Dikpati Mausumi, Suresh Akshaya, Burkepile Joan
High Altitude Observatory, National Center for Atmospheric Research, 3080 Center Green, Boulder, CO 80307-3000 United States.
Department of Astronomy, Yale University, New Haven, CT United States.
Sol Phys. 2016;291:339-355. doi: 10.1007/s11207-015-0831-8. Epub 2015 Dec 29.
The structure of the Sun's corona varies with the solar-cycle phase, from a near spherical symmetry at solar maximum to an axial dipole at solar minimum. It is widely accepted that the large-scale coronal structure is governed by magnetic fields that are most likely generated by dynamo action in the solar interior. In order to understand the variation in coronal structure, we couple a potential-field source-surface model with a cyclic dynamo model. In this coupled model, the magnetic field inside the convection zone is governed by the dynamo equation; these dynamo-generated fields are extended from the photosphere to the corona using a potential-field source-surface model. Assuming axisymmetry, we take linear combinations of associated Legendre polynomials that match the more complex coronal structures. Choosing images of the global corona from the Mauna Loa Solar Observatory at each Carrington rotation over half a cycle (1986 - 1991), we compute the coefficients of the associated Legendre polynomials up to degree eight and compare with observations. We show that at minimum the dipole term dominates, but it fades as the cycle progresses; higher-order multipolar terms begin to dominate. The amplitudes of these terms are not exactly the same for the two limbs, indicating that there is a longitude dependence. While both the 1986 and the 1996 minimum coronas were dipolar, the minimum in 2008 was unusual, since there was a substantial departure from a dipole. We investigate the physical cause of this departure by including a North-South asymmetry in the surface source of the magnetic fields in our flux-transport dynamo model, and find that this asymmetry could be one of the reasons for departure from the dipole in the 2008 minimum.
太阳日冕的结构随太阳活动周期阶段而变化，从太阳活动极大期的近似球对称到太阳活动极小期的轴向偶极。人们普遍认为，大规模日冕结构受磁场支配，而磁场很可能是由太阳内部的发电机作用产生的。为了理解日冕结构的变化，我们将一个势场源表面模型与一个循环发电机模型相结合。在这个耦合模型中，对流区内的磁场由发电机方程支配；这些由发电机产生的场利用势场源表面模型从光球层延伸到日冕。假设轴对称性，我们采用相关勒让德多项式的线性组合来匹配更复杂的日冕结构。从莫纳罗亚太阳天文台在半个周期（1986 - 1991年）内每个卡林顿自转时获取的全球日冕图像中，我们计算了高达八阶的相关勒让德多项式的系数，并与观测结果进行比较。我们表明，在极小期偶极项占主导，但随着周期的推进它会减弱；高阶多极项开始占主导。这两个边缘的这些项的振幅并不完全相同，表明存在经度依赖性。虽然1986年和1996年的极小期日冕都是偶极的，但2008年的极小期不同寻常，因为它与偶极有很大偏差。我们通过在通量传输发电机模型中在磁场的表面源中纳入南北不对称性来研究这种偏差的物理原因，发现这种不对称性可能是2008年极小期偏离偶极的原因之一。
