NONLINEAR DYNAMICS OF ACOUSTIC INSTABILITY IN A COSMIC RAY SHOCK PRECURSOR AND ITS IMPACT ON PARTICLE ACCELERATION
NONLINEAR DYNAMICS OF ACOUSTIC INSTABILITY IN A COSMIC RAY SHOCK PRECURSOR AND ITS IMPACT ON PARTICLE ACCELERATION
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宇宙射线激波前体声不稳定性的非线性动力学及其对粒子加速的影响
DOI：
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发表时间：
2009
期刊：
影响因子：
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通讯作者：
P. Diamond
中科院分区：
文献类型：
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作者：
M. Malkov;P. Diamond
在非线性区域研究了由加速粒子压力梯度驱动的激波前兆的声学不稳定性。非线性使不稳定的声波变陡，并将其转变为冲击波。这些冲击形成一个“冲击串”，但它们可能会相互融合。在两种不同的情况下，行波解得到解析。在第一种情况下，只包括声学不稳定性，特征尺度（冲击之间的距离）仅受系统大小的限制（当冲击合并时）。在第二种情况下，不稳定发展出回旋不稳定种子磁流体动力学（MHD）波。MHD波包之间的空间距离决定了声激波串的尺度。单个激波的内部结构可能由离子趋肤深度c/ωpi和这些激波附近稍超热粒子分布的弛豫长度决定。假设激波与它们之间的距离相比是任意薄的，这是由一个小的粘性项来保证的。这两种类型的解决方案进行了动态验证的数值计算。声不稳定性产生的冲击前兆中的磁流体流动对于最近提出的扩散冲击加速中的两种现象至关重要。一种现象是由于激波前兆变窄，加速率大大高于其标准（玻姆）值。第二种现象是由加速粒子产生的阿尔文波的逆级联放大长尺度磁场，并在k空间中散射到声学扰动上。
An acoustic instability of a shock precursor driven by the pressure gradient of accelerated particles is studied in the nonlinear regime. The nonlinearity steepens unstable acoustic waves and turns them into shocks. The shocks form a “shocktrain” but they may merge into each other. Traveling wave solutions are obtained analytically in two different cases. In the first case, only acoustic instability is included and the characteristic scale (distance between the shocks) is limited only by the system size (while shocks merge). In the second case, the instability develops out of cyclotron unstable seed magnetohydrodynamic (MHD) waves. The spatial distance between the MHD wave packets sets the scale of the acoustic shocktrain. The internal structure of the individual shocks is presumably determined by the ion skin depth c/ωpi and by the relaxation length of slightly superthermal particle distributions near these shocks. The shocks are assumed to be arbitrarily thin compared with the distance between them which is ensured by a small viscous term. Both types of solutions are dynamically verified by numerical calculations. The hydromagnetic flow in the shock precursor emerging from the acoustic instability is crucial for two recently suggested phenomena in diffusive shock acceleration. One phenomenon is the enhancement of the acceleration rate well above its standard (Bohm) value due to the narrowing of the shock precursor. The second phenomenon is the amplification of the long-scale magnetic field by an inverse cascade of Alfvén waves generated by accelerated particles and scattered in k-space on the acoustic perturbations.