SOLAR CYCLE

period in which several important kinds of solar activity repeat themselves. The cycle, lasting about 22 years on the average, includes two 11-year cycles of sunspots, which display opposite magnetic polarities in alternate cycles, and two peaks and two declines in each of the various phenomena that vary in the same period as sunspots. These phenomena include fluctuations in the frequency of solar prominences and faculae (bright regions in the photosphere) and of magnetic effects, including the auroras and increased radio interference on Earth. Discovery of the basic 11-year sunspot cycle, sometimes also called the solar cycle, was announced by the German amateur astronomer Samuel Heinrich Schwabe in 1843. Since that time, attempts have been made to connect the solar cycle to a variety of other phenomena, including possible slight variations in the diameter of the Sun, sequences of annual growth rings in trees, and even the rise and fall of the stock market.