Browsing by Author "Zhu, Chunming"

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    Dynamics and Evolution of Solar Eruptive Prominences
    (2014-04-24) Zhu, Chunming; Alexander, David; Toffoletto, Frank R.; Niu, Fenglin
    Solar eruptive phenomena, including eruptive solar prominences/filaments, solar flares and Coronal Mass Ejections (CMEs), have severe impact on the Earth’s space environment and human activities: so-called Space Weather. The dynamics and evolution of the prominences/filaments are important for our understanding of the initiation processes that drive CMEs and lead to drastic energy release in the solar flares. This thesis focuses on recent progresses on the destabilization and subsequent eruption of the prominences/filaments, via three primary case studies that elucidate the most important activities occurring in the eruptive prominence: eruption of a bifurcated solar filament, interchange reconnection facilitating a filament eruption, and the interaction of two distinct filaments with subsequent production of solar flares. In Chapter 2, we study a partial eruption of a bifurcated filament which exhibited clear and strong kinking motion of the filament axis (∼ 120◦ rotation). Seven mass transfer events are identified and are thought to also transfer magnetic flux from the lower to upper branch, leading to the generation of ideal instabilities, that subsequently triggered the eruption of the upper branch. In Chapter 3, we present evidence of interchange reconnection driven by the interaction of an erupting filament with a nearby coronal hole that leads to the eruption of this filament. Kinking motions in this filament serves to bring the magnetic field of its eastern leg in close contact with the unipolar magnetic field of the coronal hole where it drives the reconnection that governs the subsequent evolution of the fila- ment and coronal hole boundary. The observed EUV brightenings and bi-directional flows in the contact layer formed by this interaction, along with the occurrence of type III radio bursts that are strongly related to escaping electrons along open fields, provide corroborative evidence for the occurrence of reconnection at this location. A consequence of this interaction was the development of a complex CME, that displayed both open and closed features: we believe this is the first time such a CME configuration has been observed directly in association with a filament eruption. In Chapter 4, an interaction between two filaments, rarely reported before, is identified and studied. This complex interaction is responsible for the production of a hard x-ray coronal source as part of a C3.0 class solar flare. The observed hard x-ray coronal source occurring between the two filaments, driven by a convergence of the filaments, and a newly formed hot plasma layer, indicate that magnetic reconnection occurred between the magnetic fields associated with both filaments. The eruption of the filaments later led to the onset of a much larger solar flare, class M2.9, as expected from the standard flare model. It is interesting to note that both loop shrinkage and supra-arcade downflows (SADs) are present during this M2.9 flare.
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    Magnetic helicity injection and velocity characteristics of rotating sunspots
    (2011) Zhu, Chunming; Alexander, David
    This thesis presents calculations of the magnetic helicity injection due to rotating sunspots and a determination of the characteristics of the rotating sunspots in the active regions with simple magnetic configurations. Four active regions are investigated to study the relationship between rotating sunspots and magnetic helicity. The observations indicate that significantly more helicity is injected during the period of rotation in polarities with strong magnetic field. This may be a result of the emergence of a magnetic flux rope from below the solar surface. Moreover, our preliminary study on a large sample of 90 active regions shows that the level of flaring activity increases with the rate of helicity injection. Finally, a statistical study is carried out to determine the relation between rotating sunspots and the emergence of magnetic flux tubes. Among 82 active regions which exhibit flux emergence, 93% are associated with rotating sunspots. Among 50 active regions without well-defined flux emergence, 60% of sunspots are observed to be rotating, though relatively slowly. In addition, we find that sense of the rotation (i.e., clockwise or counter-clockwise) of the sunspots shows a weak hemispherical tendency.