Solar cycle
The Solar cycle, also known as the solar magnetic activity cycle, sunspot cycle, or Schwabe cycle, is a 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.
The current scientific consensus on climate change is that solar variations only play a marginal role in driving global climate change, since the measured magnitude of recent solar variation is much smaller than the forcing due to greenhouse gases.
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. Alfred Harrison Joy would later describe how the magnitude at which the sunspots are "tilted"—with the leading spot closer to the equator than the trailing spot―grows with the latitude of these regions.
The cycle's physical basis was elucidated by George Ellery Hale and collaborators, who in 1908 showed that sunspots were strongly magnetized. 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 these regions tend to have the opposite leading polarity.
- Leading polarities in both hemispheres flip from one sunspot cycle to the next.
In 1961 the father-and-son team of Harold and Horace Babcock established that the solar cycle is a spatiotemporal magnetic process unfolding over the Sun as a whole. They observed that the solar surface is magnetized outside of sunspots, that this magnetic field is to first order a dipole, and that this dipole undergoes polarity reversals with the same period as the sunspot cycle. Horace's Babcock Model described the Sun's oscillatory magnetic field as having a quasi-steady periodicity of 22 years. It covered the oscillatory exchange of energy between toroidal and poloidal solar magnetic field components.
Cycle history
Sunspot numbers over the past 11,400 years have been reconstructed using carbon-14 and beryllium-10 isotope ratios. The level of solar activity beginning in the 1940s is exceptional – the last period of similar magnitude occurred around 9,000 years ago. The Sun was at a similarly high level of magnetic activity for only ~10% of the past 11,400 years. Almost all earlier high-activity periods were shorter than the present episode. Fossil records suggest that the solar cycle has been stable for at least the last 700 million years. For example, the cycle length during the Early Permian is estimated to be 10.62 years and similarly in the Neoproterozoic.| Event | Start | End |
| Homeric minimum | 750 BC | 550 BC |
| Oort minimum | AD 1040 | AD 1080 |
| Medieval maximum | 1100 | 1250 |
| Wolf minimum | 1280 | 1350 |
| Spörer Minimum | 1450 | 1550 |
| Maunder Minimum | 1645 | 1715 |
| Dalton Minimum | 1790 | 1820 |
| Modern Maximum | 1933 | 2008 |
Until 2009, it was thought that 28 cycles had spanned the 309 years between 1699 and 2008, giving an average length of 11.04 years, but research then showed that the longest of these may actually have been two cycles. If so then the average length would be only around 10.7 years. Since observations began cycles as short as 9 years and as long as 14 years have been observed, and if the cycle of 1784–1799 is double then one of the two component cycles had to be less than 8 years in length. Significant amplitude variations also occur.
Several lists of proposed historical "grand minima" of solar activity exist.
Recent cycles
Cycle 25
Solar cycle 25 began in December 2019. Several predictions have been made for solar cycle 25 based on different methods, ranging from very weak to strong magnitude. A physics-based prediction relying on the data-driven solar dynamo and solar surface flux transport models seems to have predicted the strength of the solar polar field at the current minima correctly and forecasts a weak but not insignificant solar cycle 25 similar to or slightly stronger than cycle 24. Notably, they rule out the possibility of the Sun falling into a Maunder-minimum-like state over the next decade. A preliminary consensus by a solar cycle 25 Prediction Panel was made in early 2019. The Panel, which was organized by NOAA's Space Weather Prediction Center and NASA, based on the published solar cycle 25 predictions, concluded that solar cycle 25 will be very similar to solar cycle 24. They anticipate that the solar cycle minimum before cycle 25 will be long and deep, just as the minimum that preceded cycle 24. They expect solar maximum to occur between 2023 and 2026 with a sunspot range of 95 to 130, given in terms of the revised sunspot number.Cycle 24
Solar cycle 24 began on 4 January 2008, with minimal activity until early 2010. The cycle featured a "double-peaked" solar maximum. The first peak reached 99 in 2011 and the second in early 2014 at 101. Cycle 24 ended in December 2019 after 11.0 years.Cycle 23
Solar cycle 23 lasted 11.6 years, beginning in May 1996 and ending in January 2008. The maximum smoothed sunspot number observed during the solar cycle was 120.8, and the minimum was 1.7. A total of 805 days had no sunspots during this cycle.Phenomena
Because the solar cycle reflects magnetic activity, various magnetically driven solar phenomena follow the solar cycle, including sunspots, faculae/plage, network, and coronal mass ejections.Sunspots
The Sun's apparent surface, the photosphere, radiates more actively when there are more sunspots. Satellite monitoring of solar luminosity revealed a direct relationship between the solar cycle and luminosity with a peak-to-peak amplitude of about 0.1%. Luminosity decreases by as much as 0.3% on a 10-day timescale when large groups of sunspots rotate across the Earth's view and increase by as much as 0.05% for up to 6 months due to faculae associated with large sunspot groups.The best information today comes from SOHO, such as the MDI magnetogram, where the solar "surface" magnetic field can be seen.
As each cycle begins, sunspots appear at mid-latitudes, and then move closer and closer to the equator until a solar minimum is reached. This pattern is best visualized in the form of the so-called butterfly diagram. Images of the Sun are divided into latitudinal strips, and the monthly-averaged fractional surface of sunspots is calculated. This is plotted vertically as a color-coded bar, and the process is repeated month after month to produce this time-series diagram.
File:Sun - btly - 2023.png|center|thumb|upright=2.5|This version of the sunspot butterfly diagram was constructed by the solar group at NASA Marshall Space Flight Center. The newest version can be found at
While magnetic field changes are concentrated at sunspots, the entire Sun undergoes analogous changes, albeit of smaller magnitude.
File:LAMF - 2023.png|center|thumb|upright=2.5|Time vs. solar latitude diagram of the radial component of the solar magnetic field, averaged over successive solar rotation. The "butterfly" signature of sunspots is clearly visible at low latitudes. Diagram constructed by the solar group at NASA Marshall Space Flight Center. The newest version can be found at