The world's largest solar telescope made small vortices visible that are changing our understanding of the sun
The highest-resolution image of the Sun's surface (photosphere) ever captured, taken at 416 nanometers by the Inouye Solar Telescope. At the edges of the solar granules, the image reveals fringed-looking structures that display swirling motions.
© NSF/NSO/AURA/MPS
To the point
- First-ever observation of tiny vortices on the Sun's surface: Images from the 4-meter National Science Foundation (NSF) Daniel K. Inouye Solar Telescope, Hawaii, and computer simulations reveal tiny vortices on the Sun's surface that have never been seen before
- Consistency with computer simulations: The vortices observed with the telescope at the edges of the solar plasma bubbles match very closely with structures predicted by complex computer simulations based solely on the laws of physics
- Understanding solar eruptions: The discovery may help us understand how energy builds up in the Sun's magnetic field before being explosively release
- Instabilities in the solar plasma: The researchers interpret the vortices as signs of Kelvin-Helmholtz instabilities. This effect occurs where fluids flow past one another at different speeds
Researchers from the U.S. National Science Foundation National Solar Observatory (NSF NSO), the Max Planck Institute for Solar System Research (MPS) in Germany, and the High Altitude Observatory (HAO) in the USA have made a groundbreaking discovery in the field of solar physics. New images of the Sun's surface taken with the world's largest solar telescope, the NSF Daniel K. Inouye Solar Telescope, built and operated by the NSO in Hawaii, along with highly sophisticated computer simulations, reveal tiny plasma vortices that have never before been made visible.
"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 and co-author of the new publication Michiel van Noort, who, among other things, contributed to the observations and conducted the data reduction and image restoration. The researchers used a broad-band imaging camera provided by the Max-Planck-Institute for Solar System Research (MPS).
The vortices occur at the edges of so-called granules, which densely cover the Sun's visible surface. They measure between 500 and 2,000 kilometers in diameter. Taken together, they form the Sun's granulation: a pattern reminiscent of the bubbles in a boiling liquid. In fact, the granulation stems from plasma flows that rise from the Sun's hot interior, cool down, and sink back into the depths. In the new study, researchers have now succeeded for the first time in visualizing fringed structures at the edges of the granules. Time and again, these structures display swirling motions resembling breaking ocean waves. Some of these "fringes" are little more than 20 kilometers wide. Resolving these delicate structures is comparable to discerning a one euro coin from a distance of 180 kilometers.
Instabilities in the Solar Plasma
The researchers interpret the swirling plasma flows as signs of Kelvin-Helmholtz instabilities. This is a well-known effect in fluid dynamics. It occurs when two fluids flow past each other at different speeds. This generates shear forces at the interface, causing minute disturbances to grow into wave- or vortex-like flows. The effect manifests itself in a wide variety of contexts and on different scales - for example, on the surfaces of lakes or in ocean waves, in cloud formation, in the atmospheres of the giant gas planets Jupiter and Saturn, and in the interaction of the solar wind with planetary magnetospheres.
Apparently, also at the edges of solar granules adjacent plasma layers flow at different speeds, thereby creating the necessary conditions for Kelvin-Helmholtz instabilities.
A side-by-side comparison of a observation from the Inouye Solar Telescope (top left) and an image generated by state-of-the-art, physics-based computer simulations (top right). The remarkable agreement between the two allows scientists to confirm the origin of the Kelvin-Helmholtz instability, a universal physical phenomenon that occurs when adjacent layers of fluid or gas move at different speeds, creating swirling patterns at their interface. A simulated map of the Sun's surface magnetic field (bottom right) confirms that these processes physically bend and deform the boundaries of the magnetic elements.
© NSF/NSO/AURA/HAO
The remarkable agreement between the two allows scientists to confirm the origin of the Kelvin-Helmholtz instability, a universal physical phenomenon that occurs when adjacent layers of fluid or gas move at different speeds, creating swirling patterns at their interface. A simulated map of the Sun's surface magnetic field (bottom right) confirms that these processes physically bend and deform the boundaries of the magnetic elements.
Twisted Magnetic Field Lines
The tiny plasma vortices now discovered on the Sun provide a completely new perspective on the processes through which the Sun stores and releases energy in its magnetic field, for example in the form of minute bursts of radiation known as nanoflares. According to current theory, the Sun builds up magnetic energy as magnetic field lines twist and coil - similar to the mechanical energy stored in a tightly coiled metal spring. This creates a highly energetic but also unstable magnetic field architecture. The stored energy can suddenly be released. In the process, known as "magnetic reconnection", the twisted magnetic field lines snap open and reconnect.
However, it was previously unclear what actually causes the magnetic field lines to twist in the first place. The new discovery could provide part of the answer to this question. Since the vortices apparently occur constantly and wherever the magnetic field is strong enough, they could be the driving force that routinely triggers the twisting.
The analyses also show that the mini-vortices efficiently mix magnetized and non-magnetized plasma on the solar surface. This could help the magnetic field spread rapidly from the surface into the Sun's atmosphere. Driven by changes in the solar magnetic field, our star's activity fluctuates in an eleven-year cycle - which is exceptionally rapid on a cosmic scale. Such a quick change in the Sun's magnetic "framework" is only possible if magnetic flux can be efficiently transported away through the solar atmosphere. Existing models cannot explain such rapid diffusion. On this issue as well, the newly discovered vortices could advance our understanding by a decisive step.
