Numerical Studies For The Evolution Of Large-scale Coronal Structures And Fast Magnetic Reconnection

MHD models, which can help fill the gaps left by the spatially and temporally limited observations, have been used successfully to simulate many important space plasma processes and provide a powerful means for significantly advancing the understanding of such processes. In this thesis, a novel and robust numerical scheme—The space-time conservation element and solution element (CESE) method is firstly introduced and fully applied to MHD modeling. In the view of MHD simulations, we numerically studied two important processes that have very different space and time scales: the evolution of the large-scale coronal structures throughout a solar cycle and the spontaneous fast magnetic reconnection.The CESE method, which has far been applied in the fields of computational fluid dynamics, is firstly introduced to modeling resistive MHD equations, and a new 2.5-dimensional magnetic reconnection MHD model has thus been developed. The validations of this new model on some classical magnetic reconnection problems show that, the CESE numerical scheme not only has good numerical resolution but also can keep the divergence-free condition for magnetic fields during the complex evolutionary reconnection process without any special treatment. With this new model, spontaneous fast reconnection in a neutral current sheet, which provides an effective mechanism for rapid magnetic energy release, is carefully studied. The numerical results indicate that, due to the existence of a positive feedback between the reconnection inflow and the current-driven anomalous resistivity and the localization of the anomalous resistivity at the X point, fast reconnection can develop self-consistently and a Petschek-like configuration can be built up. But the reconnection process can not be sustained as a quasi-steady state. In fact, during the reconnection evolution, the diffusion region undergoes an elongation process, so that after the dynamic process is non-linearly saturated secondary tearing is subject to occur. This changes the overall reconnection topology and leads to time-dependent and bursty re- connection. The reconnection evolution is further studied in various physical situations, which also confirms the bursty nature of the spontaneous fast reconnection mechanism. It is also shown that, the larger the threshold value for the anomalous resistivity to appear, the larger the anomalous resistivity magnitude, and the smaller the plasma f3, the spontaneous fast reconnection process is likely to evolve more explosively.To avoid any potential singularities at the pole in the Spherical coordinates and any potentially simplified symmetry, especially at solar maximum, a new global solar-interplanetary MHD model (SIP-CESE model) has been developed in Cartesian coordinates, where the original CESE method is further generalized to modeling the large-scale solar corona as well as the solar wind. To advance the complex MHD equations in the sphere-shaped computational region, an unique unstructured grid system is constructed, which significantly simplifies the computational boundary condition treatment at solar surface. Due to the self-similarity in radial direction of the grid system, SIP-CESE model has also been parallelized by using the Message Passing Interface (MPI). With the SIP-CESE model, the magnetic structures of the global corona are firstly obtained for fifteen Carring-ton Rotations (CRs) spanning solar cycle 23, by using the observed line-of-sight photospheric magnetic field from the Wilcox Solar Observatory (WSO) as boundary conditions. The results illustrate that, during solar cycle 23, the heliospheric current sheet (HCS) can be approximatively described as a tilted plane relative to the solar equator in most times, with a tilt angle varying from < 20°near minimum to～90°around maximum. But the HCS displays its greatest complexity and even shows a duplex structure at solar maximum, while during the rising and declining phases, it can also present large spatial variability and north-south asymmetries at sometimes. The MHD results also show that the ratio of closed to open magnetic flux in the photosphere varies from～2—3 at solar minimum to～6 at solar maximum. Comparison between our numerical results for the coronal magnetic structures and those from the standard potential field source surface (PFSS) model, with, in addition, white-light observations further validates this new MHD model. The source surface neutral lines calculated from the MHD and PFSS models generally match each other closely; however, differences occur at different phases in the solar cycle. The location of the HCS shows good overall agreement with the bright structures in the observed white-light intensity pattern, especially around solar minimum or well after solar maximum, and this result confirms that the observed white-light streamer structures originate from a single, large-scale plasma sheet located near the HCS.