[论文解读] Sunspot rotation. I. A consequence of flux emergence

Zoe Sturrock, A. W. Hood+2 more|arXiv (Cornell University)|Aug 10, 2015

Solar and Space Plasma Dynamics参考文献 27被引用 13

一句话总结

本研究通过三维阻抗磁流体动力学（MHD）模拟表明，黑子旋转源于扭曲磁通管的浮现，其中磁张力产生不平衡力矩，驱动光球层足点的旋转运动。模拟显示磁力线最大旋转达353°，并证实了光球层以下的扭曲通过螺旋度通量被输运至日球层，解释了磁通量浮现如何直接导致观测到的黑子旋转及日冕能量积聚。

ABSTRACT

Context. Solar eruptions and high flare activity often accompany the rapid rotation of sunspots. The study of sunspot rotation and the mechanisms driving this motion are therefore key to our understanding of how the solar atmosphere attains the conditions necessary for large energy release. Aims. We aim to demonstrate and investigate the rotation of sunspots in a 3D numerical experiment of the emergence of a magnetic flux tube as it rises through the solar interior and emerges into the atmosphere. Furthermore, we seek to show that the sub-photospheric twist stored in the interior is injected into the solar atmosphere by means of a definitive rotation of the sunspots. Methods. A numerical experiment is performed to solve the 3D resistive magnetohydrodynamic (MHD) equations using a Lagrangian-Remap code. We track the emergence of a toroidal flux tube as it rises through the solar interior and emerges into the atmosphere investigating various quantities related to both the magnetic field and plasma. Results. Through detailed analysis of the numerical experiment, we find clear evidence that the photospheric footprints or sunspots of the flux tube undergo a rotation. Significant vertical vortical motions are found to develop within the two polarity sources after the field emerges. These rotational motions are found to leave the interior portion of the field untwisted and twist up the atmospheric portion of the field. This is shown by our analysis of the relative magnetic helicity as a significant portion of the interior helicity is transported to the atmosphere. In addition, there is a substantial transport of magnetic energy to the atmosphere. Rotation angles are also calculated by tracing selected fieldlines; the fieldlines threading through the sunspot are found to rotate through angles of up to 353 degrees over the course of the experiment.

研究动机与目标

• 研究观测到的黑子旋转背后的物理机制，特别是其与磁通量浮现的关联。
• 确定黑子旋转是真实的动力学效应，还是观测中的表观伪影。
• 量化磁通量管浮现过程中，从光球层以下内部向太阳大气传输的磁螺旋度与能量。
• 研究磁张力在产生不平衡力矩并驱动黑子光球层足点旋转运动中的作用。
• 验证在浮现过程中，扭转阿尔文波将扭曲从内部传递至日冕的假设。

提出的方法

• 使用拉格朗日-重映射（Lagrangian-Remap）数值格式求解三维阻抗磁流体动力学（MHD）模型，以模拟磁通管浮现。
• 模拟磁环形磁通管在太阳内部的上升过程及其向大气层的浮现。
• 计算磁螺旋度与能量通量，以量化从内部向大气层的扭曲传输。
• 追踪从计算域底部穿过光球层的磁力线，直接计算旋转角度。
• 分析垂直电流密度（j_z）与涡旋运动，评估内部的解扭与大气层中的缠绕。
• 评估磁张力的作用，以判断其是否产生不平衡力矩从而驱动旋转。

实验结果

研究问题

• RQ1 在磁通量浮现过程中，导致黑子观测旋转的物理机制是什么？
• RQ2 这种旋转是磁力驱动的真实动力学效应，还是观测中的表观伪影？
• RQ3 来自光球层以下磁通管的磁螺旋度在多大程度上传输至太阳大气层？
• RQ4 磁张力如何促进光球层足点旋转运动的产生？
• RQ5 磁力线在时间推移中穿过黑子的定量旋转角度是多少？

主要发现

• 黑子旋转由磁张力产生的不平衡力矩驱动，而非表观效应。
• 穿过黑子的磁力线在模拟过程中最大旋转达353°，与观测到的旋转角度一致。
• 内部磁螺旋度的显著部分——3.6×10¹⁹ Mx²——被输运至大气层，证实了浮现过程中的螺旋度通量。
• 内部磁螺旋度减少，而大气层磁螺旋度增加，表明内部发生解扭，大气层发生缠绕。
• 内部垂直电流密度（j_z）减小，支持光球层以下磁场的解扭。
• 旋转运动同时以两个黑子磁极源为中心，浮现后光球层形成涡旋流。