.......................... 120 8.1.3 Semiconductorjunctions . 121 8.1.4 Leakagecurrent.............................. 123 8.1.5 Semiconductordetectorgeometries . 124 8.1.6 Semiconductor detector materials . 127 8.1.7 Producinganenergyspectrum. 128 8.2 The FOXSI detectorsystem ........................... 132 8.2.1 The FOXSI DSSD ............................ 132 8.2.2 TheFOXSIVATA451readoutASIC . 133 8.2.3 PrototypetestingatISAS/JAXA . 136 8.2.4 The FOXSI readoutsystem ....................... 136 8.3 FOXSI detectortestingandcalibration . 144 8.3.1 Experimentalsetup............................ 144 8.3.2 Noisystrips ................................ 145 8.3.3 Gain calibration . 145 8.3.4