Solar cycle

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This article is about the sunspot cycle. For 28-year cycle of Julian calendar, see Solar cycle (calendar).

30 years of solar variability

The Schwabe solar cycle, Schwabe-Wolf cycle, or sunspot cycle is the eleven-year cycle of activity of the sun, discovered in 1843 by the German astronomer Heinrich Schwabe. Since 1849 it has been measured using the Wolf number, based on the number of sunspots observed on the Sun. The cycle is also associated with the reversal of the solar magnetic polarity. The solar cycle is not strictly 11 years long; it has been as short as 9 years and as long as 14 years in recent years. Other possible cycles have been suggested, and are discussed in the article on solar variation.

Times of highest activity, known as solar maxima, are characterised by larger numbers of sunspots than at other times. Times of lowest activity are known as solar minima; during solar minima the solar magnetic field is almost dipolar. There was a solar maximum in 2001, and the sun entered the minimum (start) of solar cycle 24 in the year 2007.

As the image shows, the solar constant has varied by about 0.1% or 1 W/m² from its mean value of approximately 1366 W/m² over the last two solar cycles.

The Babcock Model explains the 11 and 22 year cycles as being due to a shedding of entangled magnetic fields.