Overlapping Magnetic Activity Cycles and the Sunspot Number: Forecasting Sunspot Cycle 25 Amplitude

Abstract: The Sun exhibits a well-observed modulation in the number of spots on its disk over a period of about 11 years. From the dawn of modern observational astronomy sunspots have presented a challenge to understanding -- their quasi-periodic variation in number, first noted 175 years ago, stimulates community-wide interest to this day. A large number of techniques are able to explain the temporal landmarks, (geometric) shape, and amplitude of sunspot "cycles," however forecasting these features accurately in advance remains elusive. Recent observationally-motivated studies have illustrated a relationship between the Sun's 22-year (Hale) magnetic cycle and the production of the sunspot cycle landmarks and patterns, but not the amplitude of the sunspot cycle. Using (discrete) Hilbert transforms on more than 270 years of (monthly) sunspot numbers we robustly identify the so-called "termination" events that mark the end of the previous 11-yr sunspot cycle, the enhancement/acceleration of the present cycle, and the end of 22-yr magnetic activity cycles. Using these we extract a relationship between the temporal spacing of terminators and the magnitude of sunspot cycles. Given this relationship and our prediction of a terminator event in 2020, we deduce that Sunspot Cycle 25 could have a magnitude that rivals the top few since records began. This outcome would be in stark contrast to the community consensus estimate of sunspot cycle 25 magnitude.

• The paper introduces a novel methodology combining discrete Hilbert transforms with historical sunspot data to robustly predict SC25 amplitude.
• The study identifies termination events as pivotal, revealing a strong anti-correlation between the gap of these events and subsequent cycle strength.
• Regression analysis indicates that shorter intervals between terminators correspond to higher sunspot counts, challenging conventional forecasting models.

Sunspot Cycle 25 Prediction: Methodology and Analysis

The paper explores the intricacies of solar magnetic cycles and their relationship with sunspot number variations to forecast the amplitude of Sunspot Cycle 25 (SC25). It introduces a novel methodology combining discrete Hilbert transforms and historical sunspot data to predict a potentially high amplitude for SC25, challenging prevailing consensus within the solar research community.

The research begins by addressing the challenge presented by the overlapping of solar magnetic activity cycles with the 11-year sunspot cycle. Despite various prediction techniques developed over decades, accurate forecasting of sunspot cycle amplitude has remained elusive. Recent studies have established links between the Sun’s 22-year Hale magnetic cycle and sunspot activity, yet amplitude prediction issues persisted.

The authors propose that termination events of sunspot cycles—pointing to cessation of preceding cycle activity—play a critical role in understanding and predicting future sunspot cycle amplitude. By examining 270 years of monthly sunspot numbers through discrete Hilbert transforms, the study identifies these termination events with unprecedented robustness.

Findings from the paper suggest a strong anti-correlation between the temporal spacing of solar cycle termination events and subsequent sunspot cycle amplitude. Regression analysis reveals that shorter spacing between terminators corresponds to higher amplitude sunspot cycles. By predicting a terminator event around 2020, the study forecasts SC25 to potentially reach one of the highest amplitudes recorded, contrasting sharply with the NOAA/NASA Solar Cycle 25 Prediction Panel’s consensus of modest cycle strength.

To ensure robustness, the research scrutinized variations in smoothing windows applied during Hilbert transform calculations. The findings indicate that window parameters significantly affect terminator date determinations and subsequent cycle amplitude predictions. Nevertheless, the proposed method showcases superior predictive capability compared to traditional minimum-to-minimum cycle length metrics used historically.

The paper challenges conventional solar cycle amplitude prediction methodologies, which often regard magnetic fields as passively interacting with large-scale solar flows. The authors argue for a paradigm shift in understanding solar dynamics, emphasizing active, dynamically significant magnetic band interactions. Implications for solar dynamo models—utilizing polar magnetic field measurements as proxies—must be reconsidered if SC25 follows the prediction outlined here.

This work's prediction of SC25 as potentially one of the strongest sunspot cycles observed delineates a significant deviation from consensus forecasts, emphasizing the necessity for broader exploration of solar magnetic phenomena in predictions. As SC25 evolves, empirical validation will determine the efficacy of this novel predictive approach, potentially refining understanding of solar internal dynamics and magnetic cycle interactions. The arrival of SC24’s terminator will enable further precision in predicting SC25’s amplitude, providing crucial insights into solar cycle forecasting methodologies and their development.