電子環射束產生之迴旋脈射不穩定性：電磁波的產生與電子加速; Wave generation and electron acceleration associated with cyclotron maser instability driven by an electron ring-beam distribution in space plasmas

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Title:	電子環射束產生之迴旋脈射不穩定性：電磁波的產生與電子加速;Wave generation and electron acceleration associated with cyclotron maser instability driven by an electron ring-beam distribution in space plasmas
Authors:	李昆翰;Lee,Kun-Han
Contributors:	太空科學研究所
Keywords:	電子環射束;迴旋脈射不穩定性;電子加速;electron ring-beam distribution;cyclotron maser instability;electron acceleration
Date:	2014-03-21
Issue Date:	2014-06-19 14:00:11 (UTC+8)
Publisher:	國立中央大學
Abstract:	在太空電漿物理中，迴旋脈射不穩定性(cyclotron maser instability)是一項重要的無線電波輻射機制，尤其對太陽、行星及震波發射電波幅射，迴旋脈射更是重要的機制。其中著名的觀測現象例如地球極區千米波(auroral kilometric radiation)，木星十米波(Jovian decametric radiation)。在迴旋脈射不穩定性中，有兩樣重要的因素，一是相對論效應下的波與粒子共振條件(resonance condition)，另一個是居量反轉(population inversion)的粒子分布函數。若將前者表示為粒子速度的函數，將會是一個橢圓方程式；而後者對於迴旋脈射不穩定性的意義則是，在垂直磁場方向，高能粒子的速度空間分布密度大於低能量的粒子。根據這些特點，有許多特殊粒子分布函數能造成迴旋脈射不穩定性，例如損失錐(loss-cone)、環射束(ring-beam)、馬蹄形(horse-shoe)等分布函數。本論文利用粒子模擬程式研究迴旋脈射不穩定性。初始條件中，我們考慮了背景電漿與相對低密度的環射束電子分布。模擬結果顯示，環射束電子中，平行磁場的電子速度分量在初期造成了雙流體不穩定性(two-stream instability)，產生的電漿波與朗格繆爾(Langmuir)波和哨聲(whistler)波耦合。當雙流體不穩定性強度高時，初始電子分布很快地沿著水平磁場方向擴散至低能量區，使得後期才產生的脈射電漿波無法有效率地得到能量。相反的，當環射束電子中，垂直磁場的電子速度分量遠大於平行分量時，脈射不穩定性將能有效率地產生各種電漿波，包括X波、哨聲波和Z波，而O波雖然也有明顯的激發，但強度卻相對弱很多。當電子環射束分布變成純環狀分布(pure ring distribution)時，模擬結果顯示平行磁場方向傳播的Z波與哨聲波能很有效率地加速電子。此加速機制主要是由波的相速度與振幅控制，而這些波的特性主要受電漿頻率與電子迴旋頻率比例的調控。電子加速的幅度最低約為兩倍，最高可達八倍，初始電子能量從100仟電子伏特到500仟電子伏特。我們進一步利用試驗粒子(test-particle)模式來研究此一加速機制。計算模型中，我們考慮電子與單獨一個或是兩個、四個同時存在的平面波相互作用。計算結果發現，若只有單獨一個平面波，則在速度空間中，電子僅沿著擴散曲線(diffusion curve)移動。若系統中存在著沿平行磁場方向而又反向傳播的波，電子加速可達到最大。; 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.
Appears in Collections:	[Graduate Institute of Space Science] Department of Earth Sciences

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