Studies For Influences Of Shear Flows On Some Processes In Magnetic Reconnection

Magnetic reconnection is a fundamental phenomenon in plasma physics. It provides a theoretical basis for many phenomena in space astrophysical and magnetic confinement fusion plasmas. Through reconnection, the magnetic field topology can be changed and high-speed plasma jets can be ejected. The theories of magnetic reconnection have been developed for many years. However, debates on many specific issues still outstand due to the complexity of the problem and the insufficient data. For example, most researches on magnetic reconnection are based on the static state equilibrium due to the difficulty in energy principle description for steady state equilibria with shear flows. Nevertheless, the effect of the shear flows on magnetic reconnection and related processes is important and sometimes crucial, such as for suppressing island growth and stabilizing the tearing mode in fusion devices.Numerical simulation is a powerful method to describe complicated, particularly nonlinear, physical processes and a bridge between theory and observation. Therefore, in this thesis, we apply MHD models numerically for magnetic reconnection studies. Thus, both toroidal and poloidal shear flows, as well as Hall effects and guide fields can be taken into consideration and certain important results, particularly the observability of the Hall quadrupolar field in different cases, are obtained.The thesis then is organized as follows.In Sec.1, a brief review of magnetic reconnection studies is introduced, including the origin, the basis and current achievements and progresses. We then discuss the applications and prospects, as well as challenges and outstanding issues that need to be resolved in both space physics exploration and magnetic confinement fusion.In Sec.2, the applied numerical method is presented, especially the MHD simulation code. Then the diagnostic methods of the results are also introduced.Sheared flows perpendicular to the anti-parallel reconnecting magnetic field are often observed in magnetosphere and interplanetary plasmas. In Sec. 3, we apply the reduced resistive MHD model for compressible plasmas to investigate the nonlinear evolution of the reconnection current sheet with such flows. Our study finds that the shear flows can generate bipolar or quadrupolar out-of-plane magnetic field perturbations in a two-dimensional resistive MHD reconnection without Hall effects. The quadrupolar structure has otherwise been thought a typical Hall MHD reconnection feature caused by the in-plane electron convention. The results will challenge the conventional understanding and satellite observations of the signature of reconnection evidence in space plasmas.To further understand the effect investigated in Sec.3 on reconnection processes, we develop a Hall-MHD Model with the shear flows. In Sec.4 we have studied the effects of toroidal shear flows on magnetic reconnection with Hall effects exist and found that the quadrupolar out-of-plane magnetic field components generated both by the Hall effects and the shear flows can be added up and new reconnection patterns are formed.In Sec.5, several problems relevant to our discussions in Sections 3 and 4 are discussed:(1) First, the effect of spatial variation of the guide field is investigated. We defineαas the intensity and x as the typical length for the spatial variation of the guide field and find that the spatial variation of the guide field slows the reconnection process as x increases.(2) We use the compressible Hall-MHD model to discuss the nonlinear evolution of the tearing modes in force-free field approximation, in compare with Bian & Vekstein's recent work in questioning if the quadrupolar structure during such reconnection processes is solely due to the effect.(3) We also study the effect of the guide field of different profiles on magnetic reconnection generated quadrupole field components. And then the observability of the quadrupole field by satellite observations in the cases of (2) and (3) is discussed.In Sec. 6, Alfven resonance generated by the poloidal sheared flows are studied for coronal heating application. Coronal heating has been an outstanding problem difficulty to solve for years. There are two categories of heating models: current sheet heating and wave heating. To both kinds of models however, small scale effect of the magnetic field is important. If the sheared flows are big enough, Alfvén resonance can be excited and a thin current sheet can be formed. The both the Ohmic dissipation and wave resonance on the current can provide the heating sources for the corona.Finally, a brief summary and conclusion are given to complete the thesis.