Date of Award

Summer 2010

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Applied Physics - (Ph.D.)

Department

Federated Physics Department

First Advisor

Haimin Wang

Second Advisor

Wenda Cao

Third Advisor

Dale E. Gary

Fourth Advisor

Jeongwoo Lee

Fifth Advisor

Martin Schaden

Abstract

It is generally believed that eruptive phenomena in the solar atmosphere such as solar flares and coronal mass ejections (CMEs) occur in solar active regions with complex magnetic structures. The magnetic complexity is quantified in terms of twists, kinks, and interlinkages of magnetic field lines. Magnetic helicity has been recognized as a useful measure for these properties of a given magnetic field system. Magnetic helicity studies have been naturally directed to the energy buildup and instability leading to solar eruptions. However, in spite of many years of study, detailed aspects of initiation mechanisms of eruptive events are still not well understood. The objective of this dissertation is to understand a long-term (a few days) variation of magnetic helicity in active regions and its relationship with flares and CMEs.

The research presented in this dissertation benefited significantly from the comprehensive data now available, including SOHO/MDI full-disk longitudinal magnetograms, Hinode/SOT/SP vector magnetograms, and GOES soft X-ray data. In addition, several advanced data analysis tools were utilized such as local correlation tracking, differential affine velocity estimator, Stokes inversion, 180° ambiguity resolution, and nonlinear force-free magnetic field extrapolation. Statistical studies of flare productivity and magnetic helicity injection in ~400 active regions were carried out. The time profile of the coronal magnetic helicity in the active region NOAA 10930 was also investigated to find its characteristic variation related to the X3.4 flare on 2006 December 13. In addition, the temporal varia tion of magnetic helicity injected through the photosphere of active regions was examined related to 46 CMEs and two active-region coronal arcades building up to CMEs.

The main findings in this dissertation are as follows: (1) the study of magnetic helicity for active regions producing major flares and CMEs indicates that there is always a significant helicity injection of 1042–1043 Mx2 through the active-region photosphere over a long period of ~0.5–a few days before the flares and CMEs; (2) the study of the 2006 December 13 X3.4 flare shows that the flare is preceded by not only a large increase of negative helicity in the corona over ~1.5 days but also a noticeable injection of oppositelysigned helicity though the photospheric surface around the flaring magnetic polarity inversion line; (3) the gradual inflation stage of the two arcades is temporally associated with helicity injection from the active-region photosphere; and (4) for the 30 CMEs under investigation, it is found that there is a fairly good correlation (linear correlation coefficient of 0.71) between the average helicity injection in the CME-productive active regions and the CME speed.

Beside the scientific contribution, a major broader impact of this dissertation is the observational discovery of a characteristic variation of the pattern of magnetic helicity injection in flare/CME-productive active regions, which can be used for the improvement of solar eruption forecasting. An early warning sign of flare-CME occurrence could be implemented based on tracking of a period of monotonically increasing helicity because there was always a significant amount of helicity accumulation in active regions a few days before the major flares and CMEs under investigation.

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