solar activity(soh -ler) The collective term for time-dependent phenomena on the Sun. Sunspots, faculaeplages, filaments (or prominences), and flares all belong in this category, but the granulation and chromospheric network do not since their gross configuration is stable. The level of solar activity at present varies over an average period of approximately 11 years – the so-called solar cycle (see sunspot cycle).
the aggregate of phenomena observed on the sun and associated, for example, with the formation of sun-spots, faculae, flocculi, filaments, and prominences, with the occurrence of solar flares and disturbances in the solar corona, and with an increase in ultraviolet, X- , and corpuscular radiation. Such phenomena are usually observed in a limited region, called an active region, on the sun’s surface. The region may exist for a few days to several months.
When an active region arises, flocculi appear; they correspond to an increase in the brightness of the absorption lines of hydrogen and ionized calcium. After some time, usually a few days, small sunspots appear. The number and size of the sunspots gradually increase, and the intensity of other manifestations of solar activity become more pronounced.
The radiation excess in the hydrogen and calcium lines that characterizes the active region increases markedly when solar flares occur. Solar flares arise in the vicinity of developing or decaying sunspot groups and show up in the sudden appearance of emission in strong absorption lines (for example, the Ha and Hp lines of hydrogen or the H and K lines of ionized calcium) and in the increase in intensity of ultraviolet, X-, and corpuscular radiation. The level of emission in the radio-frequency region also increases. Small flares are observed almost daily in large sunspot groups; large flares occur comparatively rarely. Flares may last from a few minutes to several hours. The strength of the magnetic field in sunspots reaches several thousand oersteds.
The intensity of solar activity is described by such quantities as the relative sunspot number (Wolf number), the sunspot area, and the area and brightness of faculae, flocculi, filaments, and prominences. The mean annual values of these quantities vary in a periodic manner. The Wolf number, for example, has an average period of approximately 11 years; the length of the period ranges from 7.5 to 16 years. The value of the maximum of the 11-year cycle varies with a period of approximately 80 years.
Active regions are distributed on the sun’s disk in two belts that run parallel to the equator, on either side of it. The distance of the belts from the equator also varies periodically. At the beginning of the 11-year cycle, the active regions lie at their greatest distance from the solar equator. They gradually draw closer to the equator, and by the end of the cycle their average helio-graphic latitude is ±8°.
Solar activity exerts a substantial influence on terrestrial phenomena.
REFERENCESSolnechnaia sistema, vol. 1. Edited by G. Kuiper. Moscow, 1957. (Translated from English.)
Zirin, H. Solnechnaia atmosfera. Moscow, 1969. (Translated from English.)
E. A. BARANOVSKII