Superdiffusion of Cosmic Rays in Compressible Magnetized Turbulence
Superdiffusion of Cosmic Rays in Compressible Magnetized Turbulence
复制标题
可压缩磁化湍流中宇宙射线的超扩散
DOI：
10.1093/mnras/stac319
复制
发表时间：
2021
影响因子：
6.5
通讯作者：
Siyao Xu
中科院分区：
文献类型：
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作者：
Yue Hu;A. Lazarian;Siyao Xu
由于湍流磁场的复杂性，模拟宇宙射线的扩散是具有挑战性的。基于目前对各向异性磁流体动力学（MHD）湍流的理解，我们利用测试粒子研究了宇宙线在垂直于平均磁场方向上的超扩散。通过改变可压缩MHD模拟的Alfvén马赫数MA和音速马赫数MS，我们的研究涵盖了包括亚音速暖气相和超音速冷分子气体在内的广泛的天体物理条件。我们表明，自由流动的宇宙线的垂直位移增加的3/2的权力的时间旅行沿着当地磁场线。如果平行传播是扩散的，则该幂律指数变为3/4。我们发现宇宙线的平行平均自由程在超音速湍流中以M−2 A的幂律关系减小。我们调查的慢，快，和Alfvénic模式的能量分数，并确认Alfvénic模式在垂直超扩散的主导地位。特别是，快模的能量分数，这是俯仰角散射的主要代理，增加与MA，但不敏感MS ≥ 2。因此，我们的结果表明，在超音速分子云中的抑制扩散主要是由于MA的变化，而不是MS。
Owing to the complexity of turbulent magnetic fields, modeling the diffusion of cosmic rays is challenging. Based on the current understanding of anisotropic magnetohydrodynamic (MHD) turbulence, we use test particles to examine the cosmic rays’ superdiffusion in the direction perpendicular to the mean magnetic field. By changing Alfvén Mach number MA and sonic Mach number MS of compressible MHD simulations, our study covers a wide range of astrophysical conditions including subsonic warm gas phase and supersonic cold molecular gas. We show that freely streaming cosmic rays’ perpendicular displacement increases as 3/2 to the power of the time traveled along local magnetic field lines. This power-law index changes to 3/4 if the parallel propagation is diffusive. We find that the cosmic rays’ parallel mean free path decreases in a power-law relation of M−2 A in supersonic turbulence. We investigate the energy fraction of slow, fast, and Alfvénic modes and confirm the dominance of Alfvénic modes in the perpendicular superdiffusion. In particular, the energy fraction of fast mode, which is the main agent for pitch-angle scattering, increases with MA, but is insensitive to MS ≥ 2. Accordingly, our results suggest that the suppressed diffusion in supersonic molecular clouds arises primarily due to the variations of MA instead of MS .