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sunspot cycleThe semiregular fluctuation of the number of sunspots over an average period of approximately 11 years – or 22 years if the respective magnetic polarities of the p - and f -spots are considered, because these reverse at sunspot minimum for the groups of the new cycle (see sunspots). The nature of the cycle is shown in the graph, where the index of sunspot activity is the relative sunspot number. It can be seen that the rise to sunspot maximum usually occupies a shorter time than the fall to minimum and that the amount of activity may vary considerably between two consecutive cycles. There is some evidence for a modulation of the amplitude of the cycle over a period of around 80 years, but more data are required before the reality of this can be established.
The phase of the sunspot cycle also determines the mean heliographic latitude of all groups. At minimum the first groups of the new cycle appear at ±30–35°. Thereafter the latitude range moves progressively toward the equator, until by the next minimum the mean latitude is around ±7°. Then, while the equatorial groups are petering out, those of the next cycle begin to appear in their characteristic higher latitudes. This latitudinal progression is known as Spörer's law . At any one time there may be a considerable spread in latitude, but groups are seldom seen farther than 35° or closer than 5° from the equator. The butterfly diagram is a graphical representation of Spörer's law obtained by plotting the mean heliographic latitude of individual groups against time (see graph). Its appearance has been likened to successive pairs of butterfly wings, hence the name.
The underlying cause of the sunspot cycle is thought to be the interplay between a large-scale relatively weak poloidal magnetic field beneath the photosphere, differential rotation, and convection. The poloidal field, which is constrained to move with the ionized solar material, becomes increasingly distorted by differential rotation until an intense toroidal field is produced. The strength of this field is further enhanced by the perturbing effect of convection, which twists the field lines into ropelike configurations that may penetrate through the surface to form sunspots. This will occur first in intermediate latitudes, where the field's rate of shearing is greatest, and thereafter in increasingly low latitudes.
The inclination to the equator of the fields of opposite polarity associated with the p - and f -spots is such that they may drift apart in both longitude and latitude, as a result of differential rotation and the cyclonic rotation of individual supergranular cells (see supergranulation). The latitudinal drift is responsible for an accumulation in the polar regions of magnetic flux of the same polarity as the f -spots in the respective hemispheres. Thus the intense (0.2–0.4 tesla) localized fields of sunspots are gradually dispersed to form weak (1–2 × 10–4 tesla) polar fields, which reverse polarity (not necessarily synchronously) around sunspot maximum. When this happens differential rotation no longer intensifies the subphotospheric toroidal field but rather weakens it and reestablishes a poloidal field of opposite direction to its predecessor.
The sunspot cycle may therefore be regarded (if this model is correct) as a relaxation process that is continually repeating itself. There is reason to believe, however, that at least some of the features of recent cycles may be transitory. In particular, a prolonged minimum, termed the Maunder minimum, from about 1645 to 1715, suggests that there is more than one circulatory mode available to the solar dynamo.
Sunspots are the most obvious but by no means the only manifestation of solar activity to undergo a cyclical change over a period of around 11 years. It is therefore proper to restrict the use of the term sunspot cycle to consideration of the fluctuation of the number of sunspots and to use the more general term solar cycle when considering the variation in the level of solar activity as a whole.