A sunspot is a strongly magnetic and relatively dark area of the photosphere of the Sun. The darkest central area of a sunspot, the umbra, is roughly circular in shape and typically 18,000 km (11,000 mi) in diameter. The umbra is surrounded by a somewhat less dark annular region, the penumbra, roughly doubling the umbral diameter. The surface brightness of the umbral area is only about 20% of the corresponding total brightness of the undisturbed photosphere. The decreased emission corresponds to that of an (idealized) perfect blackbody radiator at the effective temperature of 3,700 K, compared to the photospheric effective temperature of 5,800 K. The corresponding relative brightness of the penumbra is about 75%, and its effective temperature is 5,400 K. Spots are distributed on the Sun mainly between ± 40¡ solar latitude. A common statistical measure of the frequency of occurrence of spots is the Wolf sunspot number, an empirical parameter derived from the daily number of individual spots and spot groups observed on the disk. Spots occur in bipolar magnetic groups, whose dipole axes tend to be oriented in the east-west direction. In such a dipole, the solar magnetic field extends as a tube of magnetic flux from below the photosphere, forms a loop above the surface in the corona, and then resubmerges into the solar interior, often in a spot of opposite magnetic polarity. Magnetic field intensities up to 4,000 gauss have been measured in spot umbrae. The development of a typical sunspot group begins when a loop of magnetic flux emerges from the solar interior through the photosphere. Several small spots become visible and move apart, growing in size over the next 5 to 10 days. The group decays over a period of weeks, but large spots may remain for months. The strong radial magnetic field of a spot's umbra apparently makes it difficult for normal energy-transporting convection processes to occur, helping to explain the relatively low temperature of the gas in a spot.
In addition, the gas-pressure gradient between the cool umbra and its surroundings is apparently sufficient to balance the tendency of magnetic field lines to push apart and distribute themselves evenly. Eventually, however, the random motions of the ionized gas near the photosphere act to disassemble the strands of the tube of magnetic flux. The number of sunspots varies over an 11-year cycle in which spots first appear at higher solar latitudes, increase in number as they appear at lower latitudes, and finally diminish and cease as they appear within 7¡ of the solar equator. The latitude spread of the spots also narrows as the cycle proceeds. This cycle forms half of a 22-year magnetic cycle in which the polarities of the leading and trailing sunspots during the second 11-year period are opposite to polarities during the first 11-year period. The physical mechanisms underlying these cycles are not yet well understood. It is interesting that dark sunspots are usually accompanied by bright areas of the photosphere called faculae (Latin for "torches"), whose magnetic fields are almost as intense as those in sunspots. The main difference seems to be that in spots the magnetic field extends more or less uniformly over a much larger area than in faculae, which consist of individual tubes of magnetic flux with diameters of less than 1000 km (620 mi).