Every two seconds, the camera on board Solar Orbiter is able to freeze the corona on another frame, and in this rare data set, it has captured the early, quiet failures that can build into a major solar flare.
In watching an M7.7-class flare on September 30, 2024, the spacecraft noted a familiar-looking structure a dark, arched filament of cooler, denser material suspended in the hot corona sitting alongside an X-shaped knot of magnetic loops. While flares can be thought of as sudden explosions, the high-cadence observation revealed something more gradual: an area that became unstable step by step, as if the flare’s “main event” had to wait for many smaller triggers to fall into place.
The crucial ingredient was magnetic reconnection, in which oppositely oriented magnetic field lines break and rejoin, releasing magnetic energy as heat, motion, and accelerated particles. Reconnection has long been a crucial component of flare models, but the challenge has been the size scale. In the corona, key structures can be no larger than a few hundred kilometers and occur on timescales of seconds that are too fast and too small for most observatories to follow. The Solar Orbiter Extreme Ultraviolet Imager solved this problem by providing a resolution of approximately ≈210 km with a 2-second cadence, effectively making the pre-flare phase an observable process rather than a deduced one.
When the instrument started its close observation about 40 minutes before the peak activity, new bright magnetic strands started appearing in almost every frame around the X-shaped structure. These strands appeared magnetically confined and twisted, as if rope fibers were being stretched. The activity did not occur in one frame; instead, it added to its complexity. The reconnection signals started weak and then became stronger, and the brightening process accelerated as the system approached the flare point. This was consistent with an avalanche process, where small fluctuations trigger a cascade.
“We were really very lucky to witness the precursor events of this large flare in such beautiful detail,” said Pradeep Chitta. “Such detailed high-cadence observations of a flare are not possible all the time because of the limited observational windows and because data like these take up so much memory space on the spacecraft’s onboard computer. We really were in the right place at the right time to catch the fine details of this flare.”
One of the brightening events, recorded before the peak of the flare, was followed by the dark filament disconnecting on one side and unwinding. The unwinding process was tracked by Solar Orbiter at speeds of hundreds of kilometers per second, with bright “sparks” of reconnection appearing along the filament. During the flare, the particles were accelerated to 40-50% of the speed of light, making it one of the clearer events to show that fast, fragmented reconnection can accelerate particles to extreme energies.
Just as telling, however, was where the energy went. In addition to plasma heating and ejection, the instruments detected signs of energy deposition further down in the atmosphere, in the form of fast ribbon-like structures and a steady “rain” of small plasma blobs that continued even after the flare’s most active period. This was a connection between coronal reconnection and transport that had only been hinted at by earlier imagery.
What this means in terms is that it is important because intense flares can interfere with technology on and around the Earth’s surface in the form of radio communications during geomagnetic storms. Mechanistically, the data had furthered the narrative because a large flare did not appear as a single clean break but as a chain reaction of a series of small events.
“Observations by Solar Orbiter reveal the heart of a flare and the importance of an avalanche-like magnetic energy release process at work,” said Miho Janvier. “One of the interesting questions is whether this process occurs in all flares, and on other flaring stars.”
