sunspots

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sunspots dark, usually irregularly shaped spots on the sun's surface that are actually solar magnetic storms. The Chinese recorded dark features on the sun seen with the naked eye in 28 BC Other observers including Kepler suspected that these events might be transits of Mercury or Venus. Galileo observed them systematically for several weeks before concluding that they had to be events taking place on the solar surface. The temperature of the spots is lower than that of the surrounding photosphere; thus the spots are darker. All but the smallest show a dark central portion (the umbra) with a lighter outer area (the penumbra). Studies of the spectra of sunspots show evidence of the Zeeman effect , indicating the presence of a large magnetic field. In addition, measurements of the Doppler effect in the spectral lines show that there is a vortex motion in sunspots similar to that of a tornado on earth. The lower temperature of the gases constituting a sunspot results from the lower pressure due to the strong magnetic field. Sunspots appear usually only between latitudes from 5° to 35° north and south of the sun's equator. Sunspots are not permanent since the sun's surface is gaseous. Because the sun rotates on its axis, a sunspot cannot be observed continuously for more than about two weeks. In 1826 amateur astronomer Heinrich Schwabe began a series of solar observations (in hopes of finding planet Vulcan). By 1843 he had collected enough data to announce the existence of the sunspot cycle. An 11-year cycle from one period of maximum activity to the next is usually observed. However, a period during which most sunspots have one magnetic polarity is followed by another period during which most have the opposite magnetic polarity; thus, the cycle actually covers 22 years. During each 11-year period sunspots appear first at higher latitudes and later at latitudes closer to the solar equator as the period progresses. The spots often form in pairs or groups, with a large, long-lived leader spot matched with one or more smaller spots of opposite magnetic polarity. A number of phenomena are associated with sunspots. Sunspot activity produces various disturbances on earth—these include magnetic storms which manifest themselves as aurorae, interference with radio reception and electric power grids, and disturbances of the magnetic compass. Periods in which an increase in sunspots is observed are called active periods. Reviewing historical records in 1890, E. Maunder noticed that sunspot counts fell drastically between 1645 and 1715. In 1976 J. Eddy correlated Maunder's data with a low frequency of aurorae and the reduced sizes of annual tree rings. This "Maunder Minimum" may have played a role in the unusually low temperatures in the northern hemisphere during this period, which is known as the Little Ice Age.

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.

sunspot Region in the Sun's photosphere which is cooler than its surroundings and appears darker. Sunspots vary in size from c.1000 to 50,000km (600 to 30,000mi), and occasionally up to c.200,000km (125,000mi). Their duration varies from a few hours to a few weeks, or months for the very biggest. Sunspots occur where there is a local strengthening of the Sun's magnetic field.

sun·spot / ˈsənˌspät/ • n. Astron. a spot or patch appearing from time to time on the sun's surface, appearing dark by contrast with its surroundings.

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