[论文解读] Magnetic Twist and Writhe of Active Regions: On the Origin of Deformed Flux Tubes

M. C. López Fuentes, P. Démoulin+3 more|Digital Access to Libraries|Nov 20, 2014

Solar and Space Plasma Dynamics参考文献 44被引用 59

一句话总结

本研究调查了22个双极活动区（RTARs）中扭曲和形变磁通量管的起源，这些活动区表现出其磁极长期旋转。利用观测到的倾角演化和磁场测量数据，检验了螺旋转 instability 或科里奥利力是否能解释形变，发现仅有35%的案例与螺旋转不稳定性一致，41%与科里奥利效应一致，表明对流层中大尺度涡流流动很可能是主导机制。

ABSTRACT

We study the long term evolution of a set of 22 bipolar active regions (ARs) in which the main photospheric polarities are seen to rotate one around the other during several solar rotations. We first show that differential rotation is not at the origin of this large change in the tilt angle. A possible origin of this distortion is the nonlinear development of a kink-instability at the base of the convective zone; this would imply the formation of a non-planar flux tube which, while emerging across the photosphere, would show a rotation of its photospheric polarities as observed. A characteristic of the flux tubes deformed by this mechanism is that their magnetic twist and writhe should have the same sign. From the observed evolution of the tilt of the bipoles, we derive the sign of the writhe of the flux tube forming each AR; while we compute the sign of the twist from transverse field measurements. Comparing the handedness of the magnetic twist and writhe, we find that the presence of kink-unstable flux tubes is coherent with no more than 35\% of the 20 cases for which the sign of the twist can be unambiguously determined. Since at most only a fraction of the tilt evolution can be explained by this process, we discuss the role that other mechanisms may play in the inferred deformation. We find that 36\% of the 22 cases may result from the action of the Coriolis force as the flux tube travels through the convection zone. Furthermore, because several bipoles overpass in their rotation the mean toroidal (East-West) direction or rotate away from it, we propose that a possible explanation for the deformation of all these flux tubes may lie in the interaction with large-scale vortical motions of the plasma in the convection zone, including also photospheric or shallow sub-photospheric large scale flows.

研究动机与目标

• 确定双极活动区（RTARs）中磁极长期相对旋转的起源，这些活动区在多个太阳自转周期内表现出磁极相对旋转。
• 检验对流层底部的螺旋转不稳定性是否能通过比较磁扭曲和缠绕符号来解释通量管的观测形变。
• 评估科里奥利力在塑造新生通量管倾角演化中的作用，特别是与乔伊定律和赤道向迁移的关系。
• 评估对流层或光球层中大尺度涡流或湍流运动作为通量管形变的替代机制的贡献。
• 确定诸如螺旋转不稳定性或科里奥利力等机制是否能在统计上显著解释RTARs中观测到的旋转行为。

提出的方法

• 使用NOAA黑子数和太阳圆盘重新出现记录，追踪22个双极活动区（RTARs）的长期演化，以监测多个太阳自转周期内的倾角变化。
• 从光球层中主要磁极的旋转方向推导缠绕符号，假设顺时针/逆时针旋转分别对应负/正缠绕。
• 从光球层中的横向磁场测量计算磁扭曲符号，利用矢量磁像仪数据推断螺旋度含量。
• 通过比较扭曲和缠绕的符号来检验螺旋转不稳定性假说，该假说预测螺旋转不稳定的管中扭曲和缠绕符号相同。
• 通过分析观测到的旋转趋势是否与对流层中行星自转引起的预期偏转一致，评估科里奥利力机制。
• 提出对流层或光球层中的大尺度涡流或湍流运动可能是导致形变的机制，特别是在螺旋转不稳定性或科里奥利力无法解释的案例中。

实验结果

研究问题

• RQ1 双极活动区中观测到的倾角旋转变化是否源于磁通量管的螺旋转不稳定性？
• RQ2 科里奥利力是否足以解释新生磁通量管中磁极的观测旋转？
• RQ3 在22个RTAR中，有多少比例可由螺旋转不稳定性或科里奥利力解释，其余部分可能由何种机制解释？
• RQ4 观测到的活动区中磁扭曲和缠绕的符号是否与螺旋转不稳定通量管的理论预测一致？
• RQ5 对流层或光球层中的大尺度涡流或湍流运动是否可能是RTAR中通量管形变的主导机制？

主要发现

• 较差自转无法解释观测到的倾角变化，排除了大尺度剪切作为磁极旋转主要驱动力的可能性。
• 在20个具有明确扭曲符号判断的RTAR中，仅有35%的案例表现出扭曲和缠绕符号一致，表明螺旋转不稳定性在大多数情况下并非主导机制。
• 约41%（22个中的9个）RTAR表现出与科里奥利力一致的旋转行为，表明其可能发挥显著但非主导作用。
• 同样比例（22个中的9个）RTAR无法由螺旋转不稳定性或科里奥利力解释，表明需要替代机制。
• 作者提出，对流层、光球层或浅层下光球层中的大尺度涡流或湍流运动可能在未解释案例中导致通量管形变。
• 本研究结论认为，单一机制（螺旋转不稳定性、科里奥利力或涡流运动）无法完全解释观测行为，多种机制可能协同作用，而涡流运动可能影响所有RTAR。