NSF PR 99-63 - October 14, 1999
National Science Foundation
National Center for Atmospheric Research
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Bright Rings Found around Sunspots Show Why Spots
Are Dark, Cast Shadow on Solar Models
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 ringsí 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 sunspotsí 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 sunís interior. Their missing heat,
they say, should appear on the sunís 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 sunís interior.
In the past, measurement of the rings has been difficult.
With the aid of the Precision Solar Photometric Telescope
at NCARís 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 NCARís Advanced Stokes
Polarimeter at the National Solar Observatory in New
Mexico, which measures the sunís 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.Ē Such flows may play an important role in
sunspot birth and growth.
Editors: Visual images are available at: ftp://ftp.ucar.edu/communications