sunspots, dark, usually irregularly shaped spots on the sun's surface that are actually solar magnetic storms. The spots are darker because the temperature of the spots is lower than that of the surrounding photosphere (the visible surface of the sun). All but the smallest sunspots show a dark central portion (the umbra) with a lighter outer area (the penumbra). 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.
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. They 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.
An 11-year cycle from one period of maximum sunspot 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. Periods in which an increase in sunspots is observed are called active periods.
A number of phenomena are associated with sunspots. Sunspot activity can produce solar flares and coronal mass ejections (eruptions of charged particles into space), which can cause various disturbances on earth—these include geomagnetic storms which manifest themselves as auroras, interference with radio reception and electric power grids, and disturbances of the magnetic compass. An extreme storm in 1859 affected the telegraph system, shocking operators. Another storm in 1989 caused a blackout in Quebec prov., Canada.
The Chinese recorded dark features on the sun seen with the naked eye in 28 B.C. 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. 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. 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 portion of the period (c.1550–1850) known as the Little Ice Age.
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.