Computer Simulations Of Collisionless Magnetic Reconnection

Magnetic reconnection is an important kinetic process and energy conversion mechanism in space plasmas. Magnetic energy which stored in current sheet is released and transferred into plasma kinetic and thermal energy through magnetic reconnection. During the process of magnetic reconnection, the topological structures of the magnetic field lines are changed. Collisionless magnetic reconnection is a widely accepted reconnection theory which is developed in the past decade. It was first proposed from computer simulations, and since then, numerous experiments and observations have proved its validity. In this dissertation, for further investigation of collisionless magnetic reconnection, two-dimensional (2-D) particle-in-cell (PIC) simulations and three-dimensional (3-D) global hybrid simulations are performed to study several important problems of collisionless magnetic reconnection, as well as its dynamic effects in experimental and magnetospheric plasmas. The results are shown as follows:1. Electron dynamics in collisionless magnetic reconnectionThe onset of collisionless magnetic reconnection is considered to be controlled by electron dynamics in the electron diffusion region, where the reconnection electric field is balanced mainly by the off-diagonal electron pressure tensor term.2-D PIC simulations are employed to investigate the self-reinforcing process of the reconnection electric field in the electron diffusion region, which is found to grow exponentially. A theoretical model is proposed to demonstrate such a process in the electron diffusion region. In addition the reconnection electric field in the pileup region, which is balanced mainly by the electromotive force term, is also found to grow exponentially and its growth rate is twice that in the electron diffusion region.2-D PIC simulations are also performed to investigate the structures of the out-of-plane magnetic field in magnetic island, which is produced during anti-parallel collisionless magnetic reconnection. Regular structures with alternate positive and negative values of the out-of-plane magnetic field along the x direction are formed in magnetic island. The generation mechanism of such structures is also proposed, which is due to the Weibel instability excited by the temperature anisotropy in magnetic island. We also investigate the transfer between electron bulk kinetic and electron thermal energy in collisionless magnetic reconnection by performing2-D PIC simulations. In the vicinity of the X line, the electron bulk kinetic energy density is much larger than the electron thermal energy density. The evolution of the electron bulk kinetic energy is mainly determined by the work done by the electric field force and electron pressure gradient force. The work done by the electron gradient pressure force in the vicinity of the X line is changed to the electron enthalpy flux. In the magnetic island, the electron enthalpy flux is transferred to the electron thermal energy due to the compressibility of the plasma in the magnetic island. The compression of the plasma in the magnetic island is the consequence of the electromagnetic force acting on the plasma as the magnetic field lines release their tension after being reconnected. Therefore, we can observe that in the magnetic island the electron thermal energy density is much larger than the electron bulk kinetic energy.2. PIC simulations of magnetic reconnection in laser-plasma experiments on Shenguang-Ⅱ facilityRecently, magnetic reconnection has been realized in high-energy-density laser-produced plasmas. Plasma bubbles with self-generated magnetic fields are created by focusing laser beams to small-scale spots on a foil. The bubbles expand into each other, which may then drive magnetic reconnection. The reconnection experiment in laser-produced plasmas has also been conducted at Shenguang-II (SG-II) laser facility, and the existence of a plasmoid was identified in the experiment. By performing2-D PIC simulations, we investigate such a process of magnetic reconnection based on the experiment on SG-II facility, and a possible explanation for the formation of the plasmoid is proposed. The results show that before magnetic reconnection occurs, the bubbles squeeze each other strongly and a very thin current sheet is formed. The current sheet is unstable to the tearing mode instability, and we can then observe the formation of plasmoid(s) in such a multiple X-lines reconnection.3.3-D global hybrid simulation of the magnetotail reconnection and relevant dynamic processesDynamics of the near-Earth magnetotail reconnection and relevant dynamic processes driven by the solar wind under a southward IMF is studied using a3-D global hybrid simulation model that includes both the dayside and nightside magnetosphere for the first time. It is found that the dayside reconnection leads to the penetration of the solar wind plasma, energy and magnetic flux through the magnetopause and thus a thinning of the plasma sheet, followed by the near-tail reconnection with3-D, multiple flux ropes. Hall electric fields in the thin current layer cause a systematic dawnward ion drift motion and thus a dawn-dusk asymmetry of the plasma sheet with a higher (lower) density on the dawn (dusk) side. Correspondingly, more reconnection and relevant dynamic processes (e.g. bursty bulk flows) occur on the duskside. The injected ions undergo the magnetic gradient and curvature first in the dipole-like field, forming a ring current. Ion particle distributions and energy spectrum reveal multiple populations/beams at various distances in the tail. Dipolarization of the tail magnetic field takes place due to the pileup of the injected magnetic fluxes and thermal pressure of injected ions, where the fast earthward flow is stopped. A vortex structure is developed at the fast flow breaking. Kinetic compressional wave turbulence is present around the dipolarization front. A sharp dip of magnetic field is seen just in front of the sharp rise of the magnetic field at the dipolarization front, mainly on the dusk side. Shear-flow type instability is found on the dusk side flank of the ring current plasma, whereas a ballooning instability appears on the dawn side. The cross-tail currents break into small-scale structures with k⊥ρi～1. Shear Alfven waves are generated from the tail reconnection, and they evolve into kinetic Alfven waves (KAWs) in the dipole field region. Correspondingly, multiple field-aligned current filaments are generated above the auroral ionosphere.