Zi-Fan Wang^1,2, Jie Jiang^3,4, Jie Zhang⁵, Jing-Xiu Wang^2,1
1. Key Laboratory of Solar Activity, National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, People’s Republic of China
2. School of Astronomy and Space Science, University of Chinese Academy of Sciences, Beijing, People’s Republic of China
3. School of Space and Environment, Beihang University, Beijing, People’s Republic of China
4. Key Laboratory of Space Environment Monitoring and Information Processing of MIIT, Beijing, People’s Republic of China
5. Department of Physics and Astronomy, George Mason University, Fairfax, VA 22030, USA

Activity complexes (ACs) are defined as a sequence of closely located and continuously emerging active regions (ARs)^[1]. The popularity of ACs indicates a general “nesting” trend of ARs’ emergence, which is an important characteristic of the solar toroidal magnetic fields. During the evolution of an AC, as the ARs emerge and cancel frequently within the AC, large unipolar flux regions of opposite signs can form at the separate edges of the AC^[2]. The poleward migration of such regions can introduce prominent poleward surges representing inhomogeneous poleward flux-transport patterns, which are usually seen on magnetic butterfly diagrams. Poleward surges are possible to cause strong short-term turbulence of polar fields and interplanetary field. Such surges may also influence the long-term evolution of the polar fields, hence the development of the subsequent solar cycle strength as described by Babcock-Leighton solar dynamo models.

Figure 1| Magnetic butterfly diagram of Solar Cycle 24 generated from the longitudinally averaged SDO/HMI synoptic maps. The poleward surge that we studied is within the black dashed ellipse.

During Cycle 24, the most prominent poleward surge originates in 2014 (Carrington Rotations, CRs 2145-2159) in the southern hemisphere (Figure 1). The flux from the surge dominates the southern polar field evolution during the latter half of Cycle 24 and provides a major contribution to the polar field during Cycle 24 minimum. To clearly show the significance of the prominent surge for the long-term evolution of the surface large-scale field, we automatically identify and analyze the ARs during CRs 2145-2159 from the SDO/HMI radial magnetic field synoptic maps. We then utilize the surface flux transport (SFT) model^[3] to simulate the migration of the ARs’ flux, reconstruct the formation and transportation of the prominent surge, and analyze the associated polar field development.

Figure 2| Stack plot of magnetograms during CRs 2145-2159 of the southern hemisphere, with the identified ARs outlined by dark curves. Each magnetogram is displayed in equal sine-latitude in the vertical axis. Dark solid boxes mark the ARs that can be regarded as ACs. Dark dashed box marks the generation of unipolar regions.

A total of 84 ARs are identified during CRs 2145-2159. The latitudes of the ARs fall between 10°S to 20°S. The ARs with lower latitudes are likely to be influential to the polar field at the cycle minimum. We show these ARs on the stack-plot constructed from the southern part of the synoptic maps (Figure 2). As can be seen, some ARs fit the concept of ACs. The ARs between 180° and 270° longitudes belong to a long-lasting AC, in which unipolar regions of large area form and evolve. There are other ARs that can be regarded as components of ACs, e.g. the ARs between 50° and 135° longitudes.

The results of the polar field development derived based on the SFT simulations are shown in Figure 3. The polar field development introduced by simulating the ARs during CRs 2145-2159 (red lines) matches the observational polar field evolution (black lines). This indicates that the ARs emerged during CRs 2145-2159 have a dominating effect on the long-term polar field evolution, especially in the southern hemisphere, while the ARs after CR 2159 have only a minor collective effect. Without the ARs during CRs 2145-2159, the polar field would remain at a low level (blue lines), and even reverse to the previous polarity at the cycle minimum. The ARs during CRs 2145-2159 dominates the polar field evolution during the latter half of Cycle 24 and the cycle minimum, and in turn determines the strength of the next cycle.

Figure 3| Temporal evolution of the polar field. Solid (dashed) lines indicate the southern (northern) hemisphere. Black lines are observational data. Red lines are results of the simulation with initial field of CR2144 and with ARs during CRs 2145-2159 assimilated. Blue lines are results of the simulation with initial field of CR 2144 but without the ARs assimilated. The vertical black dashed line near year 2015 indicates CR 2159, the last CR with ARs assimilated.

Our study shows that the prominent poleward surge is generated by the ARs primarily in the form of ACs, and causes significant long-term polar field influence. Unlike the surge which does not influence the polar field’s long-term development studied by Yeates et al.^[4], the surge we study is crucial for the development of the solar cycle. Such a “super surge” is unique to Cycle 24. Meanwhile, it is noticeable that there exists apparently similar surges in other solar cycles. We consider that these surges and the characteristics of originating ARs should be analyzed by similar means, in order to further understand the toroidal-to-poloidal processes of dynamo models.

For more information, please refer to Ref. [5].

References

[1] Gaizauskas, V., Harvey, K. L., Harvey, J. W., et al. 1983, ApJ, 265, 1056
[2] Gaizauskas, V. 2008, ApJ, 686, 1432
[3] Jiang, J., Hathaway, D. H., Cameron, R. H., et al. 2014,SSRv, 186, 491
[4] Yeates, A. R., Baker, D., & van Driel-Gesztelyi, L. 2015, SoPh, 290, 3189
[5] Wang, Z.-F., Jiang, J., Zhang, J., et al. 2020, ApJ, 904, 62