Hamburger Sternwarte, Universität Hamburg Gojenbergsweg 112, 21029 Hamburg, Germany.
Phys Rev E. 2017 Nov;96(5-1):053105. doi: 10.1103/PhysRevE.96.053105. Epub 2017 Nov 10.
We explore the phenomenon of the recently discovered inverse transfer of energy from small to large scales in decaying magnetohydrodynamical turbulence by Brandenburg et al. [Phys. Rev. Lett. 114, 075001 (2015)PRLTAO0031-900710.1103/PhysRevLett.114.075001], even for nonhelical magnetic fields. For this investigation we mainly employ the Pencil Code performing a parameter study, where we vary the Prandtl number, the kinematic viscosity, and the initial spectrum. We find that to get a decay that exhibits this inverse transfer, large Reynolds numbers (O∼10^{3}) are needed and low Prandtl numbers of the order unity Pr=1 are preferred. Compared to helical MHD turbulence, though, the inverse transfer is much less efficient in transferring magnetic energy to larger scales than the well-known effect of the inverse cascade. Hence, applying the inverse transfer to the magnetic field evolution in the Early Universe, we question whether the nonhelical inverse transfer is effective enough to explain the observed void magnetic fields if a magnetogenesis scenario during the electroweak phase transition is assumed.
我们探讨了 Brandenburgh 等人最近发现的在磁流体力学湍流衰减中能量从小尺度向大尺度反向传递的现象[Phys. Rev. Lett. 114, 075001 (2015)]，即使对于非螺旋磁场也是如此。为此，我们主要使用 Pencil Code 进行参数研究，改变普朗特数、运动粘性系数和初始谱。我们发现，要得到表现出这种反向传递的衰减，需要大的雷诺数（O∼10^{3}），并且最好是普朗特数为 1 量级的低数值。然而，与螺旋磁流体力学湍流相比，反向传递在将磁能传递到更大尺度上的效率要低得多，远不如众所周知的反向级联效应。因此，如果假设电弱相变期间存在磁发生情景，我们将反向传递应用于早期宇宙中的磁场演化，我们质疑非螺旋反向传递是否足以解释观测到的空洞磁场。
