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Concept
Solar cycle
Summary
The solar cycle, also known as the solar magnetic activity cycle, sunspot cycle, or Schwabe cycle, is a nearly periodic 11-year change in the Sun's activity measured in terms of variations in the number of observed sunspots on the Sun's surface. Over the period of a solar cycle, levels of solar radiation and ejection of solar material, the number and size of sunspots, solar flares, and coronal loops all exhibit a synchronized fluctuation from a period of minimum activity to a period of a maximum activity back to a period of minimum activity.
The magnetic field of the Sun flips during each solar cycle, with the flip occurring when the solar cycle is near its maximum. After two solar cycles, the Sun's magnetic field returns to its original state, completing what is known as a Hale cycle.
This cycle has been observed for centuries by changes in the Sun's appearance and by terrestrial phenomena such as aurora but was not clearly identified until 1843. Solar activity, driven by both the solar cycle and transient aperiodic processes, governs the environment of interplanetary space by creating space weather and impacting space- and ground-based technologies as well as the Earth's atmosphere and also possibly climate fluctuations on scales of centuries and longer.
Understanding and predicting the solar cycle remains one of the grand challenges in astrophysics with major ramifications for space science and the understanding of magnetohydrodynamic phenomena elsewhere in the universe.
Solar cycles have an average duration of about 11 years. Solar maximum and solar minimum refer to periods of maximum and minimum sunspot counts. Cycles span from one minimum to the next.
Solar observation
The idea of a cyclical solar cycle was first hypothesized by Christian Horrebow based on his regular observations of sunspots made between 1761 and 1776 from the Rundetaarn observatory in Copenhagen, Denmark. In 1775, Horrebow noted how "it appears that after the course of a certain number of years, the appearance of the Sun repeats itself with respect to the number and size of the spots".
Official source
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