Local helioseismology of magnetic activity

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. However, even though significant inroads have been made over the last two decades since the inception of the held, 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 effectively diagnose the seismic response of the solar interior to hare-induced energetic transients, and what is their underlying cause? Addressing the hrst question requires the development of MHD simulations to test observational inferences made in regions of strong surface magnetic helds. 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 lateral 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 - XL2-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 impulsive 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.