Ten solar eruptions merged and interacted on their way to Earth, helping fuel the powerful geomagnetic storm and spectacular auroras of May 2024.
When the strongest solar storm in two decades slammed into Earth over Mother’s Day weekend in 2024, scientists expected a single mega-explosion at the source.
Instead, they found a parade of ten distinct solar eruptions pounding out from the Sun across four days, compounding into one massive space weather event.
Once all this reached the Wind spacecraft, which samples the solar wind before it hits Earth, ten eruptions had become five clouds.
Simulations built on the shorter list slowed too soon after midday on May 11, while the real solar wind at Earth stayed fast for days.
Ten solar eruptions merged and interacted on their way to Earth, helping fuel the powerful geomagnetic storm and spectacular auroras of May 2024.
When the strongest solar storm in two decades slammed into Earth over Mother’s Day weekend in 2024, scientists expected a single mega-explosion at the source.
Instead, they found a parade of ten distinct solar eruptions pounding out from the Sun across four days, compounding into one massive space weather event.
New simulations trace how they caught one another in flight, squeezed together, and hit Earth as a few merged clouds instead of ten separate blows.
Each eruption throws off a cloud of magnetized gas from the sun’s outer atmosphere. Researchers call them coronal mass ejections, or CMEs.
In an interview with Earth.com, Richard Lewis, a senior research writer at the University of Iowa, was asked what he would highlight from the study.
“The extreme storm was caused by the collective evolution of the CME sequence, not one unusually strong eruption,” Lewis said.
Ten eruptions arrived as five clouds
What follows is a reconstruction, not a measurement. Shirsh Soni, a postdoctoral research fellow at Iowa, worked with colleagues at KU Leuven in Belgium plus co-authors in India and Poland.
The team estimated each eruption’s speed, size and direction from telescope images. They fed those numbers into a simulation of the space between the sun and Earth, then checked the result against what spacecraft recorded.
Coronagraph images, which block the sun’s bright disk from view, showed the first two eruptions about two hours apart on May 8.
The second was moving faster and caught the first roughly 9 million miles (15 million km) from the sun, before either had traveled a tenth of the way to Earth.
That merged cloud then ran down a slower third one near the halfway mark. Two more erupted in the same minute that evening, overlapping so completely that separating them was guesswork. The team ran them as two anyway.
Once all this reached the Wind spacecraft, which samples the solar wind before it hits Earth, ten eruptions had become five clouds.
The first shock front hit on May 10, and auroras turned up that weekend over the southern United States and northern India.
Three eruptions were overlooked
Earlier analyses of the same week worked from a shorter list. Two other teams treated the opening burst as one eruption rather than two, and none of the earlier work included the last three.
NASA’s account of that week, written days after it happened, lists at least seven.
“Several features initially treated as parts of the same eruption were actually separate CMEs, and the simulations showed that these additional eruptions significantly affected the final storm,” Lewis told Earth.com.
Comparing ultraviolet images of the sun’s surface against coronagraph views from multiple viewpoints is what settled the count, he said.
Those extra three fixed a mismatch, too. Simulations built on the shorter list slowed too soon after midday on May 11, while the real solar wind at Earth stayed fast for days. Adding the late eruptions reproduced that long fast stretch.
A sideways blast changed everything
The fourth eruption wasn’t headed for us at all. It was a filament, a ribbon of cooler gas that lifted off a different part of the sun and traveled east, and only its edge came near the others.
That edge mattered anyway. In the simulation, it shoved the slow third cloud forward until it merged with the two ahead of it. The thin corridor left behind let the next eruption expand and speed up.
Those flank encounters, and the thinner wind each eruption left for the next, helped accelerate and compress the structures, Lewis said, increasing their potential impact at Earth.
How hard the push was is unsettled. Soni’s team placed the filament 50 degrees east of the sun-Earth line at 280 miles per second (450 km/s), which amounts to a graze.
A separate 2025 analysis estimated 27 degrees east and 616 miles per second (991 km/s).
With those numbers, the graze becomes a shove, and the simulated storm arrives within two hours of the real one.
Forecasts miss the strength
The best simulation called the arrival about two hours early. On strength it reached roughly 70% of the real storm.
Forecasters can act on an arrival time. What they need next is the size.
Ahead of the 2024 storm, NOAA’s Space Weather Prediction Center warned operators of power grids and satellites to take precautions.
Earth.com asked Lewis what forecasters would miss if a merged cloud arrived tomorrow.
“Forecasters might predict the arrival but underestimate the storm’s intensity and duration,” he said.
Interactions between eruptions can strengthen and prolong the southward magnetic field a cloud carries, he said, which makes the merged structure more damaging than any single one.
A magnetic field pointing south for hours, with a fast wind behind it, is the pairing the team names as the warning sign.
The model still has blind spots
The researchers are blunt about what their setup can’t do. The simple cloud shape they use has no trailing legs, so it misses flank encounters.
To compensate, they nudged every cloud 10 degrees toward the sun-Earth line. They also held the background solar wind steady, while the real one shifts hour to hour.
Most of these eruptions came at Earth nearly head-on, the hardest angle for judging shape. The team calls its own reconstruction of that geometry particularly uncertain.
The next real test is years away
The sun is heading into the quieter half of its cycle.
Lewis said the next major test may come with the following solar maximum, eight to ten years from now, although the declining phase of the current cycle could still produce one sooner.
“We need another active region producing several closely spaced, Earth-directed CMEs to evaluate whether this modeling approach can improve real-time forecasts,” he added.
A ten-eruption reconstruction this complex has not yet been attempted in real time, while a storm was still racing toward Earth.
Doing that will require better observations, faster models, and uncertainty estimates for each eruption’s speed and direction. The researchers expect machine learning could eventually help forecasters keep pace with the interactions.
Soni’s team had the luxury of hindsight. They could rerun and adjust the ten eruptions until the simulation matched what spacecraft had recorded.
A forecaster facing the next major storm won’t have that luxury. The clock will already be running.
The full study was published in the journal The Astrophysical Journal.
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