Analysis of solar faculae using stray-light-corrected spectropolarimetric data and numerical simulations

Abstract

The objective of this thesis is to develop methods to improve the quality of restored ground-based observations, and to use such restored observations, along with 3D radiative MHD simulations, to study the properties of small-scale facular magnetic elements in the lower solar atmosphere. The importance of understanding such structures is derived from studies that report their significant contribution to the total unsigned magnetic flux through the solar photosphere, as well as solar irradiance and variability. Benefiting from advancements in instrumentation and camera readout speeds, high quality observations (slit-spectra) were collected at the 1-m Swedish Solar Telescope (SST). In addition to a significant wavelength coverage that includes numerous photospheric spectral lines, the high spatial resolution, spectral resolution, and polarimetric sensitivity of the data allow for detailed, fully height stratified spectropolarimetric inversions. Before the analysis of the ground-based data was undertaken, image restoration methods that correct for different kinds of image degradation were applied to the data in order to ensure that the spatial and spectral distribution of intensities are as close as possible to the true solar values. One such method, extended as part of this thesis, is a stray-light correction procedure, which aims to remove the component of stray light caused by uncompensated high-order wave-front aberrations from the observations. The application of this method was found to improve the granulation contrasts in the slit-spectra, as well as reduce undesirable changes to the shapes of the observed spectral lines caused by stray-light contamination. Spectropolarimetric inversions were performed using the SPINOR-LTE inversion code, producing maps of the inverted physical quantities as a function of the optical depth. The retrieved atmospheres in the observed faculae were found to be in good agreement with the results of similar studies undertaken earlier, references to which can be found in the introductory chapter. Moreover, several facular bright points were identified in the cores of strong spectral lines. The temperatures, line-of-sight magnetic field strengths and line-of-sight velocities of these bright points were analysed using the inverted maps, however, since slit-spectra do not provide information on the time-evolution of magnetic structures, 3D-MHD direct-numerical-simulations (DNS) of faculae and facular bright points were performed using the MURaM 3D radiative-MHD code. The evolution of the physical parameters in a selected facular bright point was studied using the simulated atmospheres. While some of the atmospheric parameters were found to be similar in some respects with the results of the inversions, the use of high resolution simulations with high cadence also revealed the possible existence of additional dynamical processes that were seen to participate in and possibly drive the evolution of the bright point. This thesis is divided into six chapters. In the introductory chapter, a brief account of some of the important studies that analysed the properties of the vector magnetic fields in solar faculae is provided. Emphasis is placed on novel observational techniques and numerical methods that were developed to quantify magnetic field strengths, and orientation in solar faculae. In chapters 3 and 4, attention is drawn to image restoration and its importance in ground-based observations. The chapters include a brief description of the working principle of adaptive optics (AO) and the theory behind the multi-frame blind deconvolution (MFBD) method employed to restore the spectropolarimetric observations. Subsequently, a detailed discussion of the theory of residual stray light arising from uncompensated high-order wave-front aberrations is provided, and a method to remove this residual stray-light contamination from the restored slit-spectra using Kolmogorov phase screens and synthetic, degraded images is described. The final chapters on spectropolarimetric inversions and 3D radiative-MHD simulations briefly introduce the theory behind spectral-line formation, and the equations of compressible, non-ideal magneto-hydrodynamics (MHD), respectively, in addition to describing the setup of the inversions, the choice of the spectral lines, the selection of the physical parameters to be retrieved, and the setup of the numerical simulations and the relevant boundary conditions. In the concluding chapter, the results of the stray-light correction procedure, spectropolarimetric inversions, and the numerical simulations are compiled and sequentially listed. Also, a very brief outlook is given on possible next steps.