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November 1999

Why are sunspots dark? Bright rings cast shadow on solar models

Left: A sunspot and its penumbra observed in the CaIIK (calcium) band of the solar spectrum. The brightest areas around the penumbra are vertical magnetic flux tubes. The gray, cloudlike shapes are probably convection cells that release heat previously suppressed within the spot. These lie in the ring region surrounding the sunspot, where average temperature is about 10 Kelvins (18 degrees F) higher than the quiet photosphere. Right: A sunspot observed through a blue filter. The barely visible ring appears as a subtle halo slightly brighter than the surrounding solar disk. (Images courtesy Mark Rast.)

Mark Rast. (Photo by Carlye Calvin.)

The Chinese noticed dark spots on the sun as early as 25 B.C., and Galileo gazed at them through his telescope in 1611, but they have remained cloaked in mystery over the centuries. Now HAO's Mark Rast and colleagues have found bright rings around eight sunspots. The presence of these rings sheds light on why sunspots are dark, and it could spell trouble for conventional models of turbulent diffusion. The research was published on 14 October in the journal Nature.

Scientists have been searching for the rings since they were first predicted 25 years ago. According to Mark, the rings' presence supports the idea that the spots' darkness is their magnetic fields block heat transport. "These findings could change our understanding of how heat is transported under turbulent conditions," he says.

With temperatures of 4,000 Kelvins (6,700° F), sunspots are both cooler and darker than the surrounding solar disk, or photosphere, which hovers at 6,000 K (10,300°F) when the sun is quiet. The newly discovered rings are only 1% brighter than the quiet photosphere, and they compensate for only 10% of the sunspots' missing energy. Their contribution to the amount of solar energy reaching the earth is negligible. But their existence could be earthshaking for turbulent diffusion models. Evidence of even faint "bright rings" suggests that convective heat transport around sunspots is structured and vigorous rather than evenly diffuse, as the models indicate.

"The rings suggest that either sunspots are shallow phenomena or else convective flows around the spots transport heat to the surface more efficiently than the turbulent diffusion models suggest. I believe the latter is the missing component in the models," says Mark. Such flows may play an important role in sunspot birth and growth.

Scientists have long believed that sunspots are cross-sections of magnetic, rope-like structures whose origins lie deep in the sun's interior. Their missing heat, they say, should appear on the sun's surface as a bright ring around the spot. However, the rings have never been conclusively observed. Current models explain their absence as the result of heat dispersal through homogeneous turbulence in the sun's interior.

In the past, measurement of the rings has been difficult because vertical magnetic flux tubes show up as bright splotches around sunspots, obscuring the faint rings. With the aid of the Precision Solar Photometric Telescope (PSPT) at NCAR's Mauna Loa Solar Observatory, Mark found rings around eight spots in all solar spectrum wavelengths measured. According to Mark, the bigger the spot, the hotter and brighter the ring. He then analyzed data taken with HAO's advanced Stokes polarimeter, which measures the sun's magnetic field, at the National Solar Observatory (NSO) in New Mexico. His analysis showed that only a small fraction of the rings' brightness was due to magnetic intensity of vertical flux tubes. The rest, he believes, is heat suppressed by the magnetic field within the spot, which then emerges as a subtle halo.

The PSPT was developed at NSO as part of NSF's Radiative Inputs from Sun to Earth (RISE) program, which is dedicated to understanding solar radiative variability as a possible driver of climate change.

Anatta

On the Web:
A video showing a sunspot's evolution over the course of a month can be found on the Web.


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