漂移在超新星残余激波上游的宇宙射线产生磁Turbulence
Production of Magnetic Turbulence by Cosmic Rays Drifting Upstream of Supernova Remnant Shocks
摘要：
我们提出的2D和3D PIC模拟的磁湍流生产的各向同性宇宙射线离子漂移上游的SNR冲击的结果。这些研究旨在检验最近关于短波长磁场强放大的预测，并研究磁湍流的演变及其对宇宙射线的反作用。我们观察到，一个倾斜的平行模式的增长速度比在分析理论中发现的非共振平行模式，和场扰动的增长速度是远远低于平行平面波模式的估计，可能是因为在我们的模拟中，我们不能保持Ω <<Ω_i，离子陀螺频率，所需的平面波模式出现的程度。在MHD模拟中还观察到演化的斜向惯性模在非线性阶段占主导地位。因此，我们确认湍流磁场的产生是由于上游等离子体中宇宙射线离子的漂移，但作为我们的主要结果发现，湍流的振幅饱和在约dB/B~1。湍流对粒子的反作用导致了宇宙射线和背景介质的整体流速的一致性，这是宇宙射线修正激波的一个基本特征。它占饱和的不稳定性在中等场振幅。以前发表的MHD模拟假设宇宙线电流恒定，宇宙线中没有能量或动量通量，这排除了产生的磁场对宇宙射线的反作用，因此人为地抑制了场振幅的饱和。这可以解释MHD模拟中磁场的持续增长。还没有证明强磁场放大到幅度dB >> B 0。
英文摘要：
We present results of 2D and 3D PIC simulations of magnetic turbulence production by isotropic cosmic-ray ions drifting upstream of SNR shocks. The studies aim at testing recent predictions of a strong amplification of short wavelength magnetic field and at studying the evolution of the magnetic turbulence and its backreaction on cosmic rays. We observe that an oblique filamentary mode grows more rapidly than the non-resonant parallel modes found in analytical theory, and the growth rate of the field perturbations is much slower than is estimated for the parallel plane-wave mode, possibly because in our simulations we cannot maintain omega << Omega_i, the ion gyrofrequency, to the degree required for the plane-wave mode to emerge. The evolved oblique filamentary mode was also observed in MHD simulations to dominate in the nonlinear phase. We thus confirm the generation of the turbulent magnetic field due to the drift of cosmic-ray ions in the upstream plasma, but as our main result find that the amplitude of the turbulence saturates at about dB/B~1. The backreaction of the turbulence on the particles leads to an alignment of the bulk-flow velocities of the cosmic rays and the background medium, which is an essential characteristic of cosmic-ray modified shocks. It accounts for the saturation of the instability at moderate field amplitudes. Previously published MHD simulations have assumed a constant cosmic-ray current and no energy or momentum flux in the cosmic rays, which excludes a backreaction of the generated magnetic field on cosmic rays, and thus the saturation of the field amplitude is artificially suppressed. This may explain the continued growth of the magnetic field in the MHD simulations. A strong magnetic field amplification to amplitudes dB >> B0 has not been demonstrated yet.
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