“It’s a landmark moment in solar science,” said Cole Tamburri of the University of Colorado Boulder. The moment arrived when the National Science Foundation’s Daniel K. Inouye Solar Telescope (DKIST) imaged the tiniest coronal loops ever seen on the Sun towers so slender they strain the capabilities of modern optics, and so basic they could be the key to understanding the most extreme solar eruptions.
Coronal loops are vast plasma arcs following the Sun’s magnetic field lines, usually anticipating solar flares when the lines twist, break, and re-form in what is called magnetic reconnection. Though theory has long predicted loops might be as thin as 10 kilometres, no telescope had been able to resolve them until August 8, 2024, when DKIST, under exceptional observing conditions, captured hundreds of loops measuring just an average of 48.2 kilometres wide, some as slender as 21 kilometres. This was in the waning phase of an X1.3-class flare, the most powerful type of flare the Sun emits.
The observations were made in hydrogen-alpha light (656.28 nm) using DKIST’s Visible Broadband Imager, which can resolve features down to ~24 km over 2.5 times sharper than the next-best solar telescope. “Before Inouye, we could only imagine what this scale looked like,” Tamburri said. “Now we can see it directly.” The imagery revealed dark, threadlike loops arching above bright flare ribbons, each loop potentially an elementary magnetic structure. “If that’s the case, we’re not just resolving bundles of loops; we’re resolving individual loops for the first time,” he added.
Understanding these scales is critical for flare physics. Magnetic reconnection in the corona converts stored magnetic energy into heat, light, and accelerated particles. In high-energy events, electrons can reach several MeV, and ions can exceed a GeV. Models indicate particle acceleration efficiency is controlled by reconnection rates, turbulence, and magnetic-to-thermal energy ratio (\(\sigma_T\)). Within the solar atmosphere, (\sigma_T) is approximately 100, meaning enormous magnetic energy is available per particle. The newly imaged fine loops may delineate the locations where reconnection is beginning, supplying the small-scale topology of the magnetic fields that propel the large-scale eruptions.
New DKIST research on microflares revealed reconnection along dome-shaped “fan-spine” magnetic structures that include null points where field lines cancel and unload energy. In those instances, there was rapid heating, plasma jets, and turbulence in areas just 700 km wide with substructures an order of magnitude finer. That level of detail invisible to previous instruments is now guiding models that connect micro- and macro-scale solar activity.
The technological innovation is based on DKIST’s 4-meter off-axis primary mirror, adaptive optics, and coronagraphic capabilities that support diffraction-limited imaging across visible and infrared wavelengths. Capable of resolving structures as small as 20 km, it is able to probe magnetic fields from the deep photosphere out to the outer corona. Its infrared spectropolarimeters utilize the Zeeman effect to sensitively measure magnetic fields, while its visible instruments observe rapid dynamics in the chromosphere and corona.
These capabilities are not just academic. Flares and associated coronal mass ejections can disrupt satellites, power grids, and communications on Earth. By resolving the smallest magnetic structures, DKIST data can refine space weather models, potentially improving forecasts of geomagnetic storms. As Tamburri noted, “We’re finally seeing the Sun at the scales it works on,” a leap that could translate into practical resilience for modern infrastructure.
But this advancement comes with a cloud. The U.S. federal government’s proposed 2026 budget would reduce funding for DKIST from $30 million to $13 million below the level required to sustain operations, National Solar Observatory director Christoph Keller said. These reductions are part of larger cuts to NSF and NASA astrophysics programs that imperil observatories, fellowships, and global partnerships. If implemented, they would close DKIST at the moment when it is starting to produce revolutionary science, removing not just its exclusive solar imagery but also the skill of the early-career scientists it educates.
For the moment, the August 2024 flare serves as testimony to what is possible when engineering meets atmospheric conditions and scientific opportunity. In the razor-thin arcs around erupting Sun, DKIST has opened a new frontier in solar physics one measured in tens of kilometers but with implications reaching across the solar system.
