太阳上稳定磁结构自发形成的机制
Mechanism of spontaneous formation of stable magnetic structures on the Sun
摘要：
太阳磁场的一个令人困惑的特征是在太阳对流区的高度湍流的上层形成了长寿的紧凑磁结构，如太阳黑子和孔隙。我们使用真实的辐射三维MHD模拟来研究磁场和湍流对流之间的相互作用。在模拟中，一个弱的垂直均匀磁场施加在充分发展的颗粒对流的区域，并通过顶部和底部边界的总磁通量保持恒定。模拟结果揭示了稳定磁结构的自发形成过程，这可能是理解太阳上磁自组织以及太阳孔隙和黑子形成的关键。这一过程包括两个基本步骤：1）与涡量集中和涡池型运动相关的小尺度对流磁结构的形成，以及2）由于涡吸引力，这些结构的合并，这是由表面以下磁集中周围的会聚下沉气流引起的。在由会聚等离子体运动维持的大尺度结构中，表面的磁场强度达到~1.5 kG，内部达到~6 kG;表面结构类似于太阳孔。磁结构在整个模拟运行数小时内保持稳定，没有衰减的迹象。
英文摘要：
One of the puzzling features of solar magnetism is formation of long-living compact magnetic structures; such as sunspots and pores, in the highly turbulent upper layer of the solar convective zone. We use realistic radiative 3D MHD simulations to investigate the interaction between magnetic field and turbulent convection. In the simulations, a weak vertical uniform magnetic field is imposed in a region of fully developed granular convection; and the total magnetic flux through the top and bottom boundaries is kept constant. The simulation results reveal a process of spontaneous formation of stable magnetic structures, which may be a key to understanding of the magnetic self-organization on the Sun and formation of pores and sunspots. This process consists of two basic steps: 1) formation of small-scale filamentary magnetic structures associated with concentrations of vorticity and whirlpool-type motions, and 2) merging of these structures due to the vortex attraction, caused by converging downdrafts around magnetic concentration below the surface. In the resulting large-scale structure maintained by the converging plasma motions, the magnetic field strength reaches ~1.5 kG at the surface and ~6 kG in the interior; and the surface structure resembles solar pores. The magnetic structure remains stable for the whole simulation run of several hours with no sign of decay.
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