Sandra Chapman and her team at the University of Warwick’s Centre for Fusion, Space and Astrophysics have identified a sharp cutoff moment inside every solar cycle — a point where the sun’s most violent space weather stops almost overnight — and they say counting sunspots at that moment can forecast the strength of the next cycle six to seven years before it peaks. Chapman is presenting the finding this week at the Royal Astronomical Society’s National Astronomy Meeting in Birmingham.

The team used the method to correctly call that Solar Cycle 25 would run stronger than mainstream forecasts suggested. That call was vindicated when the May 2024 “Gannon” storm pushed the aurora as far south as Devon and Cornwall in the UK — and, worldwide, as far south as Mexico and Puerto Rico.

A sudden switch, not a slow fade

The sun’s roughly 11-year cycle has been mapped since the 19th century. Sunspot counts rise, peak, and fall as the star’s magnetic field prepares to flip polarity. What Chapman’s group argues is that the descent isn’t gradual for the events that matter most to power grids and satellites.

“The sun doesn’t gently go to sleep and then gently wake up again,” Chapman said in the Warwick announcement. “Instead, we’ve discovered that the most extreme space weather switches off quite suddenly at a specific point in every solar cycle. By identifying that point, we’ve found a new way to predict how active the next solar cycle is likely to be.”

That switch-off point carries a signature. In a 2024 paper in Scientific Reports, Chapman and co-author Thierry Dudok de Wit showed that the clean cutoff of extreme space weather occurs when more than 90 percent of solar active-region areas have migrated to within about 15 degrees of the solar equator, into the band where solar differential rotation weakens. Read the fingerprint on the way down, and the shape of the next climb becomes visible.

Why a six-year lead time matters

Existing solar cycle predictions typically firm up only a year or two before solar maximum, when precursor signals in the sun’s polar magnetic fields become measurable. That short horizon leaves satellite operators, grid managers, and airlines with limited time to harden infrastructure against a strong cycle.

The stakes are not hypothetical. The March 1989 geomagnetic storm collapsed the Hydro-Québec grid in roughly 90 seconds, cutting power to six million people in a Quebec winter. The 1859 Carrington storm did worse to a much smaller technological footprint. A comparable event today would hit a far more electrified, satellite-dependent civilization.

A six-to-seven-year lead time changes the calculus. Utilities plan grid upgrades on decadal timescales. Satellite constellations are procured years in advance. Knowing whether the next cycle will hammer the ionosphere or barely nudge it lets those decisions be made with better information.

The Cycle 26 preview

An early application of the technique — with the caveat that Cycle 25 has not yet reached its switch-off — points to a moderate Solar Cycle 26 with a peak sunspot number of around 100 to 120. That would put the coming cycle at roughly the same strength as Cycle 25, or slightly weaker.

“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.”

Independent statistical work supports the moderate outlook. Time-series analyses of monthly sunspot numbers also project Cycle 26 in a similar moderate range, though through purely statistical approaches rather than through a physical switch-off mechanism.

Validated by 2024’s auroral fireworks

The method’s credibility rests on Cycle 25. Official consensus forecasts predicted a weak cycle, similar to the anemic Cycle 24. The switch-off approach flagged something stronger. The sun then delivered.

The May 2024 “Gannon” storm was the first extreme event of Cycle 25 and the largest geomagnetic storm in more than two decades. Its maximum modelled geomagnetically induced current in the UK — 68 amps at the Landulph substation in Cornwall — was recorded by the British Geological Survey. Auroras were widely visible across the continental United States, Europe, and as far south as Mexico. NOAA has continued issuing geomagnetic storm warnings through the current maximum, with X-class flares triggering radio blackouts and mid-latitude aurora alerts.

2026 could deliver another strong year for Northern Lights displays as Cycle 25 works through its declining phase — precisely the window in which the switch-off signature is expected to appear.

The sunclock behind the discovery

The switch-off finding builds on Chapman’s earlier “sunclock” concept, which uses a Hilbert transform of the sunspot record to map the sun’s irregular cycles onto a standardized clock face. Because no two solar cycles run the same length — some stretch past 12 years, others contract to nine — comparing them directly is difficult. The sunclock normalizes time, letting researchers overlay cycles and see recurring features.

When cycles are stacked on the sunclock, the abrupt cessation of extreme events lines up at a consistent phase. That regularity is what allows the sunspot count at the switch-off to carry predictive information about the next cycle. A follow-up paper in Frontiers in Astronomy and Space Sciences showed the switch-off can be estimated directly from the raw sunspot time series, without the edge effects of the Hilbert transform.

The underlying physics has now been sharpened. Chapman’s team argues that the extreme storms are powered by differential rotation twisting the emerging magnetic field, and that this driver shuts down once active regions cross into the equatorial band where the sun rotates as a nearly rigid body. That is the mechanism doing the switching.

Forecasting as infrastructure

Space weather forecasting has quietly become critical infrastructure. GPS accuracy, transoceanic aviation routes, satellite lifetimes, and grid stability all depend on knowing what the sun is about to do. The gap between the current generation of forecasts and the lead times needed for major hardening decisions has been one of the field’s chronic frustrations.

A method that pushes reliable prediction out to seven years — if it holds through independent testing on future cycles — would reshape how operators plan. It would also shift some of the risk conversation from reactive scrambling during a storm to proactive engineering during quiet periods.

The technique still needs validation across multiple cycles before it becomes an operational tool. Cycle 25’s switch-off, expected around 2028 on Chapman’s current estimate, will be the critical test.

What to watch

Three things will determine whether the switch-off method graduates from promising to operational.

First, the timing of Cycle 25’s actual switch-off point. Chapman’s current estimate places it roughly two years out. If extreme events taper on schedule, the physical model gains credibility.

Second, the sunspot count at that moment. That number, run through the correlation, will yield a firm prediction for Cycle 26’s peak. Comparing it against other forecasting methods — polar field observations, machine learning models, dynamo simulations — will show whether switch-off outperforms.

Third, the physical mechanism. A prediction that works empirically but lacks a causal explanation is fragile. The differential-rotation-and-15-degree-band argument gives switch-off a candidate mechanism; testing it against the next few cycles will show whether it holds up.

For now, the sun is doing what suns do — pumping out flares, flipping its magnetic field, and dragging the terrestrial electromagnetic environment along with it. The difference is that a small group of physicists in Coventry believe they have found the moment where the star tips its hand about what comes next.