On September 30, 2024, the Sun unleashed a powerful explosion, causing magnetic field lines to break and reconnect in a criss-cross pattern. A Sun-observing probe was there to watch it unfold, gathering unprecedented data that’s helping scientists better understand the mechanism behind solar flares.
Using the European Space Agency’s Solar Orbiter spacecraft, a team of scientists discovered that solar flares are triggered by initially weak disturbances that grow more violent, similar to avalanches on snowy mountains. The process creates a sky of raining plasma blobs that continue to fall even after the solar flare has subsided, according to a new study published in Astronomy & Astrophysics.
Magnetic avalanche
Solar flares are giant explosions on the Sun, flinging energy, light, and particles into space. They take place when energy that’s stored in twisted magnetic field lines is suddenly released. The most powerful solar flares can disrupt technologies on Earth, triggering geomagnetic storms capable of causing radio blackouts.
Scientists have observed solar flares for years, but they still lack a detailed understanding of how this colossal amount of energy is released so rapidly from the Sun. Using the high-resolution data from Solar Orbiter, scientists now have a better picture of the process that leads to the violent eruption.
Solar Orbiter zoomed in on a region of the Sun with a dark arch-like ‘filament’ of twisted magnetic fields and plasma, linked to a cross-shaped structure of brightening magnetic field lines. Scientists directed the spacecraft’s Extreme Ultraviolet Imager (EUI) toward the region roughly 40 minutes before peak flare activity.
By zooming in, observations revealed new magnetic field strands appearing in every image frame—equivalent to every two seconds or less. Each strand was magnetically contained and twisted like a rope. The region became progressively less stable, just like in an avalanche.
The twisted magnetic field strands began to break and reconnect, rapidly triggering a cascade of further instability in the region. As the strands broke, they triggered progressively stronger reconnection events and outflows of energy, which appeared as increasing brightness in the images.
Then, a sudden brightening was followed by the dark filament disconnecting from one side, launching into space while violently unrolling at high speed. Scientists first recorded the unwinding at 155 miles per second (250 kilometers per second), rising to 248 miles per second (400 km/s) at the site of disconnection. Bright sparks of reconnection appeared all along the filament in stunning high resolution as the flare erupted.
“We were really very lucky to witness the precursor events of this large flare in such beautiful detail,” Pradeep Chitta, a researcher at the Max Planck Institute for Solar System Research in Göttingen, Germany, and lead author of the paper, said in a statement. “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.”
The scientists behind the study were surprised to learn that the large flare is driven by a series of smaller reconnection events that spread rapidly in space and time, creating a cascade of increasingly violent events.
Plasma rain
Even before the flare erupted, the Solar Orbiter revealed that emissions from the Sun were slowly rising when the spacecraft first began observing the region. During the flare itself, particles were accelerated to speeds of 40 to 50% the speed of light.
The detailed observations also revealed that the energy was transferred from the magnetic field to the surrounding plasma during these reconnection events. “We saw ribbon-like features moving extremely quickly down through the Sun’s atmosphere, even before the main episode of the flare,” Chitta said. “These streams of ‘raining plasma blobs’ are signatures of energy deposition, which get stronger and stronger as the flare progresses.”
Even after the flare subsided, the rain of plasma blobs continued for some time, Chitta added.
“Solar Orbiter’s observations unveil the central engine of a flare and emphasise the crucial role of an avalanche-like magnetic energy release mechanism at work,” Miho Janvier, ESA’s Solar Orbiter co-project scientist, said in a statement. “An interesting prospect is whether this mechanism happens in all flares, and on other flaring stars.”
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