The method focuses on the number of sunspots present at a newly identified "switch-off" stage in the solar cycle. At this point, the Sun's most severe space weather appears to end abruptly. Researchers have already used the approach to produce an early estimate for Solar Cycle 26.

Initial projections suggest that Cycle 26 could be moderate, with a sunspot number of roughly 100 to 120. That would make it comparable to, or possibly weaker than, the current Solar Cycle 25. However, researchers will not be able to make a more precise forecast for about two years, and both stronger and weaker outcomes remain theoretically possible.

The findings are being presented this week at the Royal Astronomical Society's National Astronomy Meeting in Birmingham.

The Sun's Extreme Weather Stops Suddenly

Sandra Chapman, Professor of Physics and Director of the Centre for Fusion, Space and Astrophysics at the University of Warwick, said: "The Sun doesn't gently go to sleep and then gently wake up again.

"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."

Professor Chapman expects the Cycle 26 forecast to become much more accurate in about two years. By then, Solar Cycle 25 should reach the newly identified "switch off" point, allowing scientists to base their calculations on direct observations instead of projections.

The Sun moves through a cycle lasting approximately 11 years. During that time, its magnetic field reverses polarity, while the number of visible sunspots increases and then declines.

Sunspots are highly active magnetic regions on the Sun's surface. They can produce powerful solar flares and coronal mass ejections, which send energy and charged particles into space. This space weather can interfere with satellites, communications, navigation systems, and electrical power grids on Earth.

Astronomers have tracked sunspots for centuries, but every solar cycle behaves differently. Some cycles are longer or shorter than others, and their intensity can vary considerably. These differences have made it difficult to forecast the strength of an upcoming cycle.

A New Solar Clock Improves Forecasting

The new technique builds on Professor Chapman's earlier "sunclock," a system that places the Sun's irregular cycles onto a standardized clock. That work showed that extreme space weather does not slowly disappear as a cycle winds down. Instead, it ends at a clearly defined stage.

Professor Chapman and her colleagues found that the number of sunspots visible at this stage is closely connected to the highest sunspot count reached during the next solar cycle.

This relationship creates a new way to estimate the strength of a future cycle about six to seven years before it reaches its peak. Existing forecasting methods generally provide less advance notice because scientists must wait until the Sun reaches solar minimum, the quietest part of its cycle.

The technique also points to a particular phase when the magnetic field responsible for the next cycle is expected to become established. Researchers hope that this timing will help them better understand the solar dynamo, the process that creates and maintains the Sun's magnetic field.

Professor Chapman said: "We're about two years away from the switch-off point for the current Solar Cycle 25. 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."

Earlier Forecast Correctly Anticipated Cycle 25

The method previously indicated that Solar Cycle 25 would be more active than many earlier forecasts had predicted. That stronger activity helped produce the striking auroral displays seen in recent years.

The UK experienced several historic solar storms in 2024 as Cycle 25 approached its "solar maximum," the period of greatest activity. The most notable events occurred from May 10 to 13.

A cluster of enormous sunspots near solar maximum triggered the strongest geomagnetic storms to affect Earth in more than 20 years. The events produced bright and widespread auroras across the UK, with the northern lights visible as far south as Devon and Cornwall.

In 2022, Professor Chapman received the Royal Astronomical Society's Chapman Medal. The award recognized her pioneering research into the behavior of planetary magnetic fields and the ways those fields generate space weather.

Why Solar Storms Switch Off

The newly identified switch-off point also occurs when active sunspot regions move below about 15 degrees solar latitude.

During each solar cycle, sunspots form a "butterfly pattern." They first appear at higher latitudes and gradually move closer to the solar equator as the cycle progresses.

The Sun has differential rotation, meaning that different latitudes rotate at different speeds. Below about 15 degrees latitude, however, that difference in rotational speed becomes weaker. This creates a co-rotating region around the solar equator (the solar 'jet stream').

Professor Chapman believes that the most powerful coronal mass ejections are driven by differential rotation. As parts of the Sun rotate at different speeds, they twist the emerging magnetic field and build up energy. Once active sunspot regions move within 15 degrees of the equator, that twisting mechanism weakens and the main driver of extreme space weather switches off.

To test the idea, Professor Chapman examined the 27-day (average solar rotation) correlation in the aa index (geomagnetic activity at Earth) and compared it with recorded space weather events.

After the switch-off point, geomagnetic storms became less extreme and followed a 27-day pattern. This suggests they were probably being produced by co-rotating streams rather than coronal mass ejections.