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Sunspots appear as dark spots on the surface of the Sun.
Temperatures in the dark centers of sunspots drop to about 3700 K (compared
to 5700 K for the surrounding photosphere). They typically
last for several days, although very large ones may live for several weeks. Sunspots are
magnetic regions on the Sun with magnetic field strengths thousands of times stronger than
the Earth's magnetic field. Sunspots usually come in groups with two sets of spots. One
set will have positive or north magnetic field while the other set will have negative or
south magnetic field. The field is strongest in the darker parts of the sunspots - the
umbra. The field is weaker and more horizontal in the lighter part - the penumbra.
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Faculae are bright areas that are usually most easily seen near the limb,
or edge, of the solar disk. These are also magnetic areas but the magnetic field is
concentrated in much smaller bundles than in sunspots. While the sunspots tend to make the
Sun look darker, the faculae make it look brighter. During a sunspot
cycle the faculae actually win out over the sunspots and make the Sun appear slightly
(about 0.1%) brighter at sunspot maximum that at sunspot minimum.
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Granules are small (about 1000 km across) cellular features that cover the
entire Sun except for those areas covered by sunspots. These features are the tops of
convection cells where hot fluid rises up from the interior in the bright areas, spreads
out across the surface, cools and then sinks inward along the dark lanes. Individual
granules last for only about 20 minutes. The granulation pattern is continually evolving
as old granules are pushed aside by newly emerging ones
(470 kB MPEG movie from the Swedish
Vacuum Solar Telescope). The flow within the granules can
reach supersonic speeds of more than 7 km/s (15,000 mph) and produce sonic
"booms" and other noise that generates waves on the
Sun's surface.
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Supergranules are much larger versions of granules (about
35,000 km across) but are best seen in measurements of the "Doppler shift"
where light from material moving toward us is shifted to the blue while
light from material moving away from us is shifted to the red. These
features also cover the entire Sun and are continually evolving. Individual supergranules last for a day or two and have
flow speeds of about 0.5 km/s (1000 mph). The fluid flows observed in supergranules carry
magnetic field bundles to the edges of the cells where they produce the chromospheric network.
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