Ring-beam粒子分布之電漿不穩定性模擬研究

A simulation study of plasma instabilities in the presence of energetic particles with a ring-beam distribution

李昆翰(Kun-Han Lee)

指導教授 ： 李羅權 ； 大村善治

國立中央大學/地球科學學院/太空科學研究所/碩士(2008年)

摘要

本篇論文將利用粒子模擬針對各種ring-beam粒子分佈的電漿不穩定性，做一系列的討論。根據Yoon et al. (2007) 所發表的研究成果，ring-beam粒子分佈很有可能成為X-mode與O-mode以及Z-mode等波的強波源。但是，本論文的研究結果發現，ring-beam粒子分佈中，沿磁場方向的粒子速度(beam component)，將造成哨聲波的激發，其成長率與飽和能量亦相當高。只有當這些高能粒子的平均速度之pitch angle接近90度時，才能讓X-mode波能有足夠的時間被增強，而避免讓哨聲波在短時間內破壞了原始的粒子分佈。此外，我們亦發現在某些情況下，ring-beam粒子分佈也有可能激發出相當強的Z-mode波。從模擬結果可以觀察到，高能粒子沿磁場方向的速度越高，使得哨聲波的激發程度越強，因而造成哨聲波成為系統中最主要的波。

關鍵字

電子迴旋輻射 ； 極區千米波

並列摘要

A population-inversion particle distribution usually causes some instabilities in space plasmas. The theoretical study of Yoon et al. (2007) shows that a ring-beam particle distribution would lead to the amplification of X-mode, O-mode, and Z-mode waves. However, in the present study, we find that the beam component of energetic particles may cause a strong excitation of whistler waves. Only when the pitch angle of the average drift velocity of energetic particles is nearly 90 degree, the X-mode waves can be intensively amplified with little interference of whistler waves, which may destroy the initial particle distribution rapidly. In the pure ring cases (90-degree pitch angle), we also find that whistler waves with a parallel propagation can have considerably high growth rate, which is still lower than that of X-mode. In the 60-degree ring-beam cases, the near 60-degree propagation angle leads to a strong amplification of Z-mode waves, and the growth rate and saturation energy of these Z-mode waves are comparable to those of whistler waves with nearly parallel propagation. As the beam component of energetic particles increases, the excitation rate of whistler waves increases, and the whistler mode becomes the dominant wave mode. In ring-beam cases with 30-degree pitch angle, whistler waves become quite strong, and we cannot find any obvious excitation of other wave modes.

並列關鍵字

cyclotron maser ； ring beam ； akr

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