Sunspot Cycle - This cycle, lasting an average period of 11 years, involves recurring increases and decreases in the number of sunspots that occur on the sun's surface.
[New Science; v141; 14; 1994.]
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Atmospheric Chemistry Glossary

The approximately 11-year quasi-periodic The approximately 11-year quasi-periodic variation in the sunspot number. The polarity pattern of the magnetic field reverses with each cycle. Other solar phenomena, such as the 10.7-cm solar radio emission, exhibit similar cyclical behavior.