Solar Orbiter Reveals Magnetic Avalanches Drive Solar Flares

Solar flares are powerful explosions on the Sun. They occur when energy stored in tangled magnetic fields is suddenly released through a process described as ‘reconnection’. In a matter of minutes, criss-crossing magnetic field lines of opposite direction break and then reconnect. The newly reconnected field lines can quickly heat up and accelerate million-degree plasma, and even high-energy particles, away from the reconnection site, potentially creating a solar flare. The most powerful flares may start a chain of reactions that lead to geomagnetic storms on Earth, perhaps triggering radio blackouts, which is why it is so important to monitor and understand them. But the fine-grained details of how exactly this humungous amount of energy is released so rapidly has remained poorly understood. This unprecedented set of new Solar Orbiter observations – from four of the mission’s instruments working in complement to provide the most complete picture of a solar flare ever made – finally has a compelling answer. High-resolution imagery from Solar Orbiter’s Extreme Ultraviolet Imager (EUI) zoomed in to features just a few hundred kilometres across in the Sun’s outer atmosphere (its corona), capturing changes every two seconds. Three other instruments – SPICE, STIX and PHI – analysed a range of depths and temperature regimes, from the corona down to the Sun’s visible surface, or photosphere. Importantly, the observations enabled scientists to watch the buildup of events that led to the flare over the course of about 40 minutes. “We were really very lucky to witness the precursor events of this large flare in such beautiful detail,” says Pradeep Chitta of the Max Planck Institute for Solar System Research, Göttingen, Germany, and lead author of the paper. “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.” Magnetic avalanche in action When EUI first started observing the region at 23:06 Universal Time (UT), about 40 minutes before peak flare activity, a dark arch-like ‘filament’ of twisted magnetic fields and plasma was already present, connected to a cross-shaped structure of progressively brightening magnetic field lines (visible in the main video above, captioned 'Magnetic avalanche in action'). Zooming in to this feature (see video below, captioned 'Zooming in on magnetic reconnection') shows that new magnetic field strands appear in every image frame – equivalent to every two seconds or less. Each strand is magnetically contained, and they become twisted, like ropes. Then, just like in a typical avalanche, the region becomes unstable. The twisted strands begin to break and reconnect, rapidly triggering a cascade of further destabilisations in the area. This creates progressively stronger reconnection events and outflows of energy, seen as sudden and increasing brightness in the imagery. One particular brightening begins at 23:29 UT, followed by the dark filament disconnecting from one side, launching into space and at the same time violently unrolling at high speed. Bright sparks of reconnection are seen all along the filament in stunning high resolution as the main flare erupts at around 23:47 UT. “These minutes before the flare are extremely important and Solar Orbiter gave us a window right into the foot of the flare where this avalanche process began,” says Pradeep. “We were surprised by how the large flare is driven by a series of smaller reconnection events that spread rapidly in space and time.” Scientists had already proposed a simple avalanche model to explain the collective behaviour of hundreds of thousands of flares on the Sun and other stars, but it had not been clear whether a single large flare could be described by an avalanche. What this result shows is exactly that – a flare is not necessarily a single coherent eruption but can be a cascade of interacting reconnection events. Raining plasma blobs For the first time, and thanks to the simultaneous measurements by Solar Orbiter’s SPICE and STIX instruments, Pradeep’s team have been able to explore in extremely high resolution how the rapid series of reconnection events deposits energy in the outermost part of the Sun’s atmosphere. Of particular interest is high-energy X-ray emission, which is a signature of where accelerated particles have deposited their energy. Given that accelerated particles can escape into interplanetary space and pose radiation hazards to satellites, astronauts, and even Earth-based technologies, understanding how this process occurs is essential for forecasting space weather. For the 30 September flare, the emission in ultraviolet to X-rays was already slowly rising when SPICE and STIX first started observing the region. The X-ray emission rose so dramatically during the flare itself – as reconnection events increased – that particles were accelerated to speeds of 40–50% the speed of light, equivalent to about 431–540 million km/h (see video below, captioned 'X-rays blast from a solar flare'). Furthermore, the observations showed that the energy was transferred from the magnetic field to the surrounding plasma during these reconnection events.