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\t\t\t\t\t\t21\/01\/2026<\/span> 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 \u2018sky\u2019 of raining plasma blobs that continue to fall even after the flare subsides.<\/p>\n<\/div>\n The discovery was enabled by one of Solar Orbiter\u2019s most detailed views of a large solar flare, observed during the spacecraft\u2019s 30 September 2024 close approach to the Sun. It is described in a paper published today in Astronomy & Astrophysics<\/i>.<\/p>\n<\/p><\/div>\n 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 \u2018reconnection\u2019. 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.<\/p>\n 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.<\/p>\n 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 \u2013 from four of the mission\u2019s instruments working in complement to provide the most complete picture of a solar flare ever made \u2013 finally has a compelling answer.<\/p>\n High-resolution imagery from Solar Orbiter\u2019s Extreme Ultraviolet Imager (EUI) zoomed in to features just a few hundred kilometres across in the Sun\u2019s outer atmosphere (its corona), capturing changes every two seconds. Three other instruments \u2013 SPICE, STIX and PHI \u2013 analysed a range of depths and temperature regimes, from the corona down to the Sun\u2019s 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.<\/p>\n \u201cWe were really very lucky to witness the precursor events of this large flare in such beautiful detail,\u201d says Pradeep Chitta of the Max Planck Institute for Solar System Research, G\u00f6ttingen, Germany, and lead author of the paper. \u201cSuch 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\u2019s onboard computer. We really were in the right place at the right time to catch the fine details of this flare.\u201d<\/p>\n<\/p><\/div>\n When EUI first started observing the region at 23:06 Universal Time (UT), about 40 minutes before peak flare activity, a dark arch-like \u2018filament\u2019 of twisted magnetic fields and plasma was already present, connected to a cross-shaped structure of progressively brightening magnetic field lines\u00a0(visible in the main video above, captioned ‘Magnetic avalanche in action’).<\/p>\n Zooming in\u00a0to this feature (see video below, captioned ‘Zooming in on magnetic reconnection’)\u00a0shows that new magnetic field strands appear in every image frame \u2013 equivalent to every two seconds or less. Each strand is magnetically contained, and they become twisted, like ropes.<\/p>\n 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.<\/p>\n 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.<\/p>\n<\/p><\/div>\n \u201cThese 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,\u201d says Pradeep. \u201cWe were surprised by how the large flare is driven by a series of smaller reconnection events that spread rapidly in space and time.\u201d<\/p>\n 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 \u2013 a flare is not necessarily a single coherent eruption but can be a cascade of interacting reconnection events.<\/p>\n For the first time, and thanks to the simultaneous measurements by Solar Orbiter\u2019s SPICE and STIX instruments, Pradeep\u2019s 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\u2019s atmosphere.\u00a0<\/p>\n 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\u00a0space weather.<\/p>\n 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 \u2013 as reconnection events increased \u2013 that particles were accelerated to speeds of 40\u201350% the speed of light, equivalent to about 431\u2013540 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.<\/p>\n<\/p><\/div>\n \u201cWe saw ribbon-like features moving extremely quickly down through the Sun\u2019s atmosphere, even before the main episode of the flare,\u201d says Pradeep. \u201cThese streams of \u2018raining plasma blobs\u2019\u00a0are 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\u2019s the first time we see this at this level of spatial and temporal detail in the solar corona.\u201d<\/p>\n<\/p><\/div>\n 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 \u2018normal\u2019 levels. At the same time, PHI observed the imprint of the flare (see video below, captioned ‘Surface imprint of a solar flare’)\u00a0on the Sun\u2019s visible surface, completing the three-dimensional picture of the event.<\/p>\n<\/p><\/div>\n \u201cWe didn\u2019t expect that the avalanche process could lead to such high energy particles,\u201d says Pradeep. \u201cWe 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.\u201d<\/p>\n \u201cThis is one of the most exciting results from Solar Orbiter so far,\u201d says Miho Janvier, ESA\u2019s Solar Orbiter co-Project Scientist. \u201cSolar Orbiter\u2019s 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.\u201d<\/p>\n<\/p><\/div>\n Notes for editors<\/b><\/p>\n A magnetic avalanche as the central engine powering a solar flare,<\/i> by L. P. Chitta et al. is published in Astronomy and Astrophysics. DOI: 10.1051\/0004-6361\/202557253<\/p>\n 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. <\/i>\u202fThe 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.<\/i><\/p>\n<\/p><\/div>\n
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\n\t\t\t\t\t\t\t\t<\/figcaption><\/figure>\nMagnetic avalanche in action<\/h3>\n
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