The Sun just bared details so fine they were once thought unreachable plasma loops as narrow as 21 kilometers, less than the distance between Manhattan and Newark, captured in the throes of a powerful X-class solar flare. On August 8, 2024, the NSF’s Daniel K. Inouye Solar Telescope atop Haleakalā, Hawaii, recorded the highest-resolution images ever taken of such an event, resolving the smallest coronal loops ever observed and confirming decades-old theoretical predictions.
“This is the first time the Inouye Solar Telescope has ever observed an X-class flare,” said Cole Tamburri, a Ph.D. candidate at the University of Colorado Boulder and lead author of the study. “These flares are among the most energetic events our star produces, and we were fortunate to catch this one under perfect observing conditions.”
Solar flares, the solar system’s most powerful explosions, happen when magnetic field lines in the Sun’s corona get distorted to the breaking point and then reconnect an event referred to as magnetic reconnection. This temporary rearrangement releases energy in amounts equivalent to that of billions of hydrogen bombs, triggering radiation throughout the electromagnetic spectrum and, at times, coronal mass ejections that can devastate satellites, power systems, and communications on Earth.
The Inouye’s success was made possible by its engineering. Its 4.24-meter off-axis primary mirror suppresses scattered sunlight, and an adaptive optics system, founded on wavefront sensors and deformable mirrors, continually corrects for atmospheric distortion. More than 11 kilometers of coolant piping run fluid to radiate the intense heat of direct solar observation. Ten precision-aligned mirrors concentrate sunlight on instruments like the Visible Broadband Imager (VBI), which, tuned to the H-alpha wavelength of 656.28 nanometers, can view features with resolution as fine as ~24 km over 2.5 times better than the next-best solar telescope.
The glow of the flare revealed hundreds of black, stringy loops curving high over bright ribbons of emission, with mean widths of 48.2 km and some of resolution limit. The scale is historical: models had predicted for decades loops 10 to 100 km wide, but there had been no observational evidence. “Before Inouye, we could only imagine what this scale looked like,” Tamburri said. “Now we can see it directly.”
Co-author and scientist at the National Solar Observatory Maria Kazachenko said, “Knowing a telescope can theoretically do something is one thing. Actually watching it perform at that limit is exhilarating.” The researchers initially planned to study chromospheric spectral line dynamics using the Visible Spectropolarimeter, but the data from VBI were found to have ultra-fine coronal structures that can be utilized to create flare models built with advanced radiative-hydrodynamic codes.
These loops might be indicative of the fundamental building blocks of flare architecture isolated magnetic strands rather than bunches. Decoupling them might revolutionize how scientists model the onset of magnetic reconnection and energy transport from the corona to the chromosphere. In the traditional flare model, electron acceleration and coronal plasma heating are caused by reconnection; subsequent energy transfer by nonthermal electron beams and thermal conduction produces the bright ribbons seen in H-alpha. The correct cross-sectional area of loops is required for electron beam flux calculation, a poorly constrained quantity to date.
The effects have broader implications than solar physics. Having knowledge of the fine-scale structure of flares can improve space weather forecasting, giving earlier warnings for geomagnetic storms that plague modern infrastructure. X-class flares like this one have the potential to cause radio blackouts within minutes and, when paired with a coronal mass ejection, trigger geomagnetically induced currents that are destructive to transformers and debilitating power grids.
The Inouye’s achievement provides Officer with a new regime of resolution based on advances in adaptive optics that have been made for other solar telescopes, but its coronal-optimized design and aperture drive provides a new regime of resolution. As Philip Goode, research professor at NJIT’s Center for Solar-Terrestrial Research, has said of the technology, With coronal adaptive optics now in operation, this marks the beginning of a new era in solar physics, promising many more discoveries in the years and decades to come.
For Tamburri, the era was revolutionary: “It’s like going from seeing a forest to suddenly seeing every single tree.” The forest here is the Sun’s corona and the trees are the smallest magnetic loops ever seen, each a potential key to the physics of the most violent storms in the solar system.
