The evolution of solar sigmoidal active regions
Savcheva-Tasseva, Antonia Stefanova
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The formation, evolution and eruption of solar active regions is a main theme in solar physics. Ultimately the goal is predicting when, where and how an eruption will occur, which will greatly aid space weather forecasting. Special kinds of S-shaped active regions (sigmoids) facilitate this line of research, since they provide conditions that are easier to disentangle and have a high probability for erupting as flares and/or coronal mass ejections (CME). Several theories have been proposed for the formation, evolution, and eruption of solar active regions. Testing these against detailed models of sigmoidal regions can provide insight into the dominant mechanisms and conditions required for eruption. This thesis explores the behavior of solar sigmoids via both observational and magnetic modeling studies. Data from the most modern space-based solar observatories are utilized in addition to state-of-the-art three-dimensional data-driven magnetic field modeling to gain insight into the physical processes controlling the evolution and eruption of solar sigmoids. We use X-ray observations and the magnetic field models to introduce the reader to the underlying magnetic and plasma structure defining these regions. By means of a large comprehensive observational study we investigate the formation and evolution mechanism. Specifically, we show that flux cancellation is a major mechanism for building the underlying magnetic structure associated with sigmoids, namely magnetic flux ropes. We make use of topological analysis to describe the complicated magnetic field structure of the sigmoids. We show that when data-driven models are used in sync with MHD simulations and observations we can arrive at a consistent picture of the scenario for CME onset, namely the positive feedback between reconnection at a generalized X-line and the torus instability. In addition we show that topological analysis is of great use in analyzing the post-eruption flare- and CME-associated observational features. Such analysis is used to extend the standard 2D flare/CME models to 3D and to find potentially large implications of topology to understanding 3D reconnection and the seed populations of energetic particles in CMEs.
Thesis (Ph.D.)--Boston University