From flux emergence to decay

In order to gain a comprehensive understanding of the dynamic processes occurring on the Sun, it is essential to closely study of the dynamic interplay between plasma flows and magnetic fields. The interplay between plasma flows and magnetic fields within active regions is the primary driver of the formation and evolution of these regions. Once magnetic flux bundles have pierced the photosphere and become visible as dark surface structures, they extend as a canopy structure into the chromosphere and higher atmospheric layers. The emergence of magnetic features affects the typical surface flows, which are generally characterized by global large-scale flow motions and divergence centers caused by granulation. Moreover, their appearance impacts the entire atmosphere, potentially affecting the entire heliosphere causing explosive events such as flares and especially coronal mass ejections (CMEs). Nevertheless, the processes by which active regions emerge, evolve and decay remain poorly understood. What are the determining factors that make the leading or trailing part the dominant one? What processes, discernible at the photospheric or chromospheric level, contribute to the heating of the corona? What are the mechanisms by which emerging magnetic flux bundles and plasma motions on the solar surface interact with each other? What are the consequences for the evolution of active regions? What processes regulate the evolution of magnetic flux bundles as they rise into the chromosphere, beyond the observed extension in a canopy-like structure? What are the conditions that give rise to the formation of a penumbra? What are the determining factors that influence whether a penumbra fully develops into its characteristic fine filamentary structure or only exhibits a more rudimentary form? To what extent do photospheric flow fields exert an influence on those typically observed in the chromosphere? Furthermore, it is little understood at which moment the gradual process of sunspot decay begins. This raises the question of whether a critical value of the magnetic field marks the onset of penumbral decay, and consequently the decay of the entire sunspot. In this context, the role played by the surrounding plasma motions requires further scrutiny. Is it solely responsible for the advection of small-scale moving magnetic features (MMFs), or does a change in the surrounding flows and magnetic fields instigates penumbral decay? This doctoral research investigates the complex processes of emergence, evolution, and decay of active regions on the ever-changing Sun. In order to achieve this objective, surface features resulting from the influence of strong magnetic fields, such as pores and sunspots, are examined at the highest spatial, spectral, and temporal resolution, as well as with high polarimetric sensitivity. The data analyzed in this work comprises high-resolution ground-based imaging and spectro-polarimetric data. However, the high-resolution observations only unveil their full potential when placed in context with synoptic data, thus, establishing statistical significance and accurately placing them in the long-term evolution of active regions. The combination of this data enabled a detailed analysis of the three-dimensional magnetic and flow fields in the photosphere and chromosphere. Considering the complex interaction of plasma motions and magnetic fields, sunspots play a special role in this context. High-resolution observations of the evolution of sunspots, from formation to decay, provide significant insights into the dynamics of the Sun. Optimally, observations should start prior to the initial disturbance of the quiet Sun by emerging magnetic flux bundles and continue to cover the entirety of the active region life cycle. This would include, the formation of a penumbra, which transforms a simple pore into a sunspot, thereby introducing further complexity into the magnetic and flow structure. Later, the evolution into a complex active region must be observed, which consists of multiple sunspots, pores, and related features such as light bridges. These are embedded in a complex plasma flow and magnetic structure until the magnetic flux eventually dissipates and decays. Nevertheless, research should not be limited to sunspots alone, but should also encompass smaller features in the periphery and those related to spots, which should be included in the analysis. The objective of this research project is to enhance our understanding of active region dynamics. The utilization of sophisticated methods, including Background-subtracted Solar Activity Maps (BaSAMs), Local Correlation Tracking (LCT), and the Stokes Inversions based on Response function (SIR) code, enabled comprehensive studies of several active regions observed in the photosphere and chromosphere. These studies encompassed pores, sunspots with rudimentary and fully formed penumbra, light bridges, and several small-scale features during different stages of evolution. These research studies are the foundation of three first-author, peer-reviewed articles in scientific journals. 1. The first publication resulting from this doctoral research covered the topic High-resolution Imaging of Solar Pores and was published in 2023 in the journal Astronomy & Astrophysics (DOI:10.1051/0004-6361/202245410). Context. Light bridges are bright, long, and narrow features that are typically connected to the formation or decay processes of sunspots and pores. Aims. The interaction of magnetic fields and plasma flows is investigated in the trailing part of an active region, where pores and magnetic knots evolve into a complex sunspot. The goal is to identify the photospheric and chromospheric processes, which transform the mainly vertical magnetic fields of pores into a sunspot with multiple umbral cores, light bridges, and rudimentary penumbrae. Methods. Conducting observations with a broad variety of telescopes and instruments provides access to different atmospheric layers and the changing morphology of features connected to strong magnetic fields. While the Helioseismic and Magnetic Imager (HMI) of the Solar Dynamics Observatory (SDO) provides full-disk continuum images and line-of-sight magnetograms, the fine structure and flows around a pore can be deduced from high-resolution observations in various wavelengths as provided by the Goode Solar Telescope (GST) at the Big Bear Solar Observatory (BBSO). Horizontal proper motions are evaluated applying local correlation tracking (LCT) to the available time series, whereas the connectivity of sunspot features can be established using the background-subtracted activity maps (BaSAMs). Results. Photospheric flow maps indicate radial outflows, where the light bridge connects to the surrounding granulation, whereas inflows are present at the border of the pores. In contrast, the chromospheric flow maps show strong radial outflows at superpenumbral scales, even in the absence of a penumbra in the photosphere. The region in between the two polarities is characterized by expanding granules creating strong divergence centers. Variations in BaSAMs follow locations of significant and persistent changes in and around pores. The resulting maps indicate low variations along the light bridge, as well as thin hairlines connecting the light bridge to the pores and strong variations at the border of pores. Various BaSAMs demonstrate the interaction of pores with the surrounding supergranular cell. The Hα line-of-sight velocity maps provide further insights into the flow structure, with twisted motions along some of the radial filaments around the pore with the light bridge. Furthermore, flows along filaments connecting the two polarities of the active region are pronounced in the line-of-sight velocity maps. Conclusions. The present observations reveal that even small-scale changes of plasma motions in and around pores are conducive to transform pores into sunspots. In addition, chromospheric counterparts of penumbral filaments appear much earlier than the penumbral filaments in the photosphere. Penumbra formation is aided by a stable magnetic feature that anchors the advection of magnetic flux and provides a connection to the surrounding supergranular cell, whereas continuously emerging flux and strong light bridges are counteragents that affect the appearance and complexity of sunspots and their penumbrae. Author Contributions. The individual contributions by the authors are listed according to the Contributor Roles Taxonomy (credit.niso.org). R. Kamlah: conceptualization, formal analysis, investigation, software, validation, visualization, and writing – original draft; M. Verma: funding acquisition, investigation, methodology, project administration, software, supervision, validation, visualization, and writing – review & editing; C. Denker: methodology, project administration, software, supervision, validation, visualization, and writing – review & editing; H. Wang: data acquisition, investigation, project administration, supervision, and writing – review & editing. 2. The second paper that covered the topic Impact of Magnetic and Flow Fields on Penumbrae and Light Bridges of Three Leading Sunspots in an Active Region was published in 2024 in the journal Solar Physics (DOI:10.1007/s11207-024-02386-9). This study investigates penumbrae and light bridges based on photospheric and chromospheric flow fields and photospheric magnetic fields in active region NOAA 13096. The improved High-resolution Fast Imager (HiFI+) and the GREGOR Infrared Spectrograph (GRIS) acquired high-resolution imaging and spectropolarimetric data at the 1.5-meter GREGOR solar telescope at the Observatorio del Teide, Iza ˜na, Tenerife, Spain. Background-Subtracted Activity Maps (BaSAMs) have been used to locate areas of enhanced activity, Local Correlation Tracking (LCT) provides horizontal proper motions, and near-infrared full-Stokes polarim