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arXiv 1502.03799astro-ph.SR
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分层双层turbulence中双极区的形成

Bipolar region formation in stratified two-layer turbulence

Jörn Warnecke, Illa R. Losada, Axel Brandenburg, Nathan Kleeorin, Igor Rogachevskii

摘要:

这项工作提出了一个广泛的研究,以前发现的形成双极通量浓度在两层模型。我们解释的形成过程中的负有效磁压不稳定性(NEMPI),这是一个可能的机制来解释太阳黑子的起源。在我们的模拟中,我们使用笛卡尔域的等温分层气体,分为两层。在下层,湍流是被迫与横向非螺旋随机波,而在上层没有流诱导。始终在整个域中施加弱的均匀磁场。在这项研究中,我们改变分层通过改变重力加速度,磁雷诺数,施加的磁场的强度,和域的大小,以调查他们的形成过程的影响。双极磁结构的形成在很大的参数范围内发生。磁结构变得更强烈的分层,直到密度对比成为周围的$100$穿过湍流层。对于考虑的雷诺数,磁通量浓度产生在磁普朗特数在0.1和1之间。双极区域中的磁场随着较高的施加场强而增加,直到该场变得与湍流的均分场强相当。更大的水平范围使通量集中变得更强和更连贯。双极结构的大小与畴的大小无关。在双极区形成的情况下,我们发现大规模的磁场,这是一个磁流体不稳定性的指示指数增长。此外,通量浓度与强大的大规模向下和收敛流。这些发现意味着NEMPI是负责磁通量浓度。

英文摘要:

This work presents an extensive study of the previously discovered formation of bipolar flux concentrations in a two-layer model. We interpret the formation process in terms of negative effective magnetic pressure instability (NEMPI), which is a possible mechanism to explain the origin of sunspots. In our simulations, we use a Cartesian domain of isothermal stratified gas that is divided into two layers. In the lower layer, turbulence is forced with transverse nonhelical random waves, whereas in the upper layer no flow is induced. A weak uniform magnetic field is imposed in the entire domain at all times. In this study we vary the stratification by changing the gravitational acceleration, magnetic Reynolds number, strength of the imposed magnetic field, and size of the domain to investigate their influence on the formation process. Bipolar magnetic structure formation takes place over a large range of parameters. The magnetic structures become more intense for higher stratification until the density contrast becomes around $100$ across the turbulent layer. For the Reynolds numbers considered, magnetic flux concentrations are generated at magnetic Prandtl number between 0.1 and 1. The magnetic field in bipolar regions increases with higher imposed field strength until the field becomes comparable to the equipartition field strength of the turbulence. A larger horizontal extent enables the flux concentrations to become stronger and more coherent. The size of the bipolar structures turns out to be independent of the domain size. In the case of bipolar region formation, we find an exponential growth of the large-scale magnetic field, which is indicative of a hydromagnetic instability. Additionally, the flux concentrations are correlated with strong large-scale downward and converging flows. These findings imply that NEMPI is responsible for magnetic flux concentrations.

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