11year

The 11-year solar cycle, commonly referred to as the sunspot cycle or Schwabe cycle, is the roughly decadal modulation of solar activity observed in the number of sunspots on the Sun’s surface. The cycle is tracked by counting sunspots and related indicators of solar magnetism.

Cycle length is variable, typically 9 to 14 years. It comprises solar maxima, when sunspot numbers and solar activity are high, and solar minima, when sunspots are few or absent. The Sun’s global magnetic field reverses polarity during each cycle, so a complete magnetic cycle lasts about 22 years (the Hale cycle) before returning to its original polarity.

Origin and measurement: The cycle is driven by the solar dynamo, a process in the Sun’s convection zone where differential rotation and turbulent convection generate and reorganize magnetic fields. Observationally, the cycle is mediated by sunspot counts, the International Sunspot Number (also known as the Wolf number), sunspot area, and proxies such as the solar radio flux at 10.7 centimeters (F10.7).

Impacts: Solar activity influences space weather, driving geomagnetic storms, auroras, and disruptions to radio communications and satellite operations. It can affect navigation, satellite drag, and, in some cases, terrestrial power systems, though the degree of impact varies with cycle strength and solar events.

History: The periodicity was first noted by Samuel Heinrich Schwabe in 1843. The sunspot-number index was later developed by Rudolf Wolf, and the magnetic nature of sunspots and their polarity reversal was demonstrated by George Ellery Hale in the early 20th century, linking solar activity to an underlying magnetic cycle.