Something New on the Sun

Innovative telescope homes in on sunspots

The sharpest-ever visible-light images of the sun are revealing puzzling new features about sunspots, the dark regions where the sun’s powerful magnetic field is concentrated. The pictures are the first to show sun structures as small as 90 kilometers in diameter. “These images take the study of sunspots into a new regime,” says Thomas R. Rimmele of the National Solar Observatory in Sunspot, N.M. They’ve also revealed new features, some of which can’t be readily explained by any existing model of how sunspots work.

SUNSPOT MYSTERIES. The sharpest image of the sun ever taken shows part of a large sunspot recorded July 15. Filaments surround a dark center. Some of the bright filaments contain dark cores, which had not been previously observed. Scharmer et al./Nature
Three panels show small sunspots with thin dark lines, dubbed hairs, and thicker dark lines, called canals, in the surrounding solar surface. The fourth panel (bottom right) shows sunspot filaments with dark cores. Scharmer et al./Nature

The scientists used the Swedish 1-meter Solar Telescope on La Palma in the Canary Islands. Operating since May, it ranks as the world’s sharpest eye on the sun. To achieve its exquisite sharpness, the telescope uses an adaptive-optics system, which corrects for the blurring effects of Earth’s turbulent atmosphere. Nineteen elements of a pliable mirror within the telescope flex 1,000 times each second to adjust for rapid changes in the air above it.

The solar telescope has recorded both strange and familiar features within the Earthsize sunspots, which can have magnetic fields 5,000 times stronger than Earth’s. Intense magnetic fields within these huge blemishes can belch flares and clouds of ionized gas that can damage electrical power systems and Earth-orbiting satellites (SN: 11/17/01, p. 310: Available to subscribers at SOHO craft gets the lowdown on sunspots). Therefore, astronomers have been eager to learn more about sunspots.

Sunspots are cooler and darker than the rest of the sun’s surface because their magnetic fields impede hot gases from rising to the surface and radiating away their heat. The darkest, central region of a sunspot, called the umbra, features tightly bundled magnetic field lines. Long, thin filaments radiate from the umbra into a brighter surrounding region called the penumbra.

These field lines diverge from the umbra like a bundle of wheat stalks splaying outward from the tie that binds them.

The bright filaments, which range from 150 to 180 km in diameter, may represent places where flowing gases have been heated by the magnetic fields or where some of the roiling gases from the interior have risen to the surface, says study coauthor Dan Kiselman of the Royal Swedish Academy of Sciences in Stockholm.

Theorists suspect that the penumbra is key to keeping a sunspot intact. But penumbra images had been too fuzzy to reveal internal structure.

With the Swedish telescope, researchers have obtained images that clearly resolve the filaments. Surprisingly, many of the bright filaments have dark cores, report Kiselman, Göran B. Scharmer, and their colleagues in the Nov. 14 Nature.

“This is the kind of result that scientists groan about but love to see because it sends everyone back to the drawing board,” says Craig DeForest of the Southwest Research Institute in Boulder, Colo.

Scharmer and his team speculate that the dark cores might actually be cooler, denser gas that lies above the other gas in the filaments. Alternatively, the cores could be the central parts of gaseous tubes sculpted by magnetic field lines.

The researchers also describe new sunspot structures that they call hairs and canals.

“Our hope is that the new data will test and weed out sunspot models,” Kiselman says. Measuring the magnetic field strength and velocity of gas in the filaments’ dark cores may reveal yet more details on how the sun got its spots.


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