Causes of Solar Eruptions: A Comparative Study of Super-Active and Low-Active Active Regions
Date
2021
Authors
Dhakal, Suman
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Abstract
Solar Active Regions (ARs) are the areas of strong magnetic flux of opposite polarities. These are the main source-location of solar eruptions. By solar eruptions, we mean the explosive events that rapidly release energy from the magnetized solar corona resulting in flares and Coronal Mass Ejections (CMEs). During solar eruptions, a tremendous amount of plasma and energy is released from the Sun, which can produce space weather disturbances and disrupt our space missions, satellites, radio communications, and even power grids on the Earth. The understanding of the origin of solar eruptions and their propagation through interplanetary space is crucial to mitigate the damages they could produce. However, a proper understanding of the physical mechanisms leading to eruptions is still lacking, and consequently a reliable and accurate forecast is not possible yet. This dissertation, using advanced observations from AIA and HMI instruments onboard the SDO spacecraft, addresses many important issues regarding the origin of solar eruptions.First, this dissertation investigates how the evolution of ARs leads to different flare productivity. This study illustrates that though the magnetic flux emergence is important, it alone is not sufficient to increase the flare productivity of an AR. The new emergence can lead to either the interaction of like or opposite magnetic fluxes of non-conjugate pairs (magnetic poles not emerging together as a conjugate pair, as in a bipolar configuration). In the former case, the overall magnetic configuration remains simple and the flare productivity of AR does not change with emergence. In the latter case, the convergence of opposite magnetic fluxes of non-conjugate pairs results in a complex magnetic configuration with long polarity inversion line (PIL). This study suggests that the long-term shearing motion and flux cancellation, along the PIL of non-conjugate pair, produce multiple intense flares. In addition, the dissertation also analyzes the magnetic field parameters including total magnetic flux, net flux, current density, current helicity, degree of current neutralization, length of strong-gradient PIL (sgPIL), and R-value to quantify the flare drivers. Our study found the weakest correlation (0.6) between flare index (FI) and total flux content of ARs. This demonstrates that the size of the AR does not necessarily determine the flare productivity. The correlation between the FI and sgPIL/ R-value was the strongest (0.8). Such a high correlation suggests that the ARs having long PIL for a longer evolutionary period have a higher probability of producing many intense flares. Second, the dissertation provides, for the first time, the evidence that shearing motion and flux cancellation play a major role in repetitive similar solar eruptions (homologous eruptions) at different evolutionary phases of an AR. Our study shows that after an eruption, the continuation of shearing motion and flux cancellation not only store magnetic helicity and energy but also form an identically-shaped erupting structure along the same PIL. The present study also demonstrates that the homologous eruptions can have similar signatures in the pre-flare phase due to similar magnetic topology. ARs can have a similar magnetic topology for a long period due to slow changes in the magnetic configuration or the formation of similar magnetic structures. Our study supports the idea of the formation and existence of pre-eruptive magnetic flux ropes. Third, the dissertation describes an atypical solar eruption, where two closely connected magnetic structures erupted consecutively within twelve minutes. This study provides a unique opportunity to reveal the formation process, initiation, and evolution of complex eruptive structures in solar ARs. This study shows that long-term continuation of shearing motion and magnetic flux cancellation form a new low-lying magnetic structure below the existing high-lying structure. These magnetic structures are arranged along the same PIL in a double-decker configuration. The high-lying magnetic structure becomes unstable and erupts first, appearing as an expanding hot channel seen at extreme ultraviolet wavelengths. About 12 minutes later, the low-lying structure also starts to erupt and moves faster compared to the high-lying one. As a result, the two erupting structures interact and merge with each other, appearing as a single CME in the outer corona. The dissertation shows that the successive destabilization of two separate but closely spaced magnetic structures, possibly in the form of magnetic flux ropes, leads to a compound solar eruption. Further, the dissertation describes the different scenarios by which the two branches of a double-decker configuration can erupt. In short, this dissertation work has made significant progress toward our understanding of the origin of solar eruptions. The work on the flare productivity of various types of ARs will improve the prediction of the occurrence of solar flares when combined with machine learning techniques. The upcoming 4-meter Daniel K. Inoue Solar Telescope (DKIST) observations will provide an ultimate verification on the importance of flux cancellation in producing solar eruptions as suggested in this study.
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Keywords
Astrophysics, Active region, CMEs, Flares, Solar eruption