Investigation of Kinetic Physics of Magnetic Reconnection under a Finite Guide Field

Abstract

Magnetic reconnection, the process by which the magnetic energy is converted into heat and flow energy, is believed to be a very important process in space and laboratory plasmas. Among various reconnection models, the Petschek model suggests a promising explanation for the fast time scale phenomena caused by the reconnection. For the antiparallel (zero guide field) cases, the Petschek model contains a small diffusion region and four standing slow shocks that bound the outflow region. In contrast to the pure antiparallel case, theoretical and simulation studies show that in general cases with a finite field, the Petschek model is modified with complicated wave structures. Such models, however, only correspond to the two-dimensional (2D) magnetic reconnection with a long, extended X-line. Two remaining issues in the Petschek model are still poorly understood. One is the onset mechanism for the reconnection, and the other is the wave properties in the outflow region of general three-dimensional (3D) reconnection. Tearing mode instabilities are thought to play a fundamental role to trigger the reconnection. In this thesis, we extend the recently-developed gyrokinetic electron and fully kinetic ion (GeFi) simulation model to nonuniform plasmas, and use it to study tearing mode instabilities under a finite guide field. By removing the rapid gyro-motions of electrons in the calculation, the main advantage of the current GeFi model is that the realistic mass ratio between electrons and ions is allowed to be employed in the simulation code. Through an eigenmode analysis, the improved GeFi model is benchmarked against the linear theory of the tearing instability. Furthermore, our simulation results show that the ion kinetics play an important role in the linear growth rate of the tearing mode instability as well as in the nonlinear saturation of tearing modes. For the cases with multiple tearing modes, the interactions between them lead to the coalescence of neighboring magnetic islands, and form larger saturated magnetic islands, which has been suggested as a necessary condition to trigger a fast reconnection. In order to explore the properties of low frequency waves generated by the reconnection under a finite guide field, a 3D hybrid simulation is carried out for the large-scale structure of the reconnection layer. For the case with an infinitely long X-line, quasi-steady rotational discontinuities are formed behind a leading plasma bugle that propagates away from the reconnection site. Field-aligned structures are also observed in the transition region between the steady layer and the leading bulge. For the cases with a limited extent of X-line with length $<30d_{i}$, the perturbations caused by the reconnection propagate along the magnetic field line, and the wavefront of propagation has the properties of kinetic Alfv\'en wave.There are no evident discontinuities that form to bound the outflow region. Due to the propagation of the waves, a layer of accelerated plasmas is found beyond the extent of the X-line. The simulation indicates that the wave structures in the reconnection are greatly modified from that in the 2D Petschek reconnection model when the X-line has a finite length.