Sunspot activity and the long‐term variation of the Sun's open magnetic flux
Sunspot activity and the long‐term variation of the Sun's open magnetic flux
复制标题
DOI：
10.1029/2001ja000500
复制
发表时间：
2002-10
影响因子：
--
通讯作者：
Y.-M. Wang;N. Sheeley
中科院分区：
文献类型：
--
作者：
Y.-M. Wang;N. Sheeley
[1] 行星际磁场（IMF）起源于太阳的开放磁区（日冕洞），而日冕洞又主要通过活跃区场的出现和分散而形成。径向 IMF 强度与总开路通量 Φopen 成正比，在校正磁图饱和效应后，可以根据测量的光球场的源表面外推来估计总开路通量 Φopen。我们推导了 1971-2000 年间 Φopen 的长期变化，并讨论了它与太阳黑子活动的关系。 1976-1996 年期间 Φopen 的平均值比 1971-1976 年和 1996-2000 年期间高约 20-30%，主要峰值出现在 1982 年和 1991 年。在太阳黑子极小期附近，大部分开放通量驻留在大的极地冕洞中，而在太阳黑子极大期，它植根于位于活动区附近的相对较小的低纬度洞中，并被表征为通过强大的脚点场；由于空穴占据的总面积的减少被其平均场强的增加所抵消，因此 Φopen 在活动最小值和最大值之间大致保持恒定，这与总光球通量 Φtot 不同。 Φopen 的长期变化大致遵循太阳总偶极子强度的变化，其中太阳黑子最大值周围的磁四极子强度有所贡献。太阳黑子活动的全球波动导致赤道偶极子强度增加，从而导致 Φopen 和 IMF 强度增强，通常持续约 1 年。我们采用模拟来阐明活跃区域出现和光球传输过程在开放通量演化中的作用。通过一个或多个双极磁区（BMR）表示初始场配置，我们计算了其在差分旋转、超粒对流和向极体流影响下的后续演化。 Φopen 的初始值很大程度上取决于赤道偶极子强度，而赤道偶极子强度又取决于 BMR 之间的纵向相位关系。当表面流将 BMR 通量带到更高纬度时，赤道偶极子在旋转剪切和湍流扩散的综合作用下，在约 1 年的时间尺度上湮灭。剩余通量集中在极点周围，并且 Φopen 接近极限值，该极限值取决于原始 BMR 的轴对称偶极子强度。因此，极冕洞代表了周期早期出现的活跃区域的长寿命、轴对称残余物。
[1] The interplanetary magnetic field (IMF) originates in open magnetic regions of the Sun (coronal holes), which in turn form mainly through the emergence and dispersal of active region fields. The radial IMF strength is proportional to the total open flux Φopen, which can be estimated from source surface extrapolations of the measured photospheric field, after correction for magnetograph saturation effects. We derive the long-term variation of Φopen during 1971–2000 and discuss its relation to sunspot activity. The average value of Φopen was ∼20–30% higher during 1976–1996 than during 1971–1976 and 1996–2000, with major peaks occurring in 1982 and 1991. Near sunspot minimum, most of the open flux resides in the large polar coronal holes, whereas at sunspot maximum it is rooted in relatively small, low-latitude holes located near active regions and characterized by strong footpoint fields; since the decrease in the total area occupied by holes is offset by the increase in their average field strengths, Φopen remains roughly constant between activity minimum and maximum, unlike the total photospheric flux Φtot. The long-term variation of Φopen approximately follows that of the Sun's total dipole strength, with a contribution from the magnetic quadrupole around sunspot maximum. Global fluctuations in sunspot activity lead to increases in the equatorial dipole strength and hence to enhancements in Φopen and the IMF strength lasting typically ∼1 year. We employ simulations to clarify the role of active region emergence and photospheric transport processes in the evolution of the open flux. Representing the initial field configuration by one or more bipolar magnetic regions (BMRs), we calculate its subsequent evolution under the influence of differential rotation, supergranular convection, and a poleward bulk flow. The initial value of Φopen is determined largely by the equatorial dipole strength, which in turn depends on the longitudinal phase relations between the BMRs. As the surface flow carries the BMR flux to higher latitudes, the equatorial dipole is annihilated on a timescale of ∼1 year by the combined effect of rotational shearing and turbulent diffusion. The remaining flux becomes concentrated around the poles, and Φopen approaches a limiting value that depends on the axisymmetric dipole strengths of the original BMRs. The polar coronal holes thus represent the long-lived, axisymmetric remnant of the active regions that emerged earlier in the cycle.