Dynamo action and magnetic buoyancy in convection simulations with vertical shear
Dynamo action and magnetic buoyancy in convection simulations with vertical shear
复制标题
垂直剪切对流模拟中的发电机作用和磁浮力
DOI：
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发表时间：
2011
期刊：
影响因子：
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通讯作者：
P. Käpylä
中科院分区：
文献类型：
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作者：
G. Guerrero;P. Käpylä
太阳黑子形成的一个假设是，由速跃层的强径向剪切力产生的磁通量管的浮力出现。在这种情况下，磁场必须超过阈值才能变得有浮力并穿过整个对流区。我们跟踪随机种子磁场的演变，目的是研究在什么条件下可能激发发电机不稳定性，以及发电机产生的磁场是否变得浮力不稳定并如通量管环境中预期的那样出现在表面。我们对包括垂直剪切层的可压缩湍流对流进行数值模拟。与太阳速斜层一样，剪切力位于对流层和稳定层之间的界面处。我们发现剪切和对流能够放大初始磁场并形成大规模的细长磁结构。磁场强度取决于几个参数，例如剪切幅度、剪切层的厚度和位置以及磁雷诺数 ($\Rm$)。每当环形磁场的幅度达到大于接近均分值的阈值时，它就会变得有浮力并上升到对流区，在对流区中它会膨胀并形成蘑菇状结构。一些出现的事件，即那些具有初始场最大幅度的事件，能够到达域的最上层。这些事件能够改变对流模式，形成更广泛的对流单元或在场方向上拉长的对流涡流。然而，在这些事件中，该场都没有保留其初始结构。
A hypothesis for sunspot formation is the buoyant emergence of magnetic flux tubes created by the strong radial shear at the tachocline. In this scenario, the magnetic field has to exceed a threshold value before it becomes buoyant and emerges through the whole convection zone. We follow the evolution of a random seed magnetic field with the aim of study under what conditions it is possible to excite the dynamo instability and whether the dynamo generated magnetic field becomes buoyantly unstable and emerges to the surface as expected in the flux-tube context. We perform numerical simulations of compressible turbulent convection that include a vertical shear layer. Like the solar tachocline, the shear is located at the interface between convective and stable layers. We find that shear and convection are able to amplify the initial magnetic field and form large-scale elongated magnetic structures. The magnetic field strength depends on several parameters such as the shear amplitude, the thickness and location of the shear layer, and the magnetic Reynolds number ($\Rm$). Whenever the toroidal magnetic field reaches amplitudes greater a threshold value which is close to the equipartition value, it becomes buoyant and rises into the convection zone where it expands and forms mushroom shape structures. Some events of emergence, i.e. those with the largest amplitudes of the initial field, are able to reach the very uppermost layers of the domain. These episodes are able to modify the convective pattern forming either broader convection cells or convective eddies elongated in the direction of the field. However, in none of these events the field preserves its initial structure.