At the end of 2024, ESA's Solar Orbiter was lucky to witness a strong solar flare - and observed the events leading up to this firework with unprecedented precision.
A snapshot taken one second before a powerful M-class solar flare erupted on the Sun on September 30, 2024. This image is a still from a video created from images taken by Solar Orbiter's Extreme Ultraviolet Imager (EUI) instrument.
© ESA & NASA/Solar Orbiter/EUI Team
To the point
- Violent eruption: New analyses of Solar Orbiter data reveal solar flares with unparalleled views.
- "Sports photography" on the Sun: Thanks to their uniquely rapid sequence - every two seconds - , the images capture even the fastest changes preceding the eruption.
- Magnetic avalanche: Small restructuring processes in the Sun's magnetic field build up like an avalanche - and then discharge explosively.
When Solar Orbiter looked at the Sun on September 30, the space probe captured a spectacular sight: our star hurled radiation and particles into space in a violent eruption. The observation conditions could not have been better. On that very day, ESA's space probe had reached the point in its elliptical orbit closest to the Sun. Approximately 45 million kilometers-about a third of the distance between the Sun and Earth-separated it from the explosive event. The perspective was also ideal. From Solar Orbiter's point of view, the eruption occurred at the limb of the solar disk. This provided an optimal view of the events that took place before and during the eruption.
"It was extremely fortunate that Solar Orbiter, the most powerful solar observatory in space, was looking at the flare at exactly the right time and from exactly the right angle. It's impossible to plan something like this, days in advance", says Lakshmi Pradeep Chitta. Months before, the scientist from the Max Planck Institute for Solar System Research (MPS), together with colleagues from the European Space Agency (ESA) and other research institutions, had designed an observation campaign for Solar Orbiter for this day. He had not expected such an eruption.
Threat to terrestrial infrastructure
The eruption on 30 September 2024, an M7.7-class flare, was not one of the most powerful, but certainly one of the more spectacular ones. Even during periods of high solar activity, flares of this magnitude occur only sporadically. When in an event like this high-energy radiation and particles spread from the Sun toward Earth, this can cause disruptions to radio communications, for example. "More powerful solar flares can have even more far-reaching consequences, for example for satellites or the power supply. It is therefore important to understand exactly what causes such events on the Sun," explains Sami K. Solanki, MPS Director and head of the PHI instrument team for Solar Orbiter.
Changes in the Sun's magnetic field architecture provide the energy needed to catapult radiation and particles into space during an eruption. Strongly twisted, criss-crossing magnetic field lines that store a large amount of energy break open and reassemble. Researchers refer to this process as reconnection. However, how this "motor" for solar flares works in detail is still unclear. In the journal Astronomy & Astrophysics, researchers led by the MPS now describe how small reconnection processes trigger further ones, building up like an avalanche and leading to a flare.
A close look at solar flare
Four instruments on Solar Orbiter focused on the solar flare in the hours around midnight on 30 September to 1 October 2024. The EUI (Extreme Ultraviolet Imager) instrument observed the events in the corona, the Sun's hot atmosphere, with a very high spatial resolution of about 210 kilometers and an image sequence with a high cadence of two seconds. Compared to conventional image sequences from other solar observatories in space, this is akin to a kind of 'solar sports photography': fast movements and changes that were previously undetectable become visible. In addition, the instruments PHI (Polarimetric and Helioseismic Imager), SPICE (Spectral Imaging of the Coronal Environment), and STIX (Spectrometer/Telescope for Imaging X-rays) provided additional information from different layers and temperature ranges of the Sun.
About 40 minutes prior to the flare, the EUI images show a dark plasma loop extending far into the corona. This comparatively cool plasma structure is suspended in the million degrees hot corona by strongly twisted, arc-shaped magnetic field lines. Such a structure stores energy, much like a coiled spring. At around 11:47 p.m., a discharge occurs: the plasma arc rears up, lights up brightly, uncoils explosively, and charged particles are accelerated to speeds of about 40 to 50 percent of the speed of light.
Magnetic avalanche and plasma rain
For researchers, the processes that take place in the Sun's magnetic field in the minutes leading up to this explosion are even more exciting. Directly adjacent to the dark plasma loop is a delicate tangle of arc-shaped, bright plasma streams captured in the magnetic field, some of which intersect. About half an hour before the eruption, this structure begins to destabilize: initial reconnection processes begin, the threads break open, rearrange themselves, and flash brightly. Almost every second, new plasma strands form, destabilize, and trigger an avalanche of reconnection processes - until the large, dark plasma loop breaks open and the flare reaches its peak. "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."
As the new analyses impressively demonstrate, not all of the energy is released into space during the flare. Some of it is transferred to the surrounding plasma, which rains down at high speeds in the form of plasma blobs. This phenomenon has also never before been observed in the extreme ultraviolet, wavelength of radiation typically emitted by coronal plasma, in such detail. "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. This study is one of the most exciting results from Solar Orbiter so far," says Miho Janvier, ESA's Solar Orbiter co-Project Scientist. "An interesting prospect is whether this mechanism happens in all flares, and on other flaring stars", she adds.
