Date of Award
Doctor of Philosophy in Applied Physics - (Ph.D.)
Federated Physics Department
Carsten J. Denker
Philip R. Goode
Dale E. Gary
Flares and Coronal Mass Ejections (CMEs) are energetic events, which can even impact the near-Earth environment and are the principal source of space weather. Most of them originate in solar active regions. The most violent events are produced in sunspots with a complex magnetic field topology. Studying their morphology and dynamics is helpful in understanding the energy accumulation and release mechanisms for flares and CMEs, which are intriguing problems in solar physics.
The study of complex active regions is based on high-resolution observations from space missions and new instruments at the Big Bear Solar Observatory (BBSO). Adaptive optics (AG) in combination with image restoration techniques (speckle masking imaging) can achieve improved image quality and a spatial resolution (about 100 km on the solar surface) close to the diffraction limit of BBSO's 65 cm vacuum telescope. Dopplergrams obtained with a two-dimensional imaging spectrometer combined with horizontal flow maps derived with Local Correlation Tracking (LCT) provide precise measurements of the three-dimensional velocity field in sunspots. Magnetic field measurements from ground- and space-based instruments complement these data.
At the outset of this study, the evolution and morphology of a typical round sunspot are described in some detail. The sunspot was followed from disk center to the limb, thus providing some insight into the geometry of the magnetic flux system. Having established a benchmark for a stable sunspot, the attention is turned to changes of the sunspot structure associated with flares and CMEs. Rapid penumbral decay and the strengthening of sunspot umbrae are manifestations of photospheric magnetic field changes after a flare. These sudden intensity changes are interpreted as a result of magnetic reconnection during the flare, which causes the magnetic field lines to be turned from more inclined to more vertical. Strong photospheric shear flows along the flaring magnetic inversion line exist several hours before the flare. The footpoints of magnetic field lines are sheared by this motion and magnetic free energy is thus stored in the flux system. This energy is then suddenly released during the flare.
Based on multi-wavelength studies of these shear flows, a new challenging phenomenon is found. The local shear flow and magnetic shear can increase right after the flare. This apparently contradicts the principle of conservation of energy, which requires an overall decrease in the magnetic free energy that powers the flare. The explanation considers the emergence of a twisted or pre-sheared flux rope near the neutral line. Since shear flows are also detected in flare-quiet sunspots, the conclusion is made that they are associated with flare occurrence but they are not a sufficient condition for flaring.
Deng, Na, "High resolution studies of complex solar active regions" (2007). Dissertations. 799.