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arXiv 0710.2617astro-ph
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Cosmic Ray传播：平行于平均磁场的非线性扩散和垂直扩散

Cosmic Ray Propagation: Nonlinear Diffusion Parallel and Perpendicular to Mean Magnetic Field

Huirong Yan, A. Lazarian

备注：
minor changes, accepted to ApJ
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摘要：

我们考虑宇宙射线在湍流磁场中的传播。我们使用的模型的磁流体动力学湍流，在数值模拟中进行了测试，其中的湍流是在大尺度上注入和级联到小尺度。我们的注意力集中在强湍流的模型，但我们也简要地讨论了弱湍流和板Alfvénic扰动的影响。后者可能是由于宇宙射线流体本身的不稳定性而出现的，例如，宇宙射线的集束和回旋共振不稳定性。为了描述宇宙射线与磁微扰的相互作用，我们发展了一种非线性形式主义，扩展了通常用于此目的的普通准线性理论（QLT）。这使我们能够避免通常的90度散射问题，并使我们能够计算宇宙射线的平均自由程。我们将该方法应用于宇宙线在星系晕和暖电离介质（WIM）中的传播。此外，我们还讨论了垂直于平均磁场的宇宙射线的输运问题，并表明宇宙射线次扩散问题（即，传播与回溯轨迹向后，这减慢了扩散）仅对于当环境湍流远离数值模拟向我们建议的情况时的受限情况是重要的。因此，这项工作提供了形式主义，可用于计算宇宙射线在各种情况下的传播。

英文摘要：

We consider the propagation of cosmic rays in turbulent magnetic fields. We use the models of magnetohydrodynamic turbulence that were tested in numerical simulations, in which the turbulence is injected on large scale and cascades to small scales. Our attention is focused on the models of the strong turbulence, but we also briefly discuss the effects that the weak turbulence and the slab Alfvénic perturbations can have. The latter are likely to emerge as a result of instabilities with in the cosmic ray fluid itself, e.g., beaming and gyroresonance instabilities of cosmic rays. To describe the interaction of cosmic rays with magnetic perturbations we develop a non-linear formalism that extends the ordinary Quasi-Linear Theory (QLT) that is routinely used for the purpose. This allows us to avoid the usual problem of 90 degree scattering and enable our computation of the mean free path of cosmic rays. We apply the formalism to the cosmic ray propagation in the galactic halo and in the Warm Ionized medium (WIM). In addition, we address the issue of the transport of cosmic rays perpendicular to the mean magnetic field and show that the issue of cosmic ray subdiffusion (i.e., propagation with retracing the trajectories backwards, which slows down the diffusion) is only important for restricted cases when the ambient turbulence is far from what numerical simulations suggest to us. As a result, this work provides formalism that can be applied for calculating cosmic ray propagation in a wide variety of circumstances.

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