Astronomers using the world's largest solar telescope have photographed swirling eddies of plasma on the Sun's surface small enough to fit inside a mid-sized city, resolving a texture of motion no instrument had captured before. Some of the whirlpools span as little as 20 kilometers, embedded within the churning granulation that covers the solar photosphere.
The observations came from the NSF Daniel K. Inouye Solar Telescope, perched near the summit of Haleakalā on Maui, whose 4-meter mirror gives it the sharpest ground-based view of the Sun ever achieved. An international team from the National Solar Observatory, the NSF National Center for Atmospheric Research's High Altitude Observatory, and Germany's Max Planck Institute for Solar System Research found the vortices recurring across the visible surface, with instability wavelengths of roughly 50 to 65 kilometers and fine internal structure narrowing to about 20 kilometers.
The swirls are produced by the Kelvin-Helmholtz instability, the same shear-driven process that curls the crest of a breaking ocean wave and carves the banded cloud belts visible on Jupiter. It arises wherever two layers of fluid, or in this case plasma, slide past each other at different speeds, and on the Sun it appears at the ragged boundaries between neighboring magnetic structures in the photosphere.
A missing piece of a 50-year puzzle
Solar physicists have long struggled to explain why the Sun's outer atmosphere, the corona, runs roughly a million degrees hotter than the surface just beneath it, an inversion that defies ordinary thermodynamics. Existing models could not account for how magnetic fields mix and diffuse quickly enough to release that much energy. The newly resolved vortices may supply the missing mechanism: by twisting and braiding magnetic field lines at small scales, they could help trigger the tiny, frequent energy bursts known as nanoflares that some researchers believe collectively heat the corona and help drive the solar wind.
We believe that the discovery of Kelvin-Helmholtz instability in the solar photosphere is a major step forward.
David Boboltz, deputy director, National Solar Observatory
The findings, led by physicist David Kuridze, were published August 5 in the journal Nature under the title "Ubiquitous Kelvin-Helmholtz Instabilities Driving Plasma Mixing on the Sun." Researchers describe the vortices as theorized for decades but never before directly observed at this scale on the Sun.
Scientists now plan follow-up campaigns with the Inouye telescope to track how often the vortices form, how much energy each one releases, and whether their combined effect is large enough to close the long-standing gap between observed coronal temperatures and what current models predict. Refining that estimate could also sharpen forecasts of the solar wind and space weather that affect satellites, spacecraft and power grids on Earth.
