The New Jersey Institute of Technology's
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Title:	Helioseismic diagnostics of active regions and their magnetic fields
Author:	Stefan, John T.
View Online:	njit-etd2022-058 (xiv, 131 pages ~ 9.0 MB pdf)
Department:	Department of Physics
Degree:	Doctor of Philosophy
Program:	Applied Physics
Document Type:	Dissertation
Advisory Committee:	Kosovichev, Alexander G. (Committee chair) Wang, Haimin (Committee member) Fleishman, Gregory D. (Committee member) Nita, Gelu M. (Committee member) Wu, Xuejian (Committee member)
Date:	2022-12
Keywords:	Active regions Helioseismology Solar activity Sunquakes
Availability:	Unrestricted
Abstract:	While two and a half decades of nearly constant observation by the Solar and Heliospheric Observatory (SOHO) and the Solar Dynamics Observatory (SDO) spacecraft have yielded key insights into the structure and dynamics of active regions, it is still unclear if active regions can be identified before emerging on the solar surface and, once emerged, whether the subsurface structure of an active region’s magnetic field can be measured. Regarding the dynamical processes associated with active regions, the height and mechanism of sunquake excitation remains poorly understood. To answer these questions, a comprehensive survey of active region magnetic fields and their associated helioseismic signatures for both the pre-emergence and post-emergence phase is completed. For the former, deviations of the mean phase travel time of acoustic waves are used to detect the rise of magnetic flux from the solar interior. Calibration and testing of the time-distance technique is performed using simulations of submerged sound speed perturbations. A detailed case study of select active regions is performed, and the technique is then applied to a collection of 46 active regions to determine the statistical significance of mean travel time perturbations as a signature of active region emergence. Next, a novel technique is developed for the assessment of existing active region magnetic fields. By combining the travel time of acoustic waves propagating in varying directions, perturbations due to subsurface horizontal magnetic fields are isolated from structural changes. The resulting measurements provide a proxy for the magnitude of the horizontal magnetic field as well as a direct measurement of the field’s azimuth. The technique is applied to a sunspot simulation for validation, and is then used to investigate the subsurface magnetic structure of several observed sunspots. Finally, a model of solar acoustic wave propagation is constructed using the compressible form of the mass, momentum, and entropy conservation equations to study the excitation of sunquakes. The constructed model is used to determine at what height sunquakes are excited, what mode of excitation is most energetically favorable, and what properties of particle beams are relevant to sunquake excitation. The excitation height is determined from comparison of observed events with a catalogue of simulated sunquakes for a range of excitation locations and for several excitation mechanisms, which allows the excitation height and energy to be estimated. Additionally, the output of FP proton beam simulations are used to derive forcing functions for the excitation of sunquakes in the model to determine the dependence of wave front amplitude on the low-energy cut-off.