A new declining phase precursor and an early prediction of cycle 26 maximum
By Sandra Chapman (CFSA, Physics, University of Warwick)
The solar polar magnetic fields during the declining phase of each Schwabe solar cycle 'seed' the toroidal fields that drive sunspot activity of the next cycle. This paper identifies the specific phase of the cycle, and hence the timing, where this relationship should unambiguously be seen, both in models and in high resolution observations. This is central to comparing observations with solar dynamo models as well as providing a precursor method to forecast the upcoming cycle maximum.
The Hilbert transform of 13 month smoothed sunspot number (SSN) since 1749 is used to construct a uniform clock for the Schwabe solar cycle which establishes a clear switch-on and off of geomagnetic activity seen at earth [1] and which correlates with solar morphology on solar cycle scales [2]. By mapping the irregular solar cycle onto a regular clock, the timings of a clear switch-off of activity in the cycle declining phase have been found. The switch-off is when solar eruptions change in character from coronal mass ejections to high speed streams, correlating both with the sunspot active regions moving to lower solar latitudes with reduced differential rotation, and the switch-off of extreme space weather at earth. The SSN at the switch-off is found to correlate well with the following SSN maximum, providing a method for predicting the upcoming cycle maximum on a ~7 year time horizon [3].
[1] S. C. Chapman, S. W. McIntosh, R. J. Leamon, N. W. Watkins, Quantifying the solar cycle modulation of extreme space weather, Geophysical Research Letters, (2020) doi:10.1029/2020GL087795
[2] S. C. Chapman, T. Dudok de Wit, A solar cycle clock for extreme space weather. Sci Rep 14, 8249 (2024). doi:10.1038/s41598-024-58960-5
[3] S. C. Chapman, A new declining phase precursor and an early prediction of cycle 26 maximum, Ap. J. in press (2026) doi:10.3847/1538-4357/ae6859
See publication for more details:
S. C. Chapman, A new declining phase precursor and an early prediction of cycle 26 maximum, Ap. J. in press (2026) doi:10.3847/1538-4357/ae6859

Correlation of the solar maximum sunspot number (SSN) with preceding solar cycle declining phase. Linear regression (black lines) with 68% and 95% confidence bounds (dark and light green shading) of each SSN solar maximum from SILSO plotted versus preceding cycle SSN at switch-off. Black circles indicate each cycle.