Acceleration of suprathermal particles by compressional plasma wave trains in the solar wind
Acceleration of suprathermal particles by compressional plasma wave trains in the solar wind
复制标题
太阳风中压缩等离子体波列对超热粒子的加速
DOI：
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发表时间：
2010
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通讯作者：
Ming Zhang
中科院分区：
文献类型：
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作者：
Ming Zhang
本文提出了用理想压缩等离子体波列计算粒子加速度的方法。在这个模型中，超热粒子，如拾取离子，被连续注入到由一系列压缩区或稀薄区组成的波列中。粒子在波列中的动量分布会变得越来越宽，这与动量空间中的扩散非常相似。加速过程非常快，即使压缩幅度很小，也不需要太多的波循环就能达到渐近稳态动量分布。在没有大规模绝热冷却的情况下，在初始注入动量以下的渐近分布是平坦的，在注入动量以上的渐近分布与斜率为- 3的幂律分布成正比。这种分布似乎与这种加速机制中的任何模型参数无关。如果太阳风的膨胀导致大规模绝热冷却，则在注入动量以下的渐近稳态分布仍然是平坦的，在注入动量以上的渐近稳态分布是幂律分布，但斜率更陡。单独的加速过程并不能自动保证p−5幂定律。然而，由于这一过程可以迅速地从加速的粒子中建立压力，预计压缩等离子体波的振幅将会减小。在非线性波粒相互作用后的最终状态下，加速粒子和等离子体波的分布必须在大规模绝热冷却和压缩等离子体波的加速之间达到平衡。在初始太阳风压力远大于新注入粒子初始压力的情况下，平衡态粒子的分布趋于p−5分布。
[1] This paper presents a calculation of particle acceleration by an idealized compressional plasma wave train. In this model, suprathermal particles, such as pickup ions, are continuously injected into a wave train consisting of a series of compression or rarefaction regions. The momentum distribution of particles will become broader and broader as they go through the wave train, which is very similar to diffusion in momentum space. The acceleration process is very fast: it does not take too many wave cycles even with a small compression amplitude to reach an asymptotic steady state momentum distribution. In the absence of large-scale adiabatic cooling, the asymptotic distribution is flat below the initial injection momentum, and above the injection momentum, it is proportional to a power law distribution with the slope of −3. This distribution appears to be independent of any model parameters in this acceleration mechanism. If there is a prevailing large-scale adiabatic cooling by the expanding solar wind, the asymptotic steady state distribution remains to be flat below the injection momentum and it is a power law distribution but with a steeper slope above the injection momentum. The acceleration process alone does not automatically guarantee a p−5 power law. However, since the process can quickly build up pressure from the accelerated particles, it is expected that the amplitude of compressional plasma wave will be reduced. In the final state after nonlinear wave-particle interactions, the distribution of accelerated particles and plasma wave must achieve a balance between large-scale adiabatic cooling and the acceleration by compressional plasma waves. The particle distribution at the equilibrium will settle with a p−5 distribution given that the initial solar wind pressure is much larger than the initial pressure of newly injected particles.