The highest-resolution images ever taken of the sun have caught a phenomenon astronomers had never resolved before: countless small whirlpools of plasma spinning along the edges of the sun's granulated surface, some no more than about 20 kilometers across.
The vortices were captured with the NSF Daniel K. Inouye Solar Telescope on Haleakalā, Maui, working alongside advanced computer simulations built by researchers at the Max Planck Institute for Solar System Research in Germany. The U.S. National Solar Observatory and the High Altitude Observatory also contributed to the work, published Aug. 5 in the journal Nature.
An instability familiar from Earth's oceans
The swirls form where plasma flows of different speeds slide past one another at the fringes of solar granules, the cell-like patterns created by roiling convection just beneath the sun's visible surface. Physicists call the resulting turbulence a Kelvin-Helmholtz instability, the same effect that curls breaking ocean waves and stripes the cloud bands of Jupiter.
To detect the vortices, we needed to resolve structures on the solar surface about 20 kilometers in size. That is at the limit of what even the world's largest solar telescope and state-of-the-art simulations can achieve.
Michiel van Noort, Max Planck Institute for Solar System Research
Institute director Sami K. Solanki said the discovery shows how much of the sun's behavior hinges on structures at the very edge of what instruments can see: "The newly discovered plasma vortices impressively demonstrate how minute processes — at the limit of what we can resolve using all available techniques — significantly determine the nature of our star."
A possible clue to the corona's heat
Solar physicists have long struggled to explain why the sun's outer atmosphere, the corona, burns at temperatures of a million degrees or more while the visible surface beneath it is far cooler, a puzzle known as the coronal heating problem. The team suspects the vortices could help twist and braid the sun's magnetic field lines, storing energy that is later released in small bursts called nanoflares, which some researchers believe collectively heat the corona.
The researchers caution that the vortices sit at the outer edge of what current instruments can measure, and confirming how much they actually contribute to coronal heating will require further observation campaigns and refined simulations. Even so, the images offer a rare direct look at plasma physics that until now had only been predicted in theory.
