Properties and Variations of Mg II h&k Lines Over the Solar Cycle

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The University of Glasgow
Over the years, the study of Mg II h&k lines has transformed our approach to unraveling the intricate structures and dynamics of the solar chromosphere. Since the middle of the last century until space missions, our understanding has grown exponentially. The Interface Region Imaging Spectrograph (IRIS) spacecraft is one of these missions that provided us with data that allowed us to observe these lines. In this thesis, the focus is to investigate the temporal and spatial variations of Mg II h&k lines in the solar atmosphere. Additionally, we present a novel approach for automatically determining the positions of the outer minima in the red and blue wings, line emission peaks in the red and blue sides, and the central absorption cores in the spectroscopic observations obtained by IRIS. In Chapter 1, we offer a brief literature review of the multifaceted aspects of the Sun's atmospheric layers. The diverse structures of the chromosphere across the quiet sun, active regions, coronal holes, and prominences and filaments are described. The narrative progresses through a historical trajectory detailing the observations and research of Mg II h&k lines, highlighting their formation, intrinsic properties, and significance as diagnostic tools for the chromosphere. The Chapter concludes with an overview of the modelling of the Mg II h&k lines. In Chapter 2, we delve into the properties of Mg II h&k lines within the Quiet Sun at the disc centre. The emphasis is placed on comprehensive observations and data analysis, leveraging the capabilities of the Interface Region Imaging Spectrograph (IRIS). Integral to the research is the introduction of a novel algorithm for the automatic identification of Mg II h&k spectral features, which is used throughout the thesis. This algorithm is meticulously described, with insights into its prerequisites, a comparison with pre-existing methods, and an assessment of the data quality it yields using synthetic data. In Chapter 3, we use the quiet Sun rasters at disc center spanning from 2015 to 2022 obtained from IRIS. This research delves into the automatic examination of Mg II features, unraveling both consistent correlations and temporal fluctuations within these complex line profiles. The study's driving aim is to discern the nuanced interplay among radiation temperatures, integrated intensities, line widths, and the intensities of specific spectral features, all within the solar chromosphere. We aim to investigate the temporal variation of the averaged flux for all Mg II h&k (2803.53 and 2796.35 Å, respectively) features over time (the end of the 24th and the beginning of the 25th solar cycles). We seek to derive information on the spectral features of the Mg II h&k lines in the solar atmosphere. We find that the intensity in k2v has a strong correlation with k3 and k2r intensities, and that the total line integrated intensity has a good correlation with feature intensities for k2v, k3, and k2r. The relationship between the line width and the intensities at k2v, k3, and k2r is inverse. We also find that the average intensities in all five spectral features of the k line at Sun centre remained roughly constant between the end of the 24th solar cycle and the beginning of the 25th solar cycle. Similarly, we find that the k/h line intensity ratio remains constant over the same time period. In Chapter 4, we use a high-resolution, single IRIS full-Sun mosaic of the Mg II k line on February 24, 2019. The solar disc is divided into 20 concentric annuli. By averaging the pixel values within each annulus, we streamline the data, mitigate local variances and facilitate an analysis. This method results in 20 average line profiles, which form the cornerstone of our final analytical endeavors. The study shows the relationships between μ (the viewing angle on the solar disc) and various solar atmospheric properties. A robust correlation is observed between μ and properties like feature intensities of spectral features, total integrated intensities, and temperatures at the k1v, and k2v, coupled with the intensity ratio k2. All these parameters decreased as μ approached the solar limb. Conversely, parameters like line width, k1 separation, k2 separation, velocities at k2 separation, and depth at k3 exhibited an increase as the viewing angle edged towards the limb. Chapter 5 presents the conclusions, gives a summary of our findings, and discusses possible plans for future work.
Sun: chromosphere – Sun: UV radiation