Nonlinear turbulent magnetic diffusion and mean-field dynamo.
Nonlinear turbulent magnetic diffusion and mean-field dynamo.
复制标题
非线性湍流磁扩散和平均场发电机。
DOI：
10.1103/physreve.64.056307
复制
发表时间：
2001
期刊：
影响因子：
--
通讯作者：
N. Kleeorin
中科院分区：
文献类型：
--
作者：
I. Rogachevskii;N. Kleeorin
定义平均电动势的非线性系数（即，非线性湍流磁扩散、非线性有效速度、非线性kappa张量等）是针对各向异性湍流计算的。各向异性背景湍流的特定情况（即，具有零平均磁场的湍流）。结果表明，环向磁场和极向磁场具有不同的非线性湍流磁扩散系数。它表明，即使是一个均匀的湍流有一个非线性的有效速度，表现出抗磁或顺磁特性取决于湍流的各向异性和背景湍流中的磁波动的水平。反磁速度导致磁场从平均磁场较强的区域被推出，而顺磁速度导致磁场集中在磁场较强的区域。分析表明，湍流的各向异性强烈影响非线性湍流磁扩散、非线性有效速度和非线性α效应。两种类型的非线性（代数和动态）进行了讨论。代数非线性意味着平均电动势对平均磁场的非线性依赖性。动态非线性由α效应的磁性部分的微分方程确定。结果表明，对于α-Ω轴对称发电机，仅代数非线性（包括非线性α效应、非线性湍流磁扩散、非线性有效速度等）不能使发电机产生的平均磁场饱和，而代数和动态非线性的组合效应限制了平均磁场的增长。
The nonlinear coefficients defining the mean electromotive force (i.e., the nonlinear turbulent magnetic diffusion, the nonlinear effective velocity, the nonlinear kappa tensor, etc.) are calculated for an anisotropic turbulence. A particular case of an anisotropic background turbulence (i.e., the turbulence with zero-mean magnetic field) with one preferential direction is considered. It is shown that the toroidal and poloidal magnetic fields have different nonlinear turbulent magnetic diffusion coefficients. It is demonstrated that even for a homogeneous turbulence there is a nonlinear effective velocity that exhibits diamagnetic or paramagnetic properties depending on the anisotropy of turbulence and the level of magnetic fluctuations in the background turbulence. The diamagnetic velocity results in the field being pushed out from the regions with stronger mean magnetic field, while the paramagnetic velocity causes the magnetic field to be concentrated in the regions with stronger field. Analysis shows that an anisotropy of turbulence strongly affects the nonlinear turbulent magnetic diffusion, the nonlinear effective velocity, and the nonlinear alpha effect. Two types of nonlinearities (algebraic and dynamic) are also discussed. The algebraic nonlinearity implies a nonlinear dependence of the mean electromotive force on the mean magnetic field. The dynamic nonlinearity is determined by a differential equation for the magnetic part of the alpha effect. It is shown that for the alphaOmega axisymmetric dynamo the algebraic nonlinearity alone (which includes the nonlinear alpha effect, the nonlinear turbulent magnetic diffusion, the nonlinear effective velocity, etc.) cannot saturate the dynamo generated mean magnetic field while the combined effect of the algebraic and dynamic nonlinearities limits the mean magnetic field growth.