Numerical Magnetohydrodynamic Simulation Of Solar Magnetic Flux Emergence

Solar active regions are the main areas of solar eruption activities,which are the main factors affecting the solar terrestrial space weather.Magnetic flux emergence refers to the process in which the magnetic field generated by the solar dynamo rises from the convective zone,through the photosphere layer and finally enters the corona.It is believed to be the main mechanism of the formation of active regions,and it also frequently participates in the triggering process of the solar eruptions.Thus,observation and simulation of magnetic flux emergence is one of the frontier research directions of solar active regions physics.Unfortunately,we are still not able to detect the magnetic field in the solar interior,so the main research approach to study magnetic flux emergence is numerical simulations.In this paper,we first study the physical properties of the atmospheric environment of flux emergence background and the physical mechanism of the emergence process,so as to lay the foundation for establishing the numerical model of magnetic flux emergence.The process of flux emergence involves a huge spacetime scale,especially in the thin layer from the solar photosphere to the transition region,which is about one thousandth of the sun’s diameter,the changes of density and pressure span nearly 10 orders of magnitude,and the core parameters such as plasma β and Alfven velocity span multiple orders of magnitude,and the nature of the interaction between plasma and magnetic field changes extremely.These are great challenge to the accuracy of numerical scheme,mesh resolution and computational complexity.In this dissertation,we utilized a space-time conservation-element and solution-element method for magnetohydrodynamics with techniques of adaptive mesh refinement and parallel computing to simulate the process of magnetic emergence on the basis of the existing theory.We first realized a 2.5-dimensional simulation of magnetic flux emergence.The simulation results basically reproduce the classical "two-step emergence" process of magnetic flux emergence,that is,magnetic flux tube rises continuously due to the magnetic buoyancy in the first stage.However,when it reaches the photosphere,the rising of the magnetic flux tube stops and the magnetic flux keeps accumulating.Until Parker instability occurs,a part of the magnetic field can rise to the corona,expand rapidly,and then relax to a state of near force-free.The results also show that the changes of the physical quantities such as magnetic field and density in the process of the magnetic flux emergence,which is consistent with results in the literature.The physical picture of plasma force accounting for the diffusion of magnetic field in the process of magnetic emergence is given,which shows that the diffusion of magnetic field is related to the evolution of the pressure gradient of atmospheric background and Lorentz force.Furthermore,we carried out a preliminary full three-dimensional simulation to show the change of the magnetic potential pattern and two shock waves caused by the three-dimensional magnetic flux tube in the process of buoyancy in the convective zone.The results of this paper are of great significance for us to understand the physical properties of the solar atmosphere and the triggering mechanism of solar eruption activities.It provides experience for the simulation of three-dimensional magnetic emergence process in the future,and also lays a foundation for the followup simulation research,including more complex simulation and application in the study of solar eruption activities.