Other
Scientific paper
Apr 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010acasn..51..217s&link_type=abstract
Acta Astronomica Sinica, vol. 51, no.2, p. 217-218
Other
Sun: Coronal Mass Ejections, Sun: Filaments, Sun: Flares, Magnetic Fields, Sun: X-Rays, Sun: Uv Radiation
Scientific paper
Solar flares, filament eruptions and coronal mass ejections (CMEs) are energetic events that have great influences on the space weather that we live in. Storage of the energy released in these eruptions requires non-potential magnetic fields, i.e., sheared or twisted fields. In this thesis, we carry out a detailed and systematic study on the sheared magnetic field configuration as well as its important role in solar eruptions. This is an important part of our effort to understand and predict our space weather. This work primarily focuses on the analysis of multi-wavelength observations, while the nonlinear force-free field (NLFFF) modeling of one active region is also explored. The main results of this study are: (1) For the X17 flare on Oct. 28th 2003, we find that the cessation of the strong-to-weak shear change of the footpoints occurs in the middle of the impulsive phase. The phenomenon is interpreted in terms of the splitting of the sheared envelope field of the greatly sheared core rope during the early phase of the flare. (2) This shear motion of the footpoints has been found to be common, i.e., 43 out of 50 of the well-observed (by TRACE) two-ribbon flares we studied show this shear motion, we call these flares type I flares. We also find that in 10 Type I flares having both measured shear angles and corresponding hard X-ray observations, the cessation of shear change is 0~2 min earlier than the end of the impulsive phase, which may suggest that the change from the impulsive to gradual phase is related to magnetic shear change. (3) For a sample of 18 Type I flares associated with CMEs, we find that the magnetic flux and the change of shear angle of the footpoints are significantly correlated with the intensity of flare/CME events, while the initial shear angle of the footpoints is not. This observations indicate that the intensity of flare/CME events may depend on the released magnetic free energy rather than the total free energy stored prior to the flare. We also find that a linear combination of the aforementioned parameters shows a much stronger correlation with the intensity of flare/CME events than each parameter itself. (4) Hinode/XRT observations of two X-class flares which occurred in December 2006 show that one part of the sheared core field erupted, while the other stayed behind during the flares. This may explain why a large part of the filament is still seen in TRACE after the flare. We also find that the post-flare core field is clearly less sheared than the pre-flare core field, which is consistent with the idea that the energy released during the flares is stored in the highly sheared fields prior to the flare. (5) Using the flux rope insertion method, we explore the NLFFF modeling of active region 10953, which produced several small flares and filament activations. We find good NLFFF models that fit the observations before a C8.5 flare, but not for the case after the flare. The flux rope contains strongly sheared but weakly twisted magnetic fields. Before the C8.5 flare, this active region is close to an eruptive state: the axial flux in the flux rope is close to the threshold value for eruption.
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