solar activity and sunspots

solar activity and sunspots The Sun provides most of the extraterrestrial energy which reaches the Earth's surface. The average annual energy reaching the Earth from the Sun is approximately 1400 W m⁻². Average surface temperatures on the Sun are estimated to be of the order of 6000 °C. Earth-based observations of the lower latitudes (30° N–30° S) of the Sun's photosphere reveal dark patches or spots; such observations have been made since the seventeenth century. The so-called ‘sunspots’ vary in size, abundance, and duration. At any one time there may be as many as 20 or 30 such sunspots which can range in size from 10³ to 2 × 10⁵ km in diameter. Each sunspot consists of a dark central umbra, which is estimated to have a temperature in the region of 4000 °C, and a surrounding penumbra, at a temperature in the region 5000 °C.

Sunspots show pronounced cyclic behaviour, the major fluctuations having an approximately 11-year period. Maxima of successive 11-year periods show a periodic variation in intensity which reflects a periodicity of around 90 years (termed Gleissberg cycles). The cycles do not exhibit an exactly 11- or 90-year periodicity; power spectra calculated for sunspot time-series consequently exhibit broad rather than distinct peaks. An observed periodicity of 22 years (termed the Hale cycle) may be due to the coupling of adjacent 11-year cycles, which are magnetically oppositely polarized. Evidence has also been presented for the existence of a 200-year cycle. During the 11-year cycle, the abundance of sunspots and their size fluctuate in a more or less regular manner. At the start of the cycle, spots appear in both hemisphere at about 35° from the solar equator. As the cycle proceeds, the initial spots fade and new, more numerous spots occur at lower latitudes. By the end of any given cycle the sunspots are concentrated at latitudes around 5° from the solar equator.

Sunspots are typically measured using the Wolf Relative Sunspot Number. This is a measure of the number of groups of spots combined with the number of individual spots, calculated on a monthly basis. Sun-spot activity varies from zero, as was observed during the Maunder Sunspot Minimum which occurred synchronously with a period of global cooling from ad 1525 to ad 1850, up to about 200 in the highly active period which peaked in 1957. An instrumental record of sunspot cycles now extends from 1750. It includes 22 sunspots cycles, and indicates that the overall variation of energy during a cycle is of the order of a few parts per thousand (i.e., less than 1 per cent).

Associated with sunspots are brighter areas called faculae. The total solar radiance, or solar energy flux, is related to the sunspot cycle in the following manner:E = 1366.82 + 7.71 × 10⁻³ S_n Wm⁻²,

where E is the solar energy flux reaching the Earth and S_n is the number of sunspots. The fact that solar output rises with increasing sunspot numbers suggests that variations in faculae brightness are a major factor in determining total solar energy flux. Instead of exhibiting a uniform change in brightness across the entire solar spectrum, variations are observed to be most marked in the UV range.

Stephen Stokes

Bibliography

Bradley, R. S. and and Jones, P. D. (1992) Climate since a.d. 1500. Routledge, London.