Twenty kilometers across. That was the threshold scientists needed to reach on the solar surface before they could spot what they were hunting for, and it happens to sit right at the ceiling of what the biggest solar telescope on the planet can manage.
They pulled it off. Working with the NSF Daniel K. Inouye Solar Telescope, researchers have captured images of plasma vortices spinning across the face of the sun that had never been observed before, minuscule features that could account for the way our star banks, shifts and unleashes magnetic energy.
The research emerged from a collaboration between the U.S. National Science Foundation National Solar Observatory (NSF NSO), Germany’s Max Planck Institute for Solar System Research (MPS) and the High Altitude Observatory (HAO) in the USA. Observations from the Inouye telescope, the world’s largest solar telescope, built and operated by the NSO in Hawaii, were combined by the team with advanced computer simulations. A broadband camera supplied by MPS handled the imaging.
What 20 kilometers looks like from here
"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," said MPS scientist Michiel van Noort, a co-author of the new publication who worked on the observations, the data reduction and the image restoration.
A number of the freshly resolved features are barely larger than 20 kilometers end to end. Singling something that small out on the sun is about as difficult as spotting a one euro coin from 180 kilometers away.
The whirlpools live on the edges of granules
Where the vortices appear is along granule boundaries, granules being the structures that blanket the sun’s visible surface. Any single granule spans somewhere between 500 and 2,000 kilometers.
Taken together, they make up the sun’s granulation, a perpetually shifting pattern with the appearance of a liquid at a rolling boil. The analogy is apt, since granulation consists of plasma in motion. Hot plasma wells up from the sun’s interior, loses heat as it nears the surface, then sinks back down again.
The novelty here is the fringe-like structure running along those granule edges, which had never previously been resolved. Those fringes, it turns out, repeatedly develop swirling motions with the look of ocean waves caught right as they begin to break.
Physics you can watch on a lake
Kelvin-Helmholtz instabilities are what the researchers believe the swirls to be, and there is nothing exotic about that explanation. It is textbook fluid dynamics.
Such instabilities arise wherever two fluids slide past one another at differing speeds. That difference in velocity generates shear forces along the boundary, and minor disturbances there can build into waves or vortices.
The same mechanism turns up across enormously varied scales: across lake surfaces, in ocean waves, in the formation of clouds, in the atmospheres of Jupiter and Saturn, and at the meeting point of the solar wind and planetary magnetospheres. Along the borders of solar granules, adjacent plasma layers likewise seem to travel at different speeds, precisely the arrangement the instability requires.
The twist nobody could account for
This is where the significance runs deeper than a striking image. Prevailing theory says magnetic energy builds up as the sun’s magnetic field lines become twisted and coiled, much as mechanical energy is stored inside a tightly wound metal spring. The more twist there is, the more energetic and the less stable the configuration becomes.
Magnetic reconnection is how that energy is eventually released, with twisted field lines snapping open and reconnecting into a fresh arrangement. Nanoflares, tiny bursts of radiation, belong to the same picture.
The missing link has always been the opening move. What puts the twist into the field lines in the first place?
Part of that answer may lie with the vortices. According to the researchers, these small whirlpools crop up nonstop anywhere the magnetic field is sufficiently strong, which makes them a durable candidate for the mechanism that introduces twist into the sun’s field lines.
An eleven-year clock that models struggle to explain
Their analysis further indicates the mini-vortices are remarkably efficient at blending magnetized plasma with non-magnetized plasma at the solar surface. Such mixing could carry magnetic fields rapidly from the surface into the sun’s atmosphere.
The sun’s roughly eleven-year activity cycle is driven by changes in its magnetic field, and by cosmic standards that turnaround is extraordinarily quick. Remodeling a magnetic framework at that pace requires magnetic flux to be carried off through the solar atmosphere efficiently, and diffusion that fast is something current models struggle to account for. Here too, the vortices could fill a gap.
"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," said Sami K. Solanki, director of the MPS and co-author of the new publication.
That is the fair way to read the result. On its own it rewrites nothing in solar physics, and the team is careful to frame the vortices as a possible mechanism rather than a proven one. Still, the next time you hear that the sun’s magnetic cycle is a solved problem, bear in mind that a key part of it may rest on structures the size of a euro coin viewed from 180 kilometers, which have only just been photographed for the first time.