On The Influence Of Photospheric Flux Distribution And Background Solar Wind On The Coronal Flux Rope Catastrophe

Coronal mass ejections (CMEs) belong to large-scale solar active phenomena and cause strong disturbances on the geospace environment, being a main source of disastrous space weather. The observational and theoretical study of CMEs serves as an active frontier subject in the fields of solar physics and solar-terrestrial space physics. In recent years, the coronal flux rope catastrophe as one of the important mechanisms for triggering CMEs has attracted a general concern among the scientific community. On the basis of previous relevant researches, this thesis carries out numerical simulation studies of the same subject from a new aspect.After a brief review of observational features and current research status of CMEs, this thesis focuses on the latest progress in the study of theoretical models of CMEs, especially the coronal flux rope catastrophe model closely related to this thesis. On this basis, we will describe the results we have obtained in the study of coronal flux rope catastrophe models.This thesis uses a 2.5-dimensional ideal MHD model in spherical geometry and constructs an equilibrium system consisting of a background magnetic field and an isolated magnetic rope. Starting from it, we investigate the influence of the photospheric magnetic flux distribution of the background field and the background solar wind on the equilibrium properties and catastrophic behavior of the coronal flux rope system.In order to reveal the physical effects of the photospheric magnetic flux distribution on the coronal flux rope catastrophe, we first assume that the background field is a bipolar potential field, whose photospheric flux distribution is adjusted in a proper way. When the flux distribution is concentrated toward the poles, the decay of the corresponding potential field slows down with heliocentric distance, and thus the background field becomes stronger at large distance. The opposite is true if the magnetic flux distribution shifts to- wards the equator. Among various distributions there is a special one, which corresponds to a uniform radial component of the magnetic field at the photosphere. Such a distribution is the same as that of the so-called split monopole field. Next, the magnetic field within the magnetic flux rope is assumed to be force free and the rope is characterized by two magnetic flux parameters, the annular and axial fluxes, and two geometrical parameters, the height of the axis of the flux rope and the length of the vertical electric current sheet below the rope. For a given annular flux, we adjust the axial flux and calculate the geometrical parameters of the flux rope in equilibrium, and then analyze the equilibrium and catastrophic properties of the whole system. It is shown that taking the flux distribution of the split monopole field as a standard, if the photospheric flux is more concentrated to the equator, i.e., the background field becomes relatively weaker at large distance, then the geometrical parameters changes smoothly with increasing axial flux of the rope, no catastrophe takes place. Correspondingly, the magnetic energy of the system is lower than the open field energy. Under this situation, a certain critical axial flux exists for each given annular flux: the geometrical parameters change sharply with axial flux in the vicinity of the critical flux, and approach infinity once the critical flux is surpassed. As a result, the bipolar potential field becomes fully opened. On the contrary, if the photospheric flux is more concentrated to the poles than it is for the split monopole field, i.e., the background field becomes relatively stronger, then the geometrical parameters will present a variation with a jump somewhere as the axial flux of the rope increases. In other words, a catastrophe exists for the system. The magnetic energy at the catastrophic point, i.e., the catastrophic energy threshold, exceeds the open field energy. The stronger the background field is over the rope, the higher the catastrophic energy threshold and its percentage in excess of the open field energy will be, and thus the faster the flux rope erupts upward after catastrophe. These results demonstrate that the observational velocity spectrum of CMEs can be reproduced by the coronal flux rope catastrophe model merely by adjusting the photospheric flux distribution of the background field.Most previous coronal flux rope catastrophe models adopted force-free or magnetostatic equilibrium approximations without considering the the phys- ical effect of the solar wind. To incorporate the influence of the solar wind on the equilibrium and catastrophic properties of the coronal flux rope system, we assume that the background field has the same photospheric flux distribution as the dipole field, but a coronal streamer and a steady solar wind exist outside the flux rope. In addition to the annular and axial fluxes, the mass of the coronal plasma within the flux rope is taken as another parameter of the rope in order to reflect the physical effect of gravity. The simulations show that the coronal flux rope system also has a catastrophic behavior. The geometrical parameters present a variation with a jump somewhere, or a catastrophe occurs, when either the annular or the axial flux increases, or the mass within the rope decreases. The catastrophic energy threshold increases with increasing mass in the rope, and the increment equals the magnitude of the so-called excess gravitational energy associated with the mass in the rope, denned as the difference between the true gravitational energy and that for a plasma in hydrostatic equilibrium. When the contribution of the gravitational energy is deducted, the result is still larger than the open field energy by about 8%, a conclusion which is almost the same as that reached for cases without the background solar wind. This indicates that the presence of the background solar wind essentially does not affect the catastrophic energy threshold. Once a catastrophe occurs, the background solar wind exerts a significant influence on the movement of the flux rope after catastrophe. In comparison with the results obtained in terms of magnetostatic approximation, there is no artificial deceleration brought about by the static plasma ahead of the erupting flux rope, so the rope can acquire a much larger asymptotic speed, which is closer to reality. The acceleration and asymptotic speed depend on the strength of the background field: the stronger the background field is, the faster the acceleration and the higher the asymptotic speed will be. The obtained temporal profile of velocity of the flux rope is essentially consistent with the typical observed velocity profiles of CMEs. Therefore, we have found another way to reproduce the observational velocity spectrum of CMEs in terms of the coronal flux rope catastrophe model, i.e., by adjusting the strength of the background field.