Probing Magnetic Fields in the Solar Convection Zone with Time-Distance Helioseismology

Abstract

Abstract The magnetic field is responsible for most of solar activities on the Sun. Subsurface magnetic field is therefore of great interest to the solar physicists, especially the origin of it at the base of the convection zone (BCZ). Helioseismology is capable of exploring the solar interior by studying the helioseismic waves observed on the surface. To infer the magnetic field inside the Sun from solar oscillations, one requires a comprehensive understanding of the interaction between waves and the magnetic field. This dissertation contains the observation of evidence of magnetic field at the BCZ, the characteristics of helioseismic waves scattered by a sunspot, and a model for the energy budget of solar acoustic waves propagating through the active region. Time-distance method in helioseismology is employed throughout this work. First, we present the observation of the solar-cycle variation of meridional flows in the entire convection zone. Time-distance helioseismology has been applied to SOHO/MDI spacecraft data to study the temporal variation of subsurface meridional flows over 15 years. The time it take for solar acoustic wave to travel between two points on the surface in the meridional plane is measured, which can be used to infer the flow in the vicinity of the ray path that connects two surface locations. A number of systematic errors have been confronted, particularly the contamination from the surface magnetic field. Observations show difference between solar maximum and minimum at the BCZ. They suggest the presence of magnetic fields around the BCZ, and place a lower limit of 2 × 10^3 gauss for the field strength at the BCZ. Second, we characterized features of the acoustic-wave packet scattered by a sunspot in a regime where the wavelength is comparable to the size of the sunspot. Not only do we measure travel-time shifts but also wave-amplitude perturbations with respect to the quiet Sun. Plane wave packets are sorted out as the incident waves. A finite-wavelength phenomenon known as wavefront healing in scattering theory has been observed in the travel-time measurement. On the other hand, the observation shows on-axis reductions and off-axis enhancements in wave-amplitude measurement. They indicate that the sunspots may introduce a fast wave-speed perturbation and defocus the wave energy in addition to the physical absorption of waves by sunspots. Last, we propose a model for the energy budget of acoustic waves propagating through a sunspot in terms of the coefficients of absorption, emissivity reduction, and local suppression of the sunspot. Effects of three mechanisms are separated by computing the cross-correlation function (CCF) which excludes the signal that is not emitted from the starting point. We then determine the fraction of three mechanisms contributing to the energy deficit from the magnitude of CCF with a selected sunspot as a case study. The acoustic power derived from the determined coefficients is consistent with that obtained directly from the acoustic-power map within 7%.