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 active regions, 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 16 km (10 mi) to 160,000 km (100,000 mi). Larger sunspots can be visible from Earth without the aid of a telescope. They may travel at relative speeds, or proper motions, of a few hundred meters per second when they first emerge. Indicating intense magnetic activity, sunspots accompany other active region phenomena such as coronal loops, prominences, and reconnection events. Most solar flares and coronal mass ejections originate in these magnetically active regions around visible sunspot groupings. Similar phenomena indirectly observed on stars other than the Sun are commonly called starspots, and both light and dark spots have been measured.
History
The earliest record of sunspots is found in the Chinese I Ching, completed before 800 BC. The text describes that a dou and mei were observed in the sun, where both words refer to a small obscuration. The earliest record of a deliberate sunspot observation also comes from China, and dates to 364 BC, based on comments by astronomer Gan De (甘德) in a star catalogue. By 28 BC, Chinese astronomers were regularly recording sunspot observations in official imperial records. The first clear mention of a sunspot in Western literature is circa 300 BC, by the ancient Greek scholar Theophrastus, student of Plato and Aristotle and successor to the latter. The earliest known drawings of sunspots were made by English monk John of Worcester in December 1128. 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 Frisian astronomers Johannes and David Fabricius. 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. 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.
Physics
Morphology
Sunspots have two main structures: a central umbra and a surrounding penumbra. The umbra is the darkest region of a sunspot and is where the magnetic field is strongest and approximately vertical, or normal, to the Sun's surface, or photosphere. The umbra may be surrounded completely or only partially by a brighter region known as the penumbra. The penumbra is composed of radially elongated structures known as penumbral filaments and has a more inclined magnetic field than the umbra. Within sunspot groups, multiple umbrae may be surrounded by a single, continuous penumbra. 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.
Lifecycle
The appearance of an individual sunspot may last anywhere from a few days to a few months, though groups of sunspots and their associated active regions tend to last weeks or months. Sunspots expand and contract as they move across the surface of the Sun, with diameters ranging from 16 km (10 mi) to 160,000 km (100,000 mi).
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