Here’s a deceptively simple question with a complicated answer: What does a star look like?
Of course, a star is a twinkling point of light in the heavens—or, in the case of our sun, a ferociously fiery orb of glowing plasma that dominates Earth’s sky. But what about a star’s actual visible surface, its so-called photosphere? Although this is the only part of the sun we can easily see, we’re surprisingly still in the dark about the photosphere’s fine details.
Now, however, unique observations from the world’s largest and best solar telescope are shedding light on this region’s secrets. Using the U.S. National Science Foundation’s (NSF’s) four-meter Daniel K. Inouye Solar Telescope (DKIST) in Hawaii, an international team of scientists has obtained the sharpest-ever images of the sun’s surface, revealing an alien vista covered in never-before-seen whirlpools of plasma. Validated with state-of-the-art computer simulations and published in Nature, the landmark result helps explain how energy moves through the photosphere and erupts into potentially Earth-threatening space weather events.
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One thing that astronomers already knew is that the sun’s photosphere acts a bit like a pot of boiling water. Bubbling up as heat rises from below, thousand-kilometer-wide convective cells of plasma called “granules” undulate across our star’s face, with each granule outlined by canyonlike “lanes” carved by plasma flowing back down as it cools. Powerful magnetic fields loop through the photosphere as well, and where they are strongest, the plasma flattens into darker, colder “sunspots,” which occasionally unleash outbursts of plasma and radiation that can disrupt spacecraft, power grids and global telecommunications.
This tidy picture of the photosphere, however, is akin to having a crude map of Earth that shows subcontinents and seas yet leaves smaller features hidden; you might see spinning hurricanes and volcanic outbursts, but you won’t really know how they occur. The new DKIST images change that.
This visualization—produced via data from the NASA Solar Dynamics Observatory satellite, DKIST and computer simulations—shows the extreme detail of new observations revealing whirlpools of plasma on the surface of the sun.
“Observational solar physics, from early sunspot observations up to today, is essentially the story of resolving magnetic elements—their shape, size and dynamic evolution,” says David Kuridze, the paper’s co-lead author and an astronomer at the NSF’s National Solar Observatory (NSO) in Boulder, Colo. “For the first time, these observations have resolved the boundaries of individual magnetic elements. Now we know that these boundaries are not simple, smooth or randomly deformed edges but dynamic swirling patterns.”
“This is a major breakthrough in solar physics,” says Mihalis Mathioudakis, an astronomer at Queen’s University Belfast, who was not a part of the study. The team “has identified this fundamental physical process in the smallest astrophysical scales,” an achievement that applies not only to the sun but also to other stars throughout the universe. And although the discovery was confirmed by computer modeling, he adds, because the result is based on simple, straightforward imaging, “it will stand the test of time and be undisputed.”
The “tiny” whirlpools, which range in size from about 20 to nearly 200 kilometers across, arise from a well-studied effect called the Kelvin-Helmholtz instability (KHI), which occurs when two fluids slip past each other at different velocities. The resulting “shear” at the boundary as the fluids mix creates eye-catching vortices that resemble cresting ocean waves; you can see them on Earth in some cloud formations and even in cream stirred into a cup of coffee.
To see them in the solar photosphere, the team pushed DKIST to its limits, zooming in on the outskirts of a sunspot and revealing dozens. The feat, says study coauthor and NSO senior scientist Friedrich Wöger, is equivalent to seeing ants crawling on Earth’s surface from an altitude of 100 miles (about 160 km). “We did immediately recognize the ‘signature vortices’ of KHI developing in our data and were super excited about that right away!” Wöger says. “But of course, that’s not enough in a scientific research sense. Providing the evidence that ‘what looks, walks and quacks like a duck actually is a duck’ is much more difficult.”
In its paper, the team compared DKIST’s images with the results of numerical models that used basic physics equations to simulate a slice of the photosphere and found that the models accurately reproduced the numbers, dynamics and other characteristics of the KHI vortices observed on the sun.
Theorists have speculated for decades about KHI in the sun’s photosphere. They have postulated that the phenomenon could help mix magnetized and nonmagnetized plasmas when, for example, downwelling flows in lanes slide past upwelling flows in granules. This could make KHI a key part of the sun’s turbulent cascade of convection, which unfolds in an 11-year-cycle that is defined by the waxing and waning of sunspot numbers. Just as importantly, the whirlpools could be the “engine” that allows magnetic field lines to tangle, twist and finally snap like rubber bands to generate the sun’s explosive outbursts.
“The continuous whirling and twisting [of KHI vortices] is likely to ‘braid’ the magnetic fields like hair,” Wöger says. This “flux braiding” is thought to build up energy that gets rapidly released when the field lines break and reconnect in new, lower-energy configurations—expelling plasma and radiation as flares and jets that can wash over Earth and other solar system worlds as extreme bouts of space weather. Flux braiding is also a leading explanation for the enigmatic “coronal heating problem”—the fact that the diluted outer atmospheres of the sun and other stars are somehow hundreds of times hotter than their photospheres.
Finding these tiny whirlpools on the sun is only the beginning, Wöger says. Further observations with DKIST will look for more of them across wider stretches of the photosphere. And improved models will seek to better understand how they emerge and whether they are, in fact, the missing links in the still-mysterious process of flux braiding that shapes the sun and the solar system alike.
“This discovery is just one important piece to a much larger picture, whether it be the origin and evolution of space weather events [or] simply how the sun functions,” he says. “The more of these pieces we find—and in greater detail, thanks to powerful tools like the NSF’s Inouye Solar Telescope—the clearer and more complete our understanding becomes.”
