Understanding Uniturbulence: Self-cascade of MHD Waves in the Presence of Inhomogeneities
Understanding Uniturbulence: Self-cascade of MHD Waves in the Presence of Inhomogeneities
复制标题
了解单湍流：存在不均匀性时 MHD 波的自级联
DOI：
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发表时间：
2019
影响因子：
4.9
通讯作者：
M. Goossens
中科院分区：
文献类型：
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作者：
N. Magyar;T. Doorsselaere;M. Goossens
MHD湍流界普遍认为，波浪能量的非线性级联需要沿某平均磁场反向传播的alfv<s:1>晶格波包。这一事实是MHD方程在不可压缩性和均匀性假设下的一个明显结果。尽管试图在MHD湍流的背景下放宽这些假设，但湍流产生的中心思想仍然存在。然而，一旦不可压缩性和均匀性的假设被打破，普遍接受的湍流叶栅产生的图景就不是普遍的了。在本文中，我们证明了垂直的不均匀性（穿过平均磁场）导致传播波解，这些波解必然由共传播Elsässer场描述，已经在不可压缩的情况下。这些波解的一个简单例子是横过磁场的平面不连续面上的表面alfvsamn波。我们通过数值模拟展示了这些单向传播波的非线性自变形如何导致波能量在磁场中的级联。这种单向级联的存在可能对结构等离子体（如日冕和太阳风）中主要向外传播的阿尔夫萨奇波的湍流耗散率有额外的强烈影响。
It is widely accepted in the MHD turbulence community that the nonlinear cascade of wave energy requires counterpropagating Alfvénic wave packets, along some mean magnetic field. This fact is an obvious outcome of the MHD equations under the assumptions of incompressibility and homogeneity. Despite attempts to relax these assumptions in the context of MHD turbulence, the central idea of turbulence generation persists. However, once the assumptions of incompressiblity and homogeneity break down, the generally accepted picture of turbulent cascade generation is not universal. In this paper, we show that perpendicular inhomogeneities (across the mean magnetic field) lead to propagating wave solutions that are necessarily described by co-propagating Elsässer fields, already in the incompressible case. One simple example of these wave solutions is the surface Alfvén wave on a planar discontinuity across the magnetic field. We show through numerical simulations how the nonlinear self-deformation of these unidirectionally propagating waves leads to a cascade of wave energy across the magnetic field. The existence of this type of unidirectional cascade might have an additional strong effect on the turbulent dissipation rate of dominantly outward-propagating Alfvénic waves in structured plasma, as in the solar corona and solar wind.