Long term sunspot number records, extreme space weather events at earth seen in the aa index, and their solar cycle modulation

Multi-solar cycle observations of sunspot number and location on the solar disk, combined with the occurrence of  extreme geomagnetic storms at earth, can be used to identify key points in the solar cycle where the occurrence of extreme space weather events switches on and off. The variable length, approximately 11 year Schwabe cycle can be mapped onto a uniform length cycle (or solar cycle clock) using the Hilbert transform of sunspot number[1,2].  The switch on and off times of geomagnetic activity over each solar cycle can be directly identified from the sunspot number record[3], without requiring a Hilbert transform. This analysis has revealed a sharp switch on and off of geomagnetic activity, with some of the most extreme events occurring close to the switch on/off, rather than at solar maximum[4]. A detailed exploration is made of the locations of individual sunspot groups on the solar disk, hemispheric sunspot numbers, and their correlation with extreme events in the aa record. As well as informing our overall understanding of extreme space weather events, these findings can translate model predictions of sunspot number and morphology into timing of the switch-off and on, offering a route to quantitative estimates of future space weather risk.

[1] Chapman et al Quantifying the solar cycle modulation of extreme space weather GRL (2020) doi:10.1029/2020GL087795

[2] Chapman et al The Sun's magnetic (Hale) cycle and 27 day recurrences in the aa geomagnetic index. Ap. J. (2021) doi: 10.3847/1538-4357/ac069e

[3] Chapman Charting the Solar Cycle, Front. Astron. Space Sci. - Space Physics (2023) doi: 10.3389/fspas.2022.1037096

[4] Chapman et al A solar cycle clock for extreme space weather. Scientific Reports (2024) doi:10.1038/s41598-024-58960-5