sunspot

sunspot A dark area on the Sun's photosphere that is cooler than its surroundings, associated with very strong (0.4 tesla) magnetic fields. Spots generally appear in pairs or groups, the leading and following spots having opposite magnetic polarities. Spot sizes vary from small pores about 300 km across to groups spanning more than 100 000 km. The largest spots usually last longest, up to 6 months; some small spots may last for less than an hour. Sunspots are mostly confined to belts either side of the equator between about 40° and 5° latitude north and south, appearing at higher latitudes at the start of the sunspot cycle and moving towards the Sun's equator as the cycle develops. Well-developed spots have a darker interior, the umbra (2), about 1600 K cooler than the photosphere, and a lighter outer penumbra (2), which accounts for up to 70% of the spot's area and is about 500 K cooler than the photosphere. All spots start their lives as tiny dark pores, and may then develop into small penumbra-less spots arranged in pairs. In a developing group, the spots become much larger and more separated in the first two days, attaining their maximum area and complexity by the tenth day. Broad categories of spot groups can be defined. In the McIntosh scheme, which has replaced the formerly used Zürich scheme, a three-letter code describes the class of sunspot group (single, pair, complex), penumbral development of the largest spot, and compactness of the group. The Mount Wilson scheme is used to describe the magnetic field structure, which may be simple (bipolar or, if a single spot, unipolar) or complex. Spots that give rise to flares tend to be highly complex in appearance and magnetic field structure. Gas flows outwards from spots at low altitudes (the Evershed effect), and inwards at coronal altitudes.