Inouye filmed plasma vortices on the Sun that may solve a 50-year mystery

Q: What is the coronal heating problem?

The Sun's photosphere sits at roughly 5,700°C. The corona — its outer atmosphere — burns at one to three million degrees, nearly 200 times hotter. Since energy flows from hot to cold, this extreme temperature has puzzled astrophysicists for roughly 50 years. The Kelvin-Helmholtz vortices are a candidate mechanism for delivering photospheric energy into the corona.

Q: What is the Kelvin-Helmholtz instability?

A fluid dynamics phenomenon named after Lord Kelvin and Hermann von Helmholtz. When two fluid or plasma layers move at different speeds, the shear force generates waves that curl into vortices — similar to breaking ocean waves or the rippled boundary between fast and still air masses.

Q: How powerful is the Inouye Solar Telescope?

The NSF Daniel K. Inouye Solar Telescope at Haleakalā, Maui, Hawaii carries the world's largest solar mirror at 4 meters across. It can resolve features 20 km across on the Sun — a star 150 million kilometers away.

There is something strange about the Sun: move away from its surface, and the temperature rises. The photosphere — the visible layer that emits the light we see — sits at roughly 5,700 degrees Celsius. The chromosphere above it reaches tens of thousands of degrees. And the corona, the Sun’s vast outer atmosphere, burns at one to three million degrees, making it nearly 200 times hotter than the layer beneath it.

This has been one of the most durable unsolved problems in astrophysics for roughly 50 years. Heat flows from hot to cold — basic thermodynamics. A corona hotter than the surface it crowns defies intuition, and yet it appears around every star in the sky. The energy heating it has to come from somewhere. Decades of theory have proposed mechanisms; none has closed the case.

The answer may live in whirlpools 20 kilometers across.

How they did it

The NSF Daniel K. Inouye Solar Telescope — a 4-meter instrument perched near the summit of Haleakalā, Maui, and the world’s most powerful solar telescope — captured images of the Sun’s photosphere with a resolution that had not been possible before. To detect the structures the team was looking for, they needed to resolve features roughly 20 kilometers across on a star 150 million kilometers away.

At that scale, the images revealed something no prior instrument had shown: fringe-like structures at the edges of solar granules, the convective cells roughly 500 to 2,000 kilometers wide that tile the photosphere like an enormous field of slowly churning bubbles. Along those edges, adjacent layers of plasma move at different velocities. The shear between those layers triggers the Kelvin-Helmholtz instability — a fluid dynamics phenomenon that curls the boundary layer into a cascade of vortices spaced 50 to 65 kilometers apart.

Kelvin-Helmholtz instabilities are well documented elsewhere: in weather fronts where cold and warm air masses meet, in the ocean’s thermocline, in Jupiter’s cloud bands, and in Earth’s magnetopause where the solar wind meets the planet’s magnetic field. Until now, no instrument had resolved them operating on the Sun’s photosphere. The Inouye telescope’s 4-meter aperture made the detection possible for the first time.

What the vortices do

The instabilities at granule boundaries appear to act as a mechanism for transporting magnetic flux through the solar atmosphere. As layers of plasma spin around each other, the magnetic field lines threaded through them get twisted, stretched, and pushed upward. The team’s analysis suggests these vortices are a source of the “missing magnetic diffusion” that theoretical models of the solar interior have required but could not locate — a mechanism for flux to move across magnetic field boundaries faster than straightforward diffusion allows.

A second implication concerns the corona directly. Twisted, compressed magnetic field lines store energy like a coiled spring. When they reconnect — snap into a lower-energy configuration — they release that energy rapidly. These brief bursts are called nanoflares. Enough of them, distributed across the entire solar surface, could sustain the corona at the extreme temperatures observed. The Kelvin-Helmholtz vortices may be what loads the spring.

“We believe that the discovery of Kelvin-Helmholtz instability in the solar photosphere is a major step forward in our understanding of the dynamics and evolution of solar and stellar plasma,” said Dr. David Boboltz, deputy director of the National Solar Observatory.

“To detect the vortices, we needed to resolve structures on the solar surface about 20 kilometers in size,” noted co-author Michiel van Noort of the Max Planck Institute for Solar System Research, describing the observational challenge.

What it doesn’t settle

This is a suggestive finding, not a final answer. The paper documents the presence of the instabilities and their likely role in magnetic diffusion. It does not prove that Kelvin-Helmholtz vortices alone can account for the full energy budget of the corona.

The coronal heating problem has attracted dozens of proposed solutions over the decades — Alfvén wave dissipation, magnetic reconnection at larger scales, cyclotron resonance, and others. What the Kelvin-Helmholtz result provides is a plausible driver operating at the smallest spatial scale yet resolved on the solar surface — a bottom-up mechanism that complements rather than replaces the others.

The vortices were also observed in a limited wavelength window and during specific solar activity conditions. Some theoretical models suggest additional instabilities may operate below 20 km — smaller than Inouye can currently resolve. Their behavior across the full range of solar cycle conditions, particularly during periods of intense magnetic field activity, remains uncharacterized.

What comes next

The Inouye Solar Telescope is undergoing instrument upgrades to improve its spectral coverage and sensitivity. Future observation campaigns are designed to capture granule-boundary regions simultaneously at multiple atmospheric layers — linking the photospheric vortices directly to measurable energy signatures in the chromosphere and corona above them.

The Sun is currently in an active phase of its 11-year cycle, a period that produces more intense magnetic field activity at granule boundaries. The team expects this to generate denser and more energetic Kelvin-Helmholtz instabilities — observations that should quantify how much energy the vortices actually deliver upward.

Common questions about Kelvin-Helmholtz instabilities on the Sun

What is the coronal heating problem?

The Sun’s photosphere (its visible surface) sits at roughly 5,700°C. The corona — its outer atmosphere extending millions of kilometers into space — burns at one to three million degrees. Since energy flows from hot to cold, and the corona is farther from the nuclear furnace at the Sun’s core, its extreme temperature has puzzled astrophysicists for roughly five decades. The Kelvin-Helmholtz vortices are a candidate mechanism for delivering photospheric energy into the corona.

What is the Kelvin-Helmholtz instability?

It is a fluid dynamics phenomenon named after Lord Kelvin and Hermann von Helmholtz. When two fluid (or plasma) layers move at different speeds along their shared boundary, the shear force between them generates waves that curl into vortices — similar to the white foam caps on ocean waves, or the rippled cloud boundary where a fast air mass slides past a stationary one. The same process now appears to operate on the Sun’s surface.

How powerful is the Inouye Solar Telescope?

The NSF Daniel K. Inouye Solar Telescope, located at Haleakalā on Maui, Hawaii, at roughly 3,000 meters elevation, carries the world’s largest solar mirror at 4 meters across. Its resolution is sufficient to distinguish features 20 km across on the Sun — a star 150 million kilometers away.

Does this solve the coronal heating mystery?

Not definitively. The paper establishes that Kelvin-Helmholtz vortices are real, ubiquitous, and carry a plausible energy transport mechanism. What it does not yet quantify is whether they deliver enough energy to sustain the full million-degree corona. Follow-up observations during the current active phase of the solar cycle are expected to address the energy budget directly.

The Inouye team’s next step is to track the vortices through multiple atmospheric layers simultaneously — from the photosphere where they originate through the chromosphere and into the corona itself — to trace the energy pathway from its photospheric source to the million-degree plasma above.

Reference: Kuridze et al., “Ubiquitous Kelvin–Helmholtz instabilities driving plasma mixing on the Sun,” Nature, 2026. DOI: 10.1038/s41586-026-10871-3

Tags: astronomy, coronal heating, inouye solar telescope, kelvin-helmholtz instabilities, plasma vortices, solar physics