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Local Helioseismology of Magnetic Activity Local Helioseismology of Magnetic Activity A thesis submitted for the degree of: Doctor of Philosophy by Hamed Moradi B. Sc. (Hons), B. Com Centre for Stellar and Planetary Astrophysics School of Mathematical Sciences Monash University Australia February 12, 2009 Contents 1 Introduction 1 1.1 Helioseismology............................... 2 1.2 Local Helioseismology Diagnostic Tools . .... 6 1.2.1 Time-distance Helioseismology . 6 1.2.2 Helioseismic Holography . 12 1.3 Helioseismology of Sunspots and Active Regions . ..... 14 1.3.1 Sunspots .............................. 14 1.3.2 SunspotSeismology . 17 1.3.3 ForwardModelling . 21 1.4 SolarFlareSeismology........................... 26 1.4.1 Solar Flare Observations . 27 1.4.2 Seismic Emission From Solar Flares . 33 1.5 BasisforthisResearch........................... 35 2 Modelling Magneto-Acoustic Ray Propagation In A Toy Sunspot 41 2.1 Introduction................................. 43 2.2 TheMHSSunspotModel ......................... 44 2.3 MHD Ray-Path Calculations . 48 2.4 The2DRay-PathSimulations. 51 2.4.1 TheComputationalMethod. 51 2.4.2 Travel-Time and Skip-Distance Perturbations . 53 2.4.3 Binned Travel-Time Perturbation Profiles . 58 2.4.4 Comparison With Observations . 59 2.4.5 Isolating the Thermal Component of Travel Time Perturbations 61 2.5 SummaryandDiscussion . .. .. .. .. .. .. .. 63 i CONTENTS 3 The Role of Strong Magnetic Fields on Helioseismic Wave Propaga- tion 67 3.1 Surface-FocusMeasurements . 70 3.1.1 Introduction ............................ 70 3.1.2 TheMHSSunspotModel . 71 3.1.3 MHD Wave-Field Simulations . 73 3.1.4 MHD Ray-Path Simulations . 75 3.1.5 Modelling Surface-Focus Travel-Time Inhomogeneities . 76 3.1.6 TheTravelTimeProfiles . 77 3.1.7 SummaryandDiscussion . 82 3.2 Deep-FocusMeasurements. 83 3.2.1 Introduction ............................ 83 3.2.2 Common Midpoint Deep-Focusing . 83 3.2.3 ResultsandDiscussion. 86 3.2.4 Conclusion ............................. 88 4 Flare Seismology 91 4.1 Introduction................................. 95 4.1.1 Computational Seismic Holography . 96 4.2 TheSolarFlareof15January2005. 98 4.2.1 Active Region Morphology . 98 4.2.2 The Helioseismic Data . 98 4.2.3 TheAcousticSignatures. 100 4.2.4 Visible Continuum Emission . 104 4.2.5 TheSeismicWaves. 105 4.2.6 HardX-RayEmission . 106 4.2.7 SummaryandDiscussion . 109 4.3 TheSolarFlareof14August2004 . 114 4.3.1 Active Region Morphology . 114 4.3.2 The Helioseismic Data . 115 4.3.3 TheAcousticSignatures. 115 4.3.4 Visible Continuum Emission . 119 4.3.5 The Magnetic Field Topology . 121 4.3.6 TheSeismicWaves. 122 4.3.7 RadioandHXREmission . 125 4.3.8 SummaryandDiscussion . 131 ii CONTENTS 4.4 TheSolarFlareof10March2001. 133 4.4.1 Active Region Morphology . 133 4.4.2 The Helioseismic Data . 134 4.4.3 The Helioseismic Signatures . 134 4.4.4 Coronal Magnetic Field Reconstruction . 143 4.4.5 SummaryandDiscussion . 146 5 Summary and Perspective 151 5.1 ForwardModelling ............................. 152 5.2 FlareSeismology .............................. 155 Bibliography 161 iii CONTENTS iv Abstract Over the years, local helioseismology has provided us with unprecedented insights into the structure and dynamics of solar active regions, in particular sunspots, which for a long time have been known to be dominated by strong magnetic activity. How- ever, even though significant inroads have been made over the last two decades since the inception of the field, there are still a number of unanswered questions regarding physical conditions in the solar interior that need to be addressed. In this thesis, we aim to shed light on two of these open questions: i) what is the true nature and extent of the sub-surface structure of active regions and sunspots? ii) how do we ef- fectively diagnose the seismic response of the solar interior to flare-induced energetic transients, and what is their underlying cause? Addressing the first question requires the development of MHD simulations to test observational inferences made in regions of strong surface magnetic fields. We devise and apply a numerical forward model based on MHD ray theory to address some of the ambiguous and inconsistent interpretations of helioseismic travel times that have resulted from tomographic observations in the vicinity of sunspots. The resulting simulations have shown that it is feasible to use ray theory in model sunspots to produce travel-time shifts than can meaningfully be compared with observations. In order to validate the results from ray theory, we also conduct detailed comparative studies with an existing simulation code developed for analysing the interaction of linear waves with magnetic structures in nonuniform atmospheres. Together, these numerical forward models provide compelling evidence which indicates that the effect of the magnetic field on helioseismic waves can not be considered to be small near the surface, with travel time inhomogeneities observed through sunspots appearing to be dominated by MHD physics. These results are the strongest indication yet that surface magnetic fields are directly and significantly altering the magnitude and lat- eral extent of linear inversions of sunspot structure (i.e., sub-surface wave and sound v Abstract speed perturbations) made by time-distance helioseismology. On addressing the second question, we employ various local helioseismic methods to distinguish and analyse the multi-wavelength observational signatures of seismic emissions from three solar flares – X1.2-class flare of 15 January 2005, M7.4-class flare of 14 August 2004 and M6.7-class flare of 10 March 2001. In-depth correlative studies were conducted, with the resulting analysis showing that all three flares exhibited the same close spatial alignment between the sources of the seismic emission and impul- sive visible continuum emission as previous flares, reinforcing the hypothesis that the acoustic emission may be driven by radiative “back-warming” – heating of the low photosphere by intense Balmer and Paschen continuum-edge recombination radiation from the overlying ionized chromospheric medium. Detailed analysis of the magnetic field topology of the host active regions also reveal the existence of a close relationship between the heights of the coronal magnetic loops that conduct high-energy particles from the flare and the seismicity of the energetic transients. vi General Declaration In accordance with Monash University Doctorate Regulation 17/Doctor of Philosophy and Master of Philosophy regulations the following decla- rations are made: I hereby declare that this thesis contains no material which has been accepted for the award of any other degree or diploma at any university or equivalent institution and that, to the best of my knowledge and belief, this thesis contains no material pre- viously published or written by another person, except where due reference is made in the text of the thesis. This thesis includes five (5) original papers published in peer reviewed journals and one (1) unpublished paper. The core theme of the thesis is the local helioseismology of magnetic activity. The ideas, development and writing up of all the papers in the thesis were the principal responsibility of myself, the candidate, working within the Centre for Stellar and Planetary Astrophysics, School of Mathematical Sciences, under the supervision of Prof. Paul Cally and Dr. Alina Donea. The inclusion of co-authors reflects the fact that the work came from active col- laboration between researchers and acknowledges input into team-based research. In the case of thesis chapters 2 - 4, my contribution to the work involved the fol- lowing: vii General Declaration Thesis Publication title Publication Nature and extent of candidate’s chapter status contribution 2 Time-distance Published, Key ideas and initiation, development oF modelling in a 2008 (Solar the code, numerical modelling, produced simulated Phys., 251, all Figures, wrote the paper. sunspot 309-327) atmosphere. 3 Numerical models Published, Key ideas and initiation, signiFicant parts oF travel-time 2009 (ApJ, oF the numerical modelling and analysis, inhomogeneities 690, L72- produced all Figures, signiFicant In sunspots. L75). contribution to discussion and analyses oF results, wrote much oF the text. 3 Deep-focus Submitted Key ideas and initiation, numerical diagnostics oF For review, modelling and analysis, produced all sunspot structure. February Figures, wrote the paper. 2009 (ASSP). Helioseismic Published, Key ideas and initiation, data reduction analysis oF the 2007 and observational analysis, produced solar Flare- 4 (MNRAS, Figures 1-4 and 7, signiFicant induced sunquake 374, contribution to discussion and analyses oF 2005 January 1155-1163). oF results, wrote much oF the text. 15. From Gigahertz to millihertz: a Key ideas and initiation, signiFicant parts multiwavelength Published, oF data reduction and observational study oF the 2007 (Solar analysis, prepared Figures 1-4, 4 acoustically Phys., 245, signiFicant contribution to discussion active 14 August 121-139). and analyses oF results, wrote parts oF 2004 M7.4 solar the text. Flare. Key ideas and initiation, signiFicant parts Seismic Published, oF data reduction and observational emissions From a 2008 (Solar analysis, prepared Figures 1-3, 4 highly impulsive Phys., 251, signiFicant contribution to discussion M6.7 solar Flare. 613-626). and analyses oF results, wrote parts oF the text. I have renumbered sections oF submitted or published papers in order to generate a consistent presentation within the thesis.
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