A regular clock for the solar cycle variation of sunspot and geomagnetic activity

Sunspot records reveal that whilst the sun has an approximately 11 year cycle of activity, no two cycles are of the same duration. Since this activity is a direct driver of space weather at earth, this presents an operational challenge to quantifying space weather risk. The sunspot number record can be used to map the variable cycle length onto a regular 'clock' and this mapping reveals in each cycle a clear active-quiet switch-off and quiet-active switch-on of activity, with around 2% of extreme space weather events occurring within the quiet intervals of the cycles over the last 155 years [1]. Some of the most extreme geomagnetic storms have occurred around the switch-on and switch-off times, rather than at solar maximum, motivating their determination and prediction. The times of the switch-on/off can be approximately determined directly from the sunspot time-series [2] so that future switch-on and switch-off times can be directly identified in model predictions for future solar activity as characterized by sunspot number. The clock supports charting – a tool to integrate observational estimates of risk (observed events and their likelihood) with narrative reports of impacts on technological systems to improve our understanding of space weather hazard. The sunspot number Hilbert transform phase is found to correspond to solar-cycle scale evolution of sunspot latitudinal bands, so that there is a direct relationship between the well known sunspot ‘butterfly pattern’ and the intensity and character of geomagnetic activity and its switch-on/off [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, Charting the Solar Cycle, Front. Astron. Space Sci. - Space Physics,  (2023) doi:10.3389/fspas.2022.1037096

[3] S. C. Chapman, T. Dudok de Wit, A solar cycle clock for extreme space weather (preprint, 2024) doi:10.21203/rs.3.rs-3672243/v1