Solar eruptive phenomena, such as flares and coronal mass ejections (CMEs), derive their energy from complex magnetic fields and are the principal source of disturbances that affect space weather. Although physical mechanism and dynamic morphology of flares have been a subject of intense research, many aspects of the flaring process still remain unclear. The objective of this dissertation is to advance the understanding of the physics behind solar flares based on observations, simulations and nonlinear force-free (NLFF) field modeling of magnetic fields.
The data used in this study are obtained from several ground-based or space-borne instruments, including BBSO/DVMG, Hinode/SOT/SP, SDO/HMI and GOES. In addition, the advanced coronal modeling method was utilized to extrapolate NLFF magnetic fields. This dissertation focuses on the magnetic properties (magnetic inclination angle, transverse magnetic field, Lorentz force, etc.) and their evolution associated with flares. The observation is also compared with numerical MHD simulations and theoretical models.
The main findings in this dissertation are briefly summarized as follows: (1) the white-light observation of S sunspots reveal the rapid penumbral decay and central umbral/penumbral darkening associated with flares; (2) the magnetic inclination angle shows an increase in the decayed peripheral penumbra and a decrease in the central area close to the flaring polarity inversion line (PIL) after major flares; (3) magnetic field observations indicate a rapid and permanent enhancement of the transverse magnetic field in the umbral core or inner penumbral region, while that in the outer decayed penumbral region decreases; (4) the downward Lorentz force exerted on the flaring area displays a sudden enhancement after flares; (5) the footpoint of flare ribbons shows slow shearing followed by fast diverging and deshearing motion. The remote Ha brightenings appear in the first stage of the eruption while the associated hard X-ray emission occurs in the later phase. These results provide strong evidence favoring superimposed effects of both the tether-cutting model, in which a short and flat loop forms near the photosphere after the eruption, and the back reaction of the coronal magnetic field following the energy release.
The major contribution of this dissertation is: (1) First study of 3-D magnetic field change associated with flares in the perspective of magnetic inclination angle and transverse magnetic field; (2) First successful comparison of the numerical simulation, observation and theory of solar flare associated magnetic field changes; (3) First clear evidence of the two stage magnetic reconnection (implosion and explosion); (4) First evidence in support of the conservation of momentum in solar flare.
To understand the radio emission produced by electrons at the very acceleration site of a solar flare, two competing acceleration models– stochastic acceleration by cascading MHD turbulence and regular acceleration in collapsing magnetic traps were studied. It is found that the radio emission from the acceleration sites (1) has sufficiently strong intensity to be observed by currently available radio instruments and (2) has spectra and light curves which are distinctly different in these two competing models, which makes them observationally distinguishable.