The physics of twisted magnetic tubes rising in a stratified medium: Two-dimensional results
The physics of twisted magnetic tubes rising in a stratified medium: Two-dimensional results
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DOI：
10.1086/305074
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发表时间：
1998-01-10
影响因子：
4.9
通讯作者：
Moreno-Insertis, F
中科院分区：
文献类型：
--
作者：
Emonet, T;Moreno-Insertis, F
利用数值磁流体动力学（MHD）程序研究了在分层介质中上升的扭曲磁通管的物理特性。所考虑的问题是完全可压缩的（没有Boussinesq近似），包括欧姆电阻率，并且是二维的，即，在管轴的方向上没有变量的变化。我们研究了半径与外部压力标度高度之比较小的高等离子体β情况。因此，所获得的结果将是相关的了解整个太阳对流区的磁通量的运输。我们确认，一个足够的扭曲的管轴周围的持有线可以抑制管转换成两个涡卷。对于相对密度差约为1/beta（经典的帕克浮力）且半径小于压力标度高度（R-2远小于H-p（2））的管，所需的最小扭转量对应于sin（-1）[（R/H-p）（1/2）]量级的平均俯仰角。详细研究了具有这种扭曲度的管的演化，包括初始瞬态阶段、内部扭转振荡和渐近准稳定阶段。在初始阶段，管的最外面的弱磁化层从其主体撕裂并被赋予涡度。它们产生具有两个涡卷的拖尾磁化尾流。被带到尾流的总磁通量的分数是初始扭曲度的函数。在弱扭曲情况下，大部分的初始管变成涡卷。在强的初始扭转下，管仅以小的变形上升，并且没有显著的磁通量损失。在初始瞬态阶段后，尾流的形成和管体的磁通损失基本完成，在管的前部和侧部形成了一个尖锐的管内外流动界面，该区域具有磁边界层的特征。它的结构被确定为欧姆扩散和通过外部流场平流之间的平衡。它是整个管道演化过程中通过磁场产生涡量的场所。从流体动力学的角度来看，该问题构成了水中气泡上升和刚性圆柱在外部介质中运动之间的中间情况。与气泡一样，管是可变形的，实验结果（上升物体和尾流的形状）取决于韦伯数的值。在本模拟中获得的几个结构特征也观察到在上升的气泡，如中央尾巴，和裙包围的尾流。在刚性圆柱体中，边界层满足无滑移条件（由磁场在管中提供），并且在移动物体的侧边缘处形成次级滚动。
The physics of a twisted magnetic flux tube rising in a stratified medium is studied using a numerical magnetohydrodynamic (MHD) code. The problem considered is fully compressible (has no Boussinesq approximation), includes ohmic resistivity, and is two-dimensional, i.e., there is no variation of the variables in the direction of the tube axis. We study a high-plasma beta-case with a small ratio of radius to external pressure scale height. The results obtained will therefore be of relevance to understanding the transport of magnetic flux across the solar convection zone.We confirm that a sufficient twist of the held lines around the tube axis can suppress the conversion of the tube into two vortex rolls. For a tube with a relative density deficit on the order of 1/beta (the classical Parker buoyancy) and a radius smaller than the pressure scale height (R-2 much less than H-p(2)), the minimum amount of twist necessary corresponds to an average pitch angle on the order of sin(-1) [(R/H-p)(1/2)]. The evolution of a tube with this degree of twist is studied in detail, including the initial transient phase, the internal torsional oscillations, and the asymptotic, quasi-stationary phase. During the initial phase, the outermost, weakly magnetized layers of the tube are torn off its main body and endowed with vorticity. They yield a trailing magnetized wake with two vortex rolls. The fraction of the total magnetic flux that is brought to the wake is a function of the initial degree of twist. In the weakly twisted case, most of the initial tube is turned into vortex rolls. With a strong initial twist, the tube rises with only a small deformation and no substantial loss of magnetic flux. The formation of the wake and the loss of flux from the main body of the tube are basically complete after the initial transient phase.A sharp interface between the tube interior and the external flows is formed at the tube front and sides; this area has the characteristic features of a magnetic boundary layer. Its structure is determined as an equilibrium between ohmic diffusion and field advection through the external flows. It is the site of vorticity generation via the magnetic field during the whole tube evolution.From the hydrodynamical point of view, this problem constitutes an intermediate case between the rise of air bubbles in water and the motion of a rigid cylinder in an external medium. As with bubbles, the tube is deformable and the outcome of the experiment (the shape of the rising object and the wake) depends on the value of the Weber number. Several structural features obtained in the present simulation are also observed in rising air bubbles, such as a central tail, and a skirt enveloping the wake. As in rigid cylinders, the boundary layer satisfies a no-slip condition (provided for in the tube by the magnetic field), and secondary rolls are formed at the lateral edges of the moving object.