The Sun 77
FIGURE 4 (Left) Numerical simulation of cellular convection at the solar surface, performed by
Fausto Cattaneo and Andrea Malagoli; (Right) High-resolution observation of the granulation
pattern in the solar photosphere. A granule has a typical size of 1000 km, representing the surface
of an elementary convection cell. The large black area represents a sunspot, where the
temperature is cooler than the surroundings. (This image was taken by Tom Berger with the
Swedish Solar Observatory.)
∼1kms−^1. In the quiet Sun, away from active regions, the
mean photospheric magnetic field amounts to a few Gauss.
3.3 Sunspots
Sunspots are the areas with the strongest magnetic fields,
and therefore a good indicator of the solar activity (Fig. 5,
bottom). Thebutterfly diagramshows that sunspots (or ac-
tive regions) appear first at higher latitudes early in the solar
cycle and then drift equatorward toward the end of the so-
lar cycle (Fig. 5, top). Since all solar activity phenomena
are controlled by the magnetic field, they have a similar
solar cycle dependence as sunspots, such as the flare rate,
active region area, global soft X-ray brightness, and radio
emission. The appearance of dark sunspots lowers the to-
tal luminosity of the Sun only by about 0.15% at sunspot
maximum, and thus the variation of the sunlight has a neg-
ligible effect on the Earth’s climate. The variation of the
EUV emission, which affects the ionization in the Earth’s
ionosphere, however, has a more decisive impact on the
Earth’s climate.
An individual sunspot consists of a very dark central um-
bra, surrounded by a brighter, radially striated penumbra.
The darkness of sunspots is attributed to the inhibition of
convective transport of heat, emitting only about 20% of
the average solar heat flux in the umbra and being signifi-
cantly cooler (∼4500 K) than the surroundings (∼6000 K).
Their diameters range from 3600 to 50,000 km, and their
lifetime ranges from a week to several months. The mag-
netic field in the umbra is mostly vertically oriented, but
it is strongly inclined over the penumbra, nearly horizon-
tally. Current theoretical models explain the interlocking
comb structure of the filamentary penumbra with outward
submerged field lines that are pumped down by turbulent,
compressible convection of strong descending plumes.
Sunspots are used to trace the surface rotation, since
Galileo in 1611. The average sidereal differential rotation
rate isω= 14. 522 − 2 .84 sin^2 deg/daywhereis the heliographic latitude. The rotation rate of an
individual feature, however, can deviate from this average