Magnetic avalanches power solar flares

(Press-News.org) Just as avalanches on snowy mountains start with the movement of a small quantity of snow, the ESA-led Solar Orbiter spacecraft has discovered that a solar flare is triggered by initially weak disturbances that quickly become more violent. This rapidly evolving process creates a ‘sky’ of raining plasma blobs that continue to fall even after the flare subsides.

The discovery was enabled by one of Solar Orbiter’s most detailed views of a large solar flare, observed during the spacecraft’s 30 September 2024 close approach to the Sun. It is described in a paper being published on Wednesday 21 January in Astronomy & Astrophysics.

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. (see Video 1: main movie)

Zooming in to this feature 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. (see Video 2: X shapes)

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. (see Video 1: main movie)

“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. (see Video 4: STIX X-ray observations)

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. Furthermore, the observations showed 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,” says Pradeep. “These streams of ‘raining plasma blobs’ are signatures of energy deposition, which get stronger and stronger as the flare progresses. Even after the flare subsides, the rain continues for some time. It’s the first time we see this at this level of spatial and temporal detail in the solar corona.” (see Video 3: raining plasma blobs, Video 1: main movie)

After the main phase of the flare, the original cross-shape of magnetic field lines is seen to relax in the EUI images, while STIX and SPICE saw the plasma start to cool down and particle emission decrease towards ‘normal’ levels. At the same time, PHI observed the imprint of the flare[NS1]  on the Sun’s visible surface, completing the three-dimensional picture of the event. (see Video 5: surface imprints)

“We didn’t expect that the avalanche process could lead to such high energy particles,” says Pradeep. “We still have a lot to explore in this process, but that would need even higher resolution X-ray imagery from future missions to really disentangle.”

“This is one of the most exciting results from Solar Orbiter so far,” says Miho Janvier, ESA’s Solar Orbiter co-Project Scientist. “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. An interesting prospect is whether this mechanism happens in all flares, and on other flaring stars.”

“These exciting observations, captured in incredible detail and almost moment by moment, allowed us to see how a sequence of small events cascaded into giant bursts of energy,” says David Pontin of the University of Newcastle, Australia, who co-authored the paper.

He adds: “By comparing the EUI observations with magnetic-field observations, we were able to disentangle the chain of events that led to the flare. What we observed challenges existing theories for flare energy release and, together with further observations, will allow us to refine those theories to improve our understanding.”

—-

Notes for editors

 

A magnetic avalanche as the central engine powering a solar flare, by L. P. Chitta et al. is published in Astronomy and Astrophysics. DOI: 10.1051/0004-6361/202557253   https://www.aanda.org/10.1051/0004-6361/202557253

Solar Orbiter is a space mission of international collaboration between ESA and NASA, operated by ESA. The Extreme Ultraviolet Imager (EUI) instrument is led by the Royal Observatory of Belgium (ROB). The Polarimetric and Helioseismic Imager (PHI) instrument is led by the Max Planck Institute for Solar System Research (MPS), Germany.  The Spectral Imaging of the Coronal Environment (SPICE) instrument is a European-led facility instrument, led by the Institut d'Astrophysique Spatiale (IAS) in Paris, France. The STIX X-ray Spectrometer/Telescope is led by FHNW, Windisch, Switzerland.

For more information, please contact:

ESA Media Relations, media@esa.int

END