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Sunspots are temporary spots on the Sun's surface that are darker than the surrounding area. They are regions of reduced surface temperature caused by concentrations of magnetic flux that inhibit convection. Sunspots appear within , usually in pairs of opposite magnetic polarity. Their number varies according to the approximately 11-year solar cycle.
Individual sunspots or groups of sunspots may last anywhere from a few days to a few months, but eventually decay. Sunspots expand and contract as they move across the surface of the Sun, with diameters ranging from to . Larger sunspots can be visible from Earth without the aid of a telescope. They may travel at , or , of a few hundred meters per second when they first emerge.
Indicating intense magnetic activity, sunspots accompany other active region phenomena such as , solar prominence, and reconnection events. Most and coronal mass ejections originate in these magnetically active regions around visible sunspot groupings. Similar phenomena indirectly observed on other than the Sun are commonly called , and both light and dark spots have been measured.
The first clear mention of a sunspot in Western literature is circa 300 BC, by Ancient Greece scholar Theophrastus, student of Plato and Aristotle and successor to the latter." Letter to the Editor: Sunspot observations by Theophrastus revisited", and see Theophrastus' Fragment VI, De Signis Tempestatum, 11.4–5.
The earliest known Sunspot drawing were made by English monk John of Worcester in December 1128.Stefan Hughes, Catchers of the Light: The Forgotten Lives of the Men and Women Who First Photographed the Heavens, ArtDeCiel Publishing, 2012 p. 317
Sunspots were first observed telescopically in December 1610 by English astronomer Thomas Harriot. His observations were recorded in his notebooks and were followed in March 1611 by observations and reports by Frisians astronomers Johannes and David Fabricius. (2025). 9780387927893, Springer, New York. ISBN 9780387927893 After Johannes Fabricius' death at the age of 29, his reports remained obscure and were overshadowed by the independent discoveries of and publications about sunspots by Christoph Scheiner and Galileo Galilei. Galileo likely began telescopic sunspot observations around the same time as Harriot; however, Galileo's records did not start until 1612. During the next decades numerous astronomers of that era participated in the pursuit of sunspots. One of these was the famous astronomer Johannes Hevelius who recorded 19 sunspot groups during the period of the early Maunder Minimum (1653-1679) in the book Machina Coelestis.
In the early 19th Century, William Herschel was one of the first to hypothesize a connection of sunspots with temperatures on Earth and believed that certain features of sunspots would indicate increased heating on Earth. During his recognition of solar behavior and hypothesized solar structure, he inadvertently picked up the relative absence of sunspots from July 1795 to January 1800 and was perhaps the first to construct a past record of observed or missing sunspots. From this he found that the absence of sunspots coincided with high wheat prices in England. The president of the Royal Society commented that the upward trend in wheat prices was due to monetary inflation.Soon, W., and Yaskell, S.H., The Maunder Minimum and the Variable Sun-earth Connection (World Scientific Press: 2003) pp. 87–88 Years later scientists such as Richard Carrington in 1865 and John Henry Poynting in 1884 tried and failed to find a connection between wheat prices and sunspots, and modern analysis finds that there is no statistically significant correlation between wheat prices and sunspot numbers.
The temperature of the umbra is roughly 3000–4500 K, in contrast to the surrounding material at about 5780 K, leaving sunspots clearly visible as dark spots. This is because the luminance of a heated black body (closely approximated by the photosphere) at these temperatures varies greatly with temperature. Isolated from the surrounding photosphere, a single sunspot would shine brighter than the full moon, with a crimson-orange color.
In some forming and decaying sunspots, relatively narrow regions of bright material appear penetrating into or completely dividing an umbra. These formations, referred to as light bridges, have been found to have a weaker, more tilted magnetic field compared to the umbra at the same height in the photosphere. Higher in the photosphere, the light bridge magnetic field merges and becomes comparable to that of the umbra. Gas pressure in light bridges has also been found to dominate over magnetic pressure, and convective motions have been detected.
The Wilson effect implies that sunspots are depressions on the Sun's surface.
Sunspots initially appear in the photosphere as small darkened spots lacking a penumbra. These structures are known as solar pores. Over time, these pores increase in size and move towards one another. When a pore gets large enough, typically around in diameter, a penumbra will begin to form.
Early in the cycle, sunspots appear at higher latitudes and then move towards the equator as the cycle approaches maximum, following Spörer's law. Spots from two sequential cycles co-exist for several years during the years near solar minimum. Spots from sequential cycles can be distinguished by direction of their magnetic field and their latitude.
Wolf number sunspot index counts the average number of sunspots and groups of sunspots during specific intervals. The 11-year solar cycles are numbered sequentially, starting with the observations made in the 1750s.
George Ellery Hale first linked magnetic fields and sunspots in 1908. Hale suggested that the sunspot cycle period is 22 years, covering two periods of increased and decreased sunspot numbers, accompanied by polar reversals of the solar magnetic dipole field. Horace W. Babcock later proposed a qualitative model for the dynamics of the solar outer layers. The Babcock Model explains that magnetic fields cause the behavior described by Spörer's law, as well as other effects, which are twisted by the Sun's rotation.
Sunspot number is correlated with the intensity of solar radiation over the period since 1979, when satellite measurements became available. The variation caused by the sunspot cycle to solar output is on the order of 0.1% of the solar constant (a peak-to-trough range of 1.3 W·m−2 compared with 1366 W·m−2 for the average solar constant).
Since looking directly at the Sun with the naked eye permanently damages human vision, amateur observation of sunspots is generally conducted using projected images, or directly through protective filters. Small sections of very dark optical filter, such as a #14 welder's glass, are effective. A telescope eyepiece can project the image, without filtration, onto a white screen where it can be viewed indirectly, and even traced, to follow sunspot evolution. Special purpose hydrogen-alpha narrow bandpass filters and aluminum-coated glass attenuation filters (which have the appearance of mirrors due to their extremely high optical density) on the front of a telescope provide safe observation through the eyepiece.
Solar activity (and the solar cycle) have been implicated as a factor in global warming. The first possible example of this is the Maunder Minimum period of low sunspot activity which occurred during the Little Ice Age in Europe. PDF Copy However, detailed studies from multiple paleoclimate indicators show that the lower northern hemisphere temperatures in the Little Ice Age began while sunspot numbers were still high before the start of the Maunder Minimum, and persisted until after the Maunder Minimum had ceased. Numerical climate modelling indicates that volcanic activity was the main driver of the Little Ice Age.
Sunspots themselves, in terms of the magnitude of their radiant-energy deficit, have a weak effect on solar flux. The total effect of sunspots and other magnetic processes in the solar photosphere is an increase of roughly 0.1% in brightness of the Sun in comparison with its brightness at the solar-minimum level. This is a difference in total solar irradiance at Earth over the sunspot cycle of close to . Other magnetic phenomena which correlate with sunspot activity include Solar facula and the chromospheric network. The combination of these magnetic factors mean that the relationship of sunspot numbers to Total Solar Irradiance (TSI) over the decadal-scale solar cycle, and their relationship for century timescales, need not be the same. The main problem with quantifying the longer-term trends in TSI lies in the stability of the absolute radiometry measurements made from space, which has improved in recent decades but remains a problem. Analysis shows that it is possible that TSI was actually higher in the Maunder Minimum compared to present-day levels, but uncertainties are high, with best estimates in the range with a uncertainty range of .
Sunspots, with their intense magnetic field concentrations, facilitate the complex transfer of energy and momentum to the upper solar atmosphere. This transfer occurs through a variety of mechanisms, including generated WaLSA Team and magnetic reconnection events.
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