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Threads of fire. Narrow, dark cores flicker within the bright filaments of gas around sunspots. Tick marks show intervals of 1000 kilometers.

Dark Hairs on the Sun

There may be nothing new under the sun, but there's something new on its surface. A sharp-eyed telescope has spied strange ribbons of cool plasma clustered tightly around sunspots, like sperm swarming an ovum. Magnetic fields must confine the dark threads of gas, but physicists don't yet know the details.

Sunspots are blotches of magnetic unrest that often grow bigger than Earth. The spots look dark because they're up to 2000ºC cooler than the rest of the sun's 6000ºC surface. Within the black center of a sunspot, called its umbra, the sun's magnetic field pierces the surface along vertical lines. The field lines tilt horizontally in the surrounding penumbra, a complex girdle of filaments that looks like a fiery comb. Earlier photos showed patterns as small as 150 kilometers wide, but that wasn't good enough for physicists to explain how penumbras arise.

Now, the sharpest images ever taken of the sun have improved that resolution to 90 kilometers. Solar physicist Göran Scharmer and his colleagues at the Institute for Solar Physics in Stockholm, Sweden, opened the world's second-largest solar telescope in May on the Canary Islands. A deformable mirror next to the 1-meter telescope erases blurring caused by air currents. The telescope's striking photos appear in the 14 November issue of Nature. The images reveal dark cores winding along the centers of bright filaments of gas, which weave together in the penumbra. The cores persist for an hour or more, suggesting that they are sustained by convective flows within the magnetic fields. "Seeing these small structures for the first time gives us the promise of studying the underlying physical mechanisms associated with magnetic fields on the sun," says Scharmer.

The images are an exciting step toward a richer sunspot model, agrees solar physicist John Thomas of the University of Rochester in New York. He notes that such a model requires precise knowledge of gas velocities as well as the strength and tilt of magnetic fields in the penumbra, data the Swedish team hopes to obtain next year. An improved model has applications beyond the sun, Thomas adds: "Sunspots are proving grounds to test our theories of interactions between gas flows and magnetic fields, which are common in more distant astrophysical objects that we can't resolve."

Related sites
Swedish Institute for Solar Physics, with movies and photos
U.S. National Solar Observatory, including plans for 4-meter solar telescope
Basics of solar physics and sunspots