Context: The Daniel K. Inouye Solar Telescope (DKIST) has produced the highest-resolution images yet of the Sun’s surface, revealing previously unseen small-scale plasma vortices at the boundaries of solar convection cells. The findings may improve understanding of how energy and magnetic fields are transported through the solar atmosphere.
Key Findings
- Scientists observed small, rapidly moving plasma whirlpools (vortices) along the boundaries of solar granules for the first time.
- These vortices may efficiently transport energy and magnetic flux and could help braid the Sun’s magnetic field lines at small scales.
- Braided magnetic fields can store and release energy, potentially contributing to solar flares and other eruptions and helping explain the extremely high temperature of the Sun’s outer atmosphere.
- The findings were published in Nature on August 5.
Solar Photosphere and Convection Cells
- The photosphere is the visible surface of the Sun and consists of a turbulent mass of superheated plasma; the sunlight reaching Earth is energy emitted from this layer.
- The photosphere has a beehive-like pattern formed by convection cells, whose visible features are called granules.
- Each convection cell can be around 2,000 km across and typically lasts 5–10 minutes before dissipating.
- For about a century, solar images showed the boundaries of granules as blurred or frayed, leading scientists to debate whether this was a genuine feature or a limitation of telescopes.
Why Earlier Telescopes Could Not Resolve the Structures?
- The researchers found that the limitation was primarily the telescope aperture: telescopes with primary mirrors smaller than 2 metres could not resolve the fundamental small-scale perturbations at the Sun’s surface.
- The new observations were therefore made using the NSF Daniel K. Inouye Solar Telescope (DKIST) in Hawaii, which has a 4-metre primary mirror and began scientific operations in late 2019.
- DKIST is located near the 10,000-foot-high summit of Haleakalā volcano and is currently the world's largest solar telescope.
How Were the Observations Made?
- Researchers observed the magnetically active solar region NOAA 14060 using light of 416 nanometres, corresponding to visible blue-violet light.
- DKIST was paired with FastCam, a high-speed camera jointly developed by the U.S. National Solar Observatory (NSO) and Germany’s Max Planck Institute for Solar System Research (MPS).
- The camera used an exposure time of just 100 microseconds and could capture up to 740 frames per second, allowing the turbulent solar atmosphere to be recorded with high temporal resolution.
- Multi-frame blind deconvolution, an advanced image-processing technique developed/used by Michiel van Noort of MPS, was employed to remove observational blurring; multiple corrected images were then combined to obtain a sharper image.
- The resulting images covered approximately 5,800 × 4,350 km with a resolution of 19 km, enabling structures comparable to a ₹10 coin viewed from Mumbai to be distinguished from Pune.
What Did the New Images Reveal?
- The apparently frayed edges of solar granules were actually composed of numerous small plasma coils.
- These coils were approximately 25–170 km long, separated by around 65 km, and possessed their own small-scale magnetic fields.
- The coils moved along granule boundaries, twisting and interacting with one another.
- Their interaction generated rapidly changing plasma vortices or whirlpools that could change shape within seconds.
How are Solar Vortices Formed?
- A vortex generally develops at the interface between adjacent fluids or fluid layers moving at different velocities, similar to small whirlpools generated when strong wind blows across a calm river.
- The observed solar vortices are produced by the interaction of plasma flows along the boundaries of convection cells.
- Similar atmospheric vortices have previously been observed on gaseous planets such as Jupiter, but this is the first observation of such wave-like vortex activity on the Sun’s surface.
Rapid Growth and Evolution of Vortices
- The fastest-growing vortices increased to 2.7 times their original size in only 18 seconds.
- Even slower-growing vortices reached this size within about 71 seconds.
- Their rotational speeds were approximately 1.6–2.8 km/s.
- Over a period of about three minutes, vortices continuously formed, merged and fragmented, demonstrating highly dynamic behaviour.
Confirmation Through Magnetohydrodynamic Simulations
- The observations were supported by magnetohydrodynamic (MHD) computer simulations, which model the behaviour of electrically conducting fluids such as plasma under the influence of magnetic fields.
- The observed and simulated vortices showed comparable rotational speeds and energy, strengthening the interpretation that the structures are genuine physical phenomena.
- Simulations indicated that the vortices originate approximately 100–400 km below the photosphere.
Significance for Solar Physics
- The vortices provide a new mechanism for studying the interaction between solar convection and magnetic fields.
- Their turbulent motion continuously distorts and twists magnetic fields, potentially creating the small-scale motions required to braid magnetic field lines.
- Braiding can accumulate magnetic energy, which may subsequently be released higher in the solar atmosphere, contributing to solar flares and other energetic solar events.
- The vortices may therefore act as small-scale energy and magnetic-flux transport mechanisms, linking processes occurring beneath the photosphere with energetic phenomena in the upper solar atmosphere.
Why is the Sun's Corona So Hot?
- The corona, the Sun’s outer atmosphere, reaches temperatures of millions of kelvin, much hotter than the underlying photosphere.
- This apparently counterintuitive temperature structure is known as the coronal heating problem.
- Magnetic-field interactions, reconnection and the dissipation of magnetic energy are among the mechanisms studied to explain coronal heating.
- The newly observed vortices could provide the small-scale motions and magnetic braiding necessary for transferring energy into the upper atmosphere.
Way Forward
- Scientists now need to determine how much energy these small-scale instabilities transport through the solar atmosphere.
- Further research will examine how these tiny vortices are connected to larger and more energetic solar phenomena, including flares and other atmospheric eruptions.
