The NSF Inouye Solar Telescope has observed its first X-class solar flare and the smallest coronal loops ever imaged.
These high-resolution images of a solar flare may help researchers better understand the Sun’s magnetic architecture and improve space weather forecasting.
Using the U.S. National Science Foundation (NSF) Daniel K. Inouye Solar Telescope, built and operated by the NSF National Solar Observatory (NSO), astronomers captured dark coronal loop strands with unprecedented clarity during the decay phase of an X1.3-class flare.
This marks a potential breakthrough in resolving the fundamental scale of solar coronal loops and pushing the limits of flare modelling into an entirely new realm.
What are coronal loops, and why are they important for studying the Sun?
Coronal loops are arches of plasma that follow the Sun’s magnetic field lines, often preceding solar flares that trigger sudden releases of energy associated with some of these magnetic field lines twisting and snapping.
This burst of energy fuels solar storms that can impact Earth’s critical infrastructure. Astronomers at the Inouye observe sunlight at the H-alpha wavelength (656.28 nm) to view specific features of the Sun, revealing details not visible in other types of solar observations.
“This is the first time the Inouye Solar Telescope has ever observed an X-class flare,” said Cole Tamburri, the study’s lead author, who is supported by the Inouye Solar Telescope Ambassador Program.
“These flares are among the most energetic events our star produces, and we were fortunate to catch this one under perfect observing conditions.”
How the Inouye Solar Telescope produced the clearest images ever
The researchers focused on the razor-thin magnetic field loops (hundreds of them) woven above the flare ribbons. On average, the loops measured about 48 km across, but some were right at the telescope’s resolution limit.
“Before Inouye, we could only imagine what this scale looked like,” Tamburri commented. “Now we can see it directly. These are the smallest coronal loops ever imaged on the Sun.”
The Inouye’s Visible Broadband Imager (VBI) instrument, tuned to the H-alpha filter, can resolve features down to ~24 km. That is over two and a half times sharper than the next-best solar telescope, and it is that leap in resolution that made this solar flare discovery possible.
While the original research plan involved studying chromospheric spectral line dynamics with Inouye’s Visible Spectropolarimeter (ViSP) instrument, the VBI data revealed something unexpected – ultra-fine coronal structures that can directly inform flare models built with complex radiative-hydrodynamic codes.
Theories have long suggested coronal loops could be anywhere from 10 to 100 km in width, but confirming this range observationally has been impossible – until now.
Tamburri explained: “We’re finally peering into the spatial scales we’ve been speculating about for years. This opens the door to studying not just their size, but their shapes, their evolution, and even the scales where magnetic reconnection occurs.”
A landmark moment in solar flare science
Perhaps the biggest discovery from this research is the idea that these loops might be elementary structures, which are the fundamental building blocks of solar flare architecture.
The imagery itself is breath-taking: dark, threadlike loops arching in a glowing arcade, bright flare ribbons etched in almost impossibly sharp relief – a compact triangular one near the centre, and a sweeping arc-shaped one across the top.
Even a casual viewer, Tamburri suggested, would immediately recognise the complexity.
He concluded: “It’s a landmark moment in solar flare science. We’re finally seeing the Sun at the scales it works on.”
