Studies Of Collisionless Magnetic Reconnection By Using PIC Simulation

Magnetic reconnection plays a crucial role in a lot of rapid macroscale plasma process, it is necessary to investigate magnetic reconnection in-depth.Numerical simulation is an important method to describe physical processes based on computer science, and the particle in cell (PIC) simulation is one of it. The PIC method is based on Newton's law and Maxwell's equations, calculates the fields in the simulation domain statistically, then the fields as forces act on the particles, move and accelerate them. The system will evolve after many times of circulation. In this thesis, two-dimensional collisionless magnetic reconnection is studied by using PIC simulations, and some important results are obtained.In periodic simulations of magnetic reconnection we find that, when the initial current sheet is symmetric: (1) electron flow is symmetrical about the current sheet out of the diffusion region along the separatrix; (2) the perpendicular electric field on the separatrix is weak and its scale is much larger than the local electron debye length; (3) the structure of the guiding magnetic field B_y is quadrapole. When thereis a temperature gradient perpendicular to the current sheet, i.e., the initial current sheet is asymmetric: (1) in the low and hot (magnetosphere) side electron flows out of the region rapidly and formed a strong electron jet, and in the high and cold density (magneto-sheath) side no electron jet is observed; (2) a large amplitude perpendicular electrostatic field structure appears nearly along the separatrix in the magnetosphere side, and its space scale is very small. For the case with the initial temperature ratio T_αs/T_αm=0.2, the half width of this structure is about three times the local electron debye length. This structure is not seen in the magneto-sheath side; (3) the guiding magnetic B_y field appears dipolar structure and mainly in themagneto-sheath side. These perpendicular electric field and guiding field structures were observed in January, 2004, when the polar satellite moving through the magnetopause.Based on the PIC code with periodic boundary condition, we developed a code with open boundary. We studied the magnetic reconnection by using the open boundary model and find that: (1) if the imposed boundary condition of the magnetic field at the in-flow boundary is the zero-gradient (ZG) boundary condition, plasmas are blocked from entering the system and the reconnection rate will decrease to a very low level. Under both EM and MS boundary conditions, the system can reach a quasi-steady state, and the steady reconnection rate under the EM boundary is aboutE_r≈0.08 which is lower than under the MS boundary where E_r≈0.13.(2) whenthe initial current sheet thickness is chosen to ion skin depth, the peak value of reconnection rate is independent of the system size, for the quasisteady state, the length of the ion diffusion region is linear with the system length, and the reconnection rate is linear with the inverse of the system length; (3) when the initialsystem size is L_x×L_x=50d_i×25d_i,the relationship between the maximum reconnection rate and the initial current sheet thickness is about log₁₀E_r-log₁₀(d_i/a)^1/2. For the systems where a≥2d_i, the reconnection tends to be intermittent.