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.
Definition
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.
Observational history
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". The solar cycle however would not be clearly identified until 1843 when Samuel Heinrich Schwabe noticed a periodic variation in the average number of sunspots after 17 years of solar observations. Schwabe continued to observe the sunspot cycle for another 23 years, until 1867. In 1852, Rudolf Wolf designated the first numbered solar cycle to have started in February 1755 based on Schwabe's and other observations. Wolf also created a standard sunspot number index, the Wolf number, which continues to be used today.
Between 1645 and 1715, very few sunspots were observed and recorded. This was first noted by Gustav Spörer and was later named the Maunder minimum after the wife-and-husband team Annie S. D. Maunder and Edward Walter Maunder who extensively researched this peculiar interval.
In the second half of the nineteenth century Richard Carrington and Spörer independently noted the phenomena of sunspots appearing at different heliographic latitudes at different parts of the cycle. (See Spörer's law.) Alfred Harrison Joy would later describe how the magnitude at which the sunspots are "tilted"—with the leading spot(s) closer to the equator than the trailing spot(s)―grows with the latitude of these regions. (See Joy's law.)
The cycle's physical basis was elucidated by George Ellery Hale and collaborators, who in 1908 showed that sunspots were strongly magnetized (the first detection of magnetic fields beyond the Earth). In 1919 they identified a number of patterns that would collectively become known as Hale's law: In the same heliographic hemisphere, bipolar active regions tend to have the same leading polarity. In the opposite hemisphere, i.e., across the equator, these regions tend to have the opposite leading polarity. Leading polarities in both hemispheres flip from one sunspot cycle to the next. Hale's observations revealed that the complete magnetic cycle—which would later be referred to as a Hale cycle—spans two solar cycles, or 22 years, before returning to its original state (including polarity). Because nearly all manifestations are insensitive to polarity, the 11-year solar cycle remains the focus of research; however, the two halves of the Hale cycle are typically not identical: the 11-year cycles usually alternate between higher and lower sums of Wolf's sunspot numbers (the Gnevyshev-Ohl rule).