Other
Scientific paper
Dec 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007phdt........44s&link_type=abstract
PhD Thesis, 2007, 198 pages
Other
1
Coronal Mass Ejections, Filaments, Solar Flares, Magnetic Fields, X-Rays, Uv/Euv Radiation
Scientific paper
In this thesis, we study the evolution of the highly sheared magnetic fields before, during, and after solar flares, in order to address the fundamental questions in solar flare research: how does the magnetic free energy stored and released? The work primarily focuses on the analysis of multi-wavelength data, while non-linear force free field (NLFFF) modeling of one active region is also explored.
A detailed analysis of the strong-to-weak shear motion of the flare footpoints (TRACE) in an X17 flare (2003 Oct 28) shows that the cessation of this shear change occurs in the middle of the impulsive phase. The observations are interpreted in terms of the splitting of the sheared envelope field of the greatly sheared core rope during the early phase of the flare, based on a 3D version of the standard flare model. 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, this type of flares are called type I flare. We also found that: for a subset of 24 type I flares, the initial and final shear angles of the footpoints are mainly in the range from 50° to 80° and 15° to 55°, respectively; in 10 of the 14 flares having both measured shear angle and corresponding hard X-ray observations, the cessation of shear change is 0--2 minutes earlier than the end of the impulsive phase, which may suggest that the change from impulsive to gradual phase is related to magnetic shear change. For a sample of 18 Type I flares associated with CMEs, we found that the magnetic flux and the change of shear angle of the foopoints significantly correlated with the intensity of flare/CME events, while the initial shear angle of the foopoints does not. This observation indicates 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 found that a linear combination of several parameters shows a much better correlation with the intensity of flare/CME events than each parameter itself.
The aforementioned work are mainly based on TRACE observations, in which we infer the sheared field information from the flare footpoints. However, the X-ray telescope (XRT) on board the newly launched Hinode spacecraft provides direct observations of highly sheared coronal loops at various stages. XRT observations of two X-class flares occurred last December show that part of the sheared core field erupted, and another part of the sheared core field stayed behind during the flares, which may explain why a large part of the filament is still seen by 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. At last, we explored the NLFFF modeling of a simple bipolar active region (NOAA 10953), which produced several small flares (mostly B class and one C8.5 class) and filament activations from April 30 to May 3 in 2007. These events appear to be associated with the frequent flux cancellations (SOHO/MDI) that occurred in the region close to the polarity inversion line. We constructed a series of NLFFF models for this active region at three times, using the flux-rope insertion method. The models are constructed based on MDI magetograms, and constrained by Hα filaments and highly sheared loops observed by XRT. We find good NLFFF models that fit the observations before the C8.5 flare, but not for the case after the flare. The flux rope contains highly 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 upper limit for eruption.
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