This paper introduces and analyzes two quantitative proxies to study the axis-nonsymmetrical component of the solar magnetic field over centennial timescales. The first proxy, the "27-day component," is derived using a Fourier transform on daily solar activity data to isolate the energy corresponding to the Sun's rotation period. The second proxy identifies "Recurrent Large Sunspot Structures" (ReLaSS) through an algorithm that tracks large, persistent sunspot groups as they rotate with the Sun.
The study compares these two proxies and reveals significant, previously unknown regularities in solar activity. A key finding is a dramatic shift in their relationship: after decades of strong correlation (1885–1967), the two proxies became strongly anti-correlated from 1967 to 2009. Furthermore, the analysis shows that the overall contribution of both proxies to solar nonsymmetry is lower in the modern era than it was a century ago, suggesting the influence of cycles longer than the centennial Gleissberg cycle. These findings highlight peculiar centennial waves in solar asymmetry that need to be incorporated into solar dynamo models.
Investigating Solar Asymmetry: Key Findings on Sunspot Structures and Rotational Cycles
Introduction: Quantifying Solar Asymmetry: The paper aims to quantify the Sun's longitudinal asymmetry—the tendency for magnetic activity and sunspots to cluster at specific longitudes. This is done by developing two distinct proxies for this non-symmetrical behavior.
Data & Methods: Two Proxies for Asymmetry: The analysis uses daily sunspot number and sunspot area data from 1870 onward.
(Section 3.1) 27-Day Signal: A Fourier transform is applied to the data within a 22-year sliding window to isolate the signal strength of phenomena recurring at the Sun's ~27-day rotation period. This captures the effect of any persistent, rotating feature.
(Section 2) Recurrent Large Sunspot Structures (ReLaSS): An algorithm identifies large sunspot groups that reappear on the solar disk after rotating around the far side. This directly measures the contribution of long-lived, large-scale active regions.
Results: A Shift in Correlation: A major finding is the changing relationship between the two proxies.
(Section 4) 1885-1967: The 27-day component and the ReLaSS contribution were strongly correlated, indicating they were driven by the same phenomena.
(Section 4) 1967-2009: The relationship flipped to a strong anti-correlation. A relative increase in the 27-day component was accompanied by a decrease in the ReLaSS component, and vice-versa. This highlights a fundamental, unexplained shift in the nature of solar asymmetry.
Results: A Precursor Signal: In the period of low solar activity between 1915-1935, the 27-day component was disproportionately strong compared to the overall activity. This preceded the significant rise in solar activity in the 1940s, suggesting this proxy may act as a predictive signal for future cycles.
Conclusion: Evidence for Long-Term Cycles: The contribution of both proxies to the Sun's overall asymmetry is lower in the modern era (1990-2010) than it was approximately 100 years earlier. This suggests the existence of regularities and cycles in solar asymmetry that operate on scales longer than the known 80-100 year Gleissberg cycle.
Conclusion: Implications for Solar Science: The study concludes that the Fourier-based and ReLaSS-based methods capture slightly different aspects of solar asymmetry. The discovery of the correlation shift and long-term trends provides new regularities that must be explained by and incorporated into solar dynamo models to improve the prediction of solar cycles.