湍流应力引起的大尺度磁通量集中
Large-scale magnetic flux concentrations from turbulent stresses
摘要：
在这项研究中，我们提供了第一个数值演示的平均洛伦兹力和由此形成的大规模磁结构的湍流的影响。使用三维直接数值模拟（DNS）的强制湍流，我们表明，强加的平均磁场导致的湍流磁流体压力和张力的减少。通过确定将湍流雷诺应力和麦克斯韦应力之和与平均磁场的麦克斯韦应力相关联的相关函数来量化这种现象。使用这样一个参数化，我们通过二维和三维的平均场数值模拟，等熵密度分层层变得不稳定的存在下，一个统一的施加磁场。这种大尺度的不稳定性导致了环状磁结构的形成，这些环状磁结构集中在分层层的顶部。在三维空间中，这些结构类似于太阳中双极磁区的外观。DNS和平均场数值模拟的结果与理论预测吻合得很好。我们讨论我们的模型的背景下，分布式太阳发电机的活动区域和太阳黑子可能是相当浅的现象。
英文摘要：
In this study we provide the first numerical demonstration of the effects of turbulence on the mean Lorentz force and the resulting formation of large-scale magnetic structures. Using three-dimensional direct numerical simulations (DNS) of forced turbulence we show that an imposed mean magnetic field leads to a decrease of the turbulent hydromagnetic pressure and tension. This phenomenon is quantified by determining the relevant functions that relate the sum of the turbulent Reynolds and Maxwell stresses with the Maxwell stress of the mean magnetic field. Using such a parameterization, we show by means of two-dimensional and three-dimensional mean-field numerical modelling that an isentropic density stratified layer becomes unstable in the presence of a uniform imposed magnetic field. This large-scale instability results in the formation of loop-like magnetic structures which are concentrated at the top of the stratified layer. In three dimensions these structures resemble the appearance of bipolar magnetic regions in the Sun. The results of DNS and mean-field numerical modelling are in good agreement with theoretical predictions. We discuss our model in the context of a distributed solar dynamo where active regions and sunspots might be rather shallow phenomena.
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