Physicists at the University of Warwick have discovered a precise moment in the Sun's 11-year cycle when the most violent space weather abruptly shuts off — and the number of sunspots left behind at this turning point can predict the strength of the next solar cycle up to seven years in advance.

The discovery, presented this week at the Royal Astronomical Society's National Astronomy Meeting in Birmingham, offers the earliest known forecasting method for solar storms, which can disrupt satellites, communications, navigation systems, and electrical power grids on Earth.

"The Sun doesn't gently go to sleep and then gently wake up again," said Professor Sandra Chapman, Director of the Centre for Fusion, Space and Astrophysics at the University of Warwick. "Instead, we've discovered that the most extreme space weather switches off quite suddenly at a specific point in every solar cycle."

This "switch-off" point occurs when active sunspot regions drift below approximately 15 degrees of solar latitude. At this boundary, the Sun's differential rotation — the mechanism believed to drive the most powerful coronal mass ejections — weakens, causing extreme space weather to cease almost overnight.

Using the new technique, researchers have produced an early estimate for Solar Cycle 26, suggesting it could be "moderate" with a sunspot number of roughly 100 to 120 — comparable to or slightly weaker than the current Cycle 25. The method previously forecast that Cycle 25 would be more active than many earlier predictions, which proved accurate as the Sun produced historic geomagnetic storms in May 2024 that made the northern lights visible as far south as Devon and Cornwall in the UK.

Professor Chapman expects the forecast to become significantly more precise in about two years, once the current Solar Cycle 25 reaches its own switch-off point and scientists can base calculations on direct observations rather than projections.

"We're about two years away from the switch-off point for the current Solar Cycle 25," Chapman said. "At the moment, we have to estimate where that point will be, but once we reach it we can use observations alone to make a much more precise prediction for Solar Cycle 26. That will still give us around seven years' warning of how strong the cycle is likely to be."

The research builds on Chapman's earlier "sunclock" system, which places the Sun's irregular cycles onto a standardized clock, and her pioneering work on planetary magnetic fields, for which she earned the Royal Astronomical Society's Chapman Medal in 2022.

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