1947’s Record Sunspot Could Have Sparked Flare Up to 100 Times Stronger Than Any Seen

A sunspot cluster the size of dozens of Earths appeared on the Sun in April 1947, and new calculations suggest it could have unleashed a flare roughly 100 times more powerful than anything astronomers have ever directly measured. It remains the largest sunspot group ever recorded, yet it never actually produced a flare that made history. A new study asks what might have happened if it had.

Researchers who study the Sun’s magnetic behavior estimate that this 1947 giant could, in theory, have released a flare with energy reaching a few times 10 to the 34th power ergs, the unit scientists use to measure how much energy a flare gives off. The most powerful flares directly observed since 1975 reached energies of roughly 4 times 10 to the 32nd power ergs or more. That gap means the 1947 sunspot’s theoretical ceiling could be dozens of times, possibly over a hundred times, stronger than anything humans have actually watched erupt from the Sun.

To be clear, nobody is claiming this monster flare actually happened. No flare of that scale was recorded in 1947, and the study’s authors, writing in the journal Philosophical Transactions of the Royal Society A, are careful to say they are not rewriting history. Instead, they are testing the Sun’s outer limits, asking how big a flare that sprawling, tangled sunspot group could plausibly have produced if conditions had lined up just right. The answer matters because it helps scientists judge whether the Sun could ever produce the kind of extreme outburst that drives severe space weather, the sort of event that can disrupt power grids and satellites on a scale the world has never experienced.

How Scientists Rebuilt the Sun’s Biggest Sunspot Flare

Flares happen when the tangled magnetic fields hovering above sunspots suddenly snap, releasing bursts of energy. Bigger, messier sunspot clusters generally store more magnetic energy, and more stored energy means more potential for a major flare. But nobody has ever directly measured a flare from the largest sunspots in history, because the instruments capable of capturing that kind of detail didn’t exist yet in 1947, let alone in 1859.

So the research team, led by scientists at the Max Planck Institute for Solar System Research along with colleagues from the University of Colorado Boulder and the National Solar Observatory, built a bridge between past and present. They pulled from a catalog of more than 300 major flares captured between 2010 and 2016 by NASA’s Solar Dynamics Observatory, a spacecraft that watches the Sun around the clock. Those were the major M- and X-class flares, the strongest categories astronomers track. That catalog tracked flare ribbons, bright streaks that show up in the Sun’s lower atmosphere during a flare, marking where snapped magnetic field lines touched down. Bigger ribbons generally mean more energy got released.

Using that modern data, the team worked out a relationship between the size of a sunspot cluster and the size of the flare ribbons it could produce during its most extreme outbursts, focusing on the extreme upper end of that relationship, the rare cases where a given sunspot cluster generated unusually large flare ribbons. They then applied that relationship to the biggest sunspot groups in the historical record, including the 1859 Carrington sunspot, the 1947 giant, and several powerful modern storms from 1989, 2000, 2003, 2014 and 2024, to see what those spots could theoretically have unleashed under worst-case conditions.

Testing the Method on Storms Scientists Actually Measured

Before trusting this approach on events from the 1800s, the researchers checked it against a modern storm that had already been measured directly: a massive sunspot cluster from October 2014 known as AR 12192, which produced a flurry of intense flares. The predicted ribbon sizes and flare energies lined up well with what satellites recorded from that region, giving the team confidence the method held up against real measurements before applying it to sunspots no instrument had ever captured.

They also tested it against the Bastille Day storm of July 2000 and the so-called Halloween storms of October and November 2003, two of the most disruptive space weather events of the modern satellite era. In both cases, the predicted energy ranges matched reasonably well with independent measurements from other instruments tracking the Sun’s brightness output during flares. That consistency across multiple real-world storms suggested the method could be pushed further back in time with reasonable confidence.

When applied to the Carrington storm of 1859, long regarded as the benchmark for extreme solar activity because it triggered auroras as far south as the Caribbean and set telegraph equipment on fire, the calculations produced an energy estimate close to other scientists’ independent reconstructions of that famous event. That agreement matters. It means the new method isn’t spitting out random numbers; it’s landing in the same range as decades of prior research on the Sun’s most infamous storm.

But the 1947 sunspot group dwarfed even Carrington’s in raw size. When the same math was applied, the results pointed toward energies edging into “superflare” territory, a level of activity that satellite observations of other stars suggest happens on sun-like stars roughly once per century.

Why Sunspot Size Doesn’t Guarantee a Dangerous Flare

One twist complicates the picture. AR 12192, the giant 2014 sunspot cluster used to test the method, produced a string of powerful flares, but none of them erupted outward into space as a full-blown blast capable of striking Earth. Scientists studying that event separately have noted its flares stayed unusually contained, likely because strong magnetic fields overhead kept the energy from escaping. Meanwhile, the 1989 sunspot region behind the famous Quebec blackout was actually even larger than the 2014 cluster, yet it produced an intensely disruptive, eruptive blast, showing that huge sunspots don’t always stay bottled up.

That contrast suggests raw size isn’t destiny. A sunspot’s magnetic structure, not just its footprint on the Sun’s surface, determines whether stored energy stays locked in place or blasts toward Earth. The study raises another wrinkle worth watching, too: sunspot clusters sometimes emerge in tight-knit groups or even merge together, a phenomenon nicknamed “nesting.” A 2024 storm nicknamed the Mother’s Day event involved several sunspot regions merging into one another, and the researchers note that clustering like this could, in principle, push flare energies even higher than what a single giant sunspot could produce alone.

Solar physics has long wrestled with a strange contradiction: telescopes pointed at other sun-like stars keep spotting superflares far more violent than anything the Sun has shown us directly, yet nobody knows whether our own star can truly produce that kind of outburst. This new analysis doesn’t settle that debate. But it does show that the Sun’s largest known sunspot, sitting quietly in the historical record for nearly 80 years, may have carried far more destructive potential than it ever revealed. Whether the Sun ever cashes in that potential is a separate question, and one that keeps space weather scientists watching every new sunspot with a healthy dose of caution.