Scientists unlock the secret trigger of massive solar explosions

Scientists have finally captured direct evidence of how solar flares ignite and grow into some of the most powerful explosions in the solar system. The European Space Agency's Solar Orbiter spacecraft observed a massive solar flare on September 30, 2024, and revealed that the explosion began not with a sudden burst, but with a series of small magnetic disturbances that rapidly built upon each other—much like snow tumbling down a mountainside.

The discovery provides the clearest picture yet of what triggers these violent events and why they can hurl radiation and particles across space at speeds reaching 40 to 50 percent the speed of light. Understanding this process could eventually help scientists predict dangerous solar storms before they strike Earth.

Background

Solar flares have long puzzled researchers. These sudden eruptions on the Sun's surface release enormous amounts of energy in just minutes, sometimes sending dangerous radiation toward Earth and disrupting satellites, power grids, and communications systems. Yet scientists have struggled to understand exactly how the Sun builds up enough energy to trigger such massive explosions.

The Sun's surface is dominated by powerful magnetic fields that constantly shift and twist. When these fields become unstable, they can release tremendous energy. But the precise mechanism that sets off a flare has remained unclear. Previous observations gave scientists only glimpses of what happened, usually catching the flare after it was already underway.

Solar Orbiter changed that. The spacecraft, which orbits closer to the Sun than any other European mission, carries instruments capable of capturing details just a few hundred kilometers across. During its close approach in late September 2024, the mission's Extreme Ultraviolet Imager began recording a region of the Sun about 40 minutes before a major flare erupted.

Key Details

The Avalanche Effect

What Solar Orbiter captured was remarkable. About 40 minutes before the main explosion, a dark, arch-shaped structure made of twisted magnetic fields and plasma hung above the Sun's surface. This filament was connected to a cross-shaped pattern of magnetic field lines that were gradually growing brighter.

As the spacecraft watched, something unexpected happened. New magnetic field strands appeared in nearly every image frame—roughly every two seconds or less. Each strand twisted like a tightly wound rope, confined by magnetic forces. As more strands formed and twisted together, the region became increasingly unstable.

Then the cascade began. The twisted magnetic structures started breaking and reconnecting in rapid succession. Each break triggered another nearby, creating a spreading chain of disruptions. Each one was stronger than the last, visible as sudden bursts of brightness spreading across the region.

"We were surprised by how the large flare is driven by a series of smaller reconnection events that spread rapidly in space and time," said one of the scientists leading the study.

This process continued for several minutes. At 11:29 p.m. Universal Time, a particularly intense brightening occurred. Soon after, the dark filament detached on one side and shot outward into space, violently unrolling as it moved. Bright flashes of energy release appeared along its length as the main flare erupted around 11:47 p.m.

Extreme Particle Acceleration

While the spacecraft's imaging instrument captured this visual sequence, three other instruments on Solar Orbiter monitored what was happening at different depths in the Sun's atmosphere and at different temperatures. They revealed that as the magnetic reconnection events multiplied, particles were being accelerated to extreme speeds.

During the peak of the flare, particles reached speeds equivalent to 431 to 540 million kilometers per hour. This acceleration happened as the magnetic field transferred its energy to the surrounding plasma—the hot gas that makes up much of the Sun's outer atmosphere.

The observations also showed something unexpected: streams of glowing plasma blobs continued to rain down through the Sun's atmosphere long after the main flare had peaked. This "raining plasma" effect had never been observed in such detail before.

What This Means

The discovery answers a long-standing question about how the Sun releases such tremendous amounts of energy so quickly. Rather than a single catastrophic event, a solar flare appears to be a cascade of smaller events that amplify each other—similar to how a small shift of snow can trigger an entire avalanche.

This understanding could have practical applications. The most powerful solar flares can trigger geomagnetic storms on Earth, causing radio blackouts and damaging power infrastructure. If scientists can confirm that this avalanche mechanism occurs in all solar flares, they may be able to develop better warning systems by watching for the early, subtle magnetic disturbances that precede the main explosion.

However, researchers acknowledge significant gaps remain. The observations raised new questions about why particles accelerate to such extreme speeds and whether the avalanche mechanism works the same way in all types of solar flares. Future missions with even higher resolution X-ray imaging may help answer these questions.

For now, the Solar Orbiter findings represent a major step forward in understanding one of the Sun's most violent phenomena. The spacecraft's unprecedented view of a solar flare's buildup and ignition has given scientists a window into the central engine that powers these cosmic explosions.