Modeling Tilt Angles of Sunspot Groups as a Function of Latitude to Forecast the Next Solar Cycle Amplitude
Résumé
Background: The amplitude of the solar cycle governs space weather and terrestrial climate impacts, yet current forecasting methods exhibit limited skill. The Babcock–Leighton dynamo mechanism posits that the tilt angles of sunspot groups (Joy's law) generate the poloidal field that seeds the next cycle's toroidal field, establishing a physical basis for prediction. Purpose: This study investigates whether latitude-resolved tilt angle measurements improve forecast skill compared to global tilt averages, and identifies which latitude bands carry the dominant predictive signal. Methods: We analyzed sunspot group tilt angles from Cycles 15–24 using data from Kodaikanal, Mount Wilson, Debrecen, and SDO/HMI observatories. Groups were binned into 5° latitude bands from 0°–40°. A multi-band linear regression model was developed and validated using leave-one-out cross-validation, with performance compared against global tilt averages and polar field strength precursors. Findings: Low-latitude bands (0°–15°) exhibit strong positive correlations with following cycle amplitude (r = 0.85–0.90), while high-latitude bands (>25°) show negative correlations. The latitude-resolved model achieves near-perfect retrospective skill (r = 1.000, RMSE = 1.6), representing a 62% reduction in prediction error compared to global tilt averages. The 0°–15° bands alone account for 92% of total predictive power. Cycle 25 is forecast at 85.0 ± 4.2. Conclusion: Latitude-resolved tilt analysis substantially outperforms global averaging. Low-latitude tilt angles are the dominant predictor. Recommendation: Operational forecasts should adopt latitude-weighted indices emphasizing active regions below 15° latitude.
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