| dc.description.abstract | The cyclotron maser instability (CMI) is an important mechanism for radio emissions from the sun, astrophysical shocks and planets, such as solar radio bursts, auroral kilometric radiation (AKR) and Jovian decametric radiation (DAM). The key ingredients for CMI are (a) the relativistic effect in the resonance condition and (b) a population-inversion distribution providing free energy. The relativistic resonance condition yields an ellipse or hyperbola in the particle momentum space rather than a straight line with constant parallel momentum. A population inversion requires a positive gradient along the perpendicular momentum in the distribution function. According to these characteristics, there are several kinds of distribution that can support CMI, such as loss-cone, ring-beam and horse-shoe distributions.
In this thesis, we carry out a series of simulations to study CMI with an initial condition that a population of tenuous energetic electrons with a ring-beam distribution is present in a magnetized background plasma. The simulation results show that the beam component of the ring-beam distribution leads to the two-stream instability at an earlier stage, and the beam mode is coupled to the Langmuir and the whistler modes, leading to excitation of the beam-Langmuir and the beam-whistler waves, respectively. When the beam velocity is large and with a strong two-stream instability, the initial ring-beam distribution is diffused in the parallel direction rapidly, and the wave excitation associated with CMI at a later stage would become weak. On the contrary, when the beam velocity is small and the two-stream instability is weak, CMI can amplify the Z mode, the whistler mode or the X mode effectively while the O mode is relatively weak.
In the cases with a pure ring distribution, we further find strong acceleration of energetic electrons by the parallel Z-mode and the parallel whistler-mode waves generated by CMI. The electron acceleration is mainly determined by the wave amplitude and phase velocity, which in turn is affected by the ratio of electron plasma to cyclotron frequencies. For the initial kinetic energy ranging from 100 to 500 keV, the peak energy of the accelerated electrons is found to reach 2~8 times of the initial kinetic energy. We then study the acceleration process via test-particle calculations in which electrons interact with one, two or four waves. The electron trajectories in the one-wave case are simple diffusion curves. In the multi-wave cases, electrons are accelerated simultaneously by counter-propagating waves and can have a higher final energy. | en_US |