Charged particel transport in interstellar plasmas: Improved theory for scattering of cosmic rays in turbulent fields

宇宙射线、带电尘埃粒子和湍流磁场是星际介质的重要组成部分。理解和预测带电粒子在湍流介质中的传输是许多相关学科的关键挑战，从磁约束聚变研究到天体物理学。在天体物理学中，它是宇宙射线在星际磁场中的散射、加速和空间扩散。这个建议的目的是双重的：首先，我们想研究宇宙磁场中的快速粒子的传输，应用最近在基于渗流理论的托卡马克物理学背景下发展的标度概念。这是可以做到的，因为虽然湍流尺度是非常不同的，垂直于背景场的散射机制基本上是相同的。第二，我们要发展一个改进的弱湍流输运理论的快速带电粒子在部分湍流磁场中，避免通常的准线性近似的粒子轨道，并适当地考虑到最近的非线性改进。采用了磁湍流的三个统计性质：（1）准定常湍流，（2）存在有限的湍流解相关时间，（3）空间均匀湍流。与标度理论相反，弱湍流输运理论经常利用Corrsin独立性假设，允许人们用两个二阶相关函数的乘积来表示四阶相关函数。这个提议的两个主要目标是：（1）分别从弱湍流和标度输运理论计算宇宙射线散射率;（2）比较它们，以验证或反驳Corrsin假设对宇宙湍流磁场的有效性。这样改进的输运理论将被应用于选定的星际现象，如宇宙射线粒子的扩散冲击加速在存在大尺度平行空间梯度的引导磁场。

Cosmic rays, charged dust particles and turbulent magnetic fields are important constituents of the interstellar medium. Understanding and predicting the transport of charged particles in turbulent media is a key challenge in a number of related disciplines, ranging from magnetic confinement fusion research to astrophysics. In astrophysics, it is the scattering, acceleration and spatial diffusion of cosmic rays in interstellar magnetic fields. The purpose of this proposal is two-fold:First, we want to investigate the transport of fast particles in cosmic magnetic fields, applying scaling concepts which were developed recently in the context of tokamak physics based on the percolation theory. This can be done since although the turbulent scales are extremely different, the scattering mechanisms perpendicular to the background field are basically the same.Secondly, we want to develop an improved weak turbulence transport theory of fast charged particles in a partially turbulent magnetic field, that avoids the usually made quasilinear approximation to the particle orbits, and properly takes into account recent nonlinear improvements. Three statistical properties of the magnetic turbulence are adopted: (1) quasi-stationary turbulence, (2) the existence of a finite turbulence decorrelation time, and (3) spatially homogenous turbulence. In contrast to the scaling theory, weak turbulence transport theories often make use of the Corrsin independence hypothesis allowing one to express a fourth order correlation function in terms of the product of two second order correlation functions. Two central goals of this proposal are (1) the calculation of cosmic ray scattering rates from weak turbulence and scaling transport theories, respectively, and (2) their comparison to verify or disprove the validity of the Corrsin hypothesis for cosmic turbulent magnetic fields. The so improved transport theory will be applied to selected interstellar phenomena such as the diffusive shock acceleration of cosmic ray particles in the presence of large scale parallel spatial gradients of the guide magnetic field.