| Summary: | 博士 === 國立成功大學 === 物理學系碩博士班 === 101 === Alfvén waves are low-frequency transverse waves propagating in a magnetized plasma. The presence of neutral particles may modify the wave frequency and cause damping of Alfvén waves. MHD discontinuity is a stationary thin layer through which the magnetic field, plasma density, pressure, and flow velocity may have a significant jump. In the first part of this thesis, we investigate the effect of ion-neutral collisions on the damping of Alfvén waves in a homogeneous plasma. In the second part of this thesis, 1-D Riemann problem is used to study the generation and evolution of MHD discontinuities associated with magnetic reconnection.
First, the effects on Alfvén waves depend on two parameters: (1) , the ratio of neutral density and ion density , and (2) , the ratio of neutral collisional frequency by ions , , to the wave frequency . Most of previous studies examined only the limiting case with a relatively large neutral collisional frequency or . In the Chapter 2 of this thesis, the dispersion relation for Alfvén waves is solved for all values of and . It is found for the first time that there is a “forbidden zone” in the parameter space, where the real frequency of Alfvén waves becomes zero and Alfvén waves become evanescent. Approximate solutions in the limit as well as are obtained. We also discuss the propagation and damping of Alfvén waves in the ionosphere and in the solar chromosphere, where the “forbidden zone” is identified.
Second, we use 1-D hybrid code to simulate the generation and evolution of MHD discontinuities associated with magnetic reconnection in a current sheet. As a result of the leakage of slow shock (SS), the ion parallel temperature and temperature anisotropy tends to increase, where is the ion parallel (perpendicular) beta. The propagation rotational discontinuity (RD) and slow shock (SS) lead to formation of various compound structures in the reconnection outflow region. Four types of compound structure are found in our simulations: (a) RD-SS compound structure: the RD is attached to the leading part of SS, (b) SS-RD (DD) compound structure: RD is attached to the rear part of SS, (c) SS-RD-SS compound structure: RD is trapped inside SS, and (d) switch-off slow shock (SSS) with a rotational wave train. The type of compound structure generated depends on the initial ion beta and magnetic shear angle .
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