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, researchers at the National Center for Atmospheric Research (NCAR) in Boulder, Colorado, have found bright rings around eight sunspots. The presence of these rings sheds light on why sunspots are dark. The research is published this week (October 14) in the journal Nature. NCAR's primary sponsor is the National Science Foundation (NSF).
According to lead author, Mark Rast, the rings' presence supports the idea that the spots appear dark because their magnetic fields block heat transport.
With temperatures of 4000 Kelvin ( 3700 Celsius), sunspots are both cooler and darker than the surrounding solar disk, or photosphere, which hovers at 6000 K (5700 degree C) when the sun is quiet. The newly discovered rings are only 1% brighter, or ten degrees hotter, 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 evidence of even faint rings suggests that convective heat transport around sunspots is structured and vigorous rather than evenly diffuse, as the models indicate.
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 turbulence in the sun's interior.
In the past, measurement of the rings has been difficult. With the aid of the Precision Solar Photometric Telescope at NCAR's Mauna Loa Solar Observatory in Hawaii, Rast found rings around eight spots. According to Rast, the bigger the spot, the hotter and brighter the ring. He then analyzed data taken with NCAR's Advanced Stokes Polarimeter at the National Solar Observatory in New Mexico, which measures the sun's magnetic field.
"The existence of even such faint rings suggests that either sunspots are shallow phenomena," says Rast, "or convective flows around the spots transport heat to the surface more efficiently than earlier models suggest." Such flows may play an important role in sunspot birth and growth.
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