Sunspots are temporary phenomena on the Sun's photosphere that appear as spots darker than the surrounding areas. They are regions of reduced surface temperature caused by concentrations of magnetic field flux that inhibit convection. Sunspots usually appear 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 phenomena such as coronal loops, prominences, and reconnection events. Most solar flares and coronal mass ejections originate in 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.
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 () in a star catalogue. By 28 BC, Chinese astronomers were regularly recording sunspot observations in official imperial records.
Sunspots were first observed telescopically in late 1610 by English astronomer Thomas Harriot and Frisian astronomers Johannes and David Fabricius, who published a description in June 1611. After Johannes Fabricius' death at the age of 29, the book remained obscure and was eclipsed by the independent discoveries of and publications about sunspots by Christoph Scheiner and Galileo Galilei, few months later.
In the early 19th Century, William Herschel was one of the first to equate sunspots with the abundance of heating and cooling it was capable of causing on Earth. He believed that the "great shallows (sunspots' penumbrae) ridges (bright, elevated extended features resembling faculae) nodules (bright, elevated, yet smaller features resembling luculi) and corrugations (less luminous, rough, mottled, dark features) instead of small indentations (depressed, extended dark features) on the sun would let in large amounts of heat into Earth. On the other hand, "pores, small indentations -central regions of dark, depressed spots - and the nodules' and ridges' absence," meant less heat touching Earth. During his recognition of solar behavior and hypothesized solar structure, he inadvertently picked up the relative absence altogether of spots on the Sun from July, 1795 to January, 1800. He was perhaps the very first to construct a past record or observed or missing sunspots and found that, in England at least, the absence of sunspots coincided with high wheat prices. Herschel read his paper before the Royal Society. He was completely misinterpreted and heartily ridiculed before that body.
Sunspots have two parts: its center umbra, the darkest part, where the magnetic field is approximately vertical (normal to the Sun's surface) and the surrounding penumbra, which is lighter, where the magnetic field is more inclined.
The temperature of the umbra is roughly 3,000-4,500 K (2,700-4,200 °C), in contrast to the penumbra at about 5,780 K (5,500 °C) leaving sunspots clearly visible as dark spots, occasionally visible even to the naked eye. This is because the luminance (which is essentially "brightness" in visible light) 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.
Any given appearance of a sunspot may last anywhere from a few days to a few months, though groups of sunspots and their active regions tend to last weeks or months, but all do eventually decay and disappear. 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).
Although the details of sunspot generation are still a matter of research, it appears that sunspots are the visible counterparts of magnetic flux tubes in the Sun's convective zone that get "wound up" by differential rotation. If the stress on the tubes reaches a certain limit, a loop of the tube may project through the photosphere, the Sun's visible surface. Convection is inhibited at the puncture points; the energy flux from the Sun's interior decreases, and with it, surface temperature, causing the surface area through which the magnetic field passes to look dark against the bright background of the photosphere.
The Wilson effect implies that sunspots are depressions on the Sun's surface. Observations using the Zeeman effect show that prototypical sunspots come in pairs with opposite magnetic polarity. From cycle to cycle, the polarities of leading and trailing (with respect to the solar rotation) sunspots change from north/south to south/north and back. Sunspots usually appear in groups.
Magnetic pressure should tend to remove field concentrations, causing the sunspots to disperse, but sunspot lifetimes are measured in days to weeks. In 2001, observations from the Solar and Heliospheric Observatory (SOHO) using sound waves traveling below the photosphere (local helioseismology) were used to develop a three-dimensional image of the internal structure below sunspots; these observations show that a powerful downdraft underneath each sunspot, forms a rotating vortex that sustains the concentrated magnetic field.
Solar cycle duration is typically about eleven years, varying from just under 10 to just over 12 years. Over the solar cycle, sunspot populations rise quickly and then fall more slowly. The point of highest sunspot activity during a cycle is known as solar maximum, and the point of lowest activity as solar minimum. This period is also observed in most other solar activity and is linked to a variation in the solar magnetic field that changes polarity with this period.
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.
The 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 numbers also change over long periods. For example during the period known as the modern maximum from 1900 to 1958 the solar maxima trend of sunspot count was upwards; for the following 60 years the trend was mostly downwards. Overall, the Sun was last as active as the modern maximum over 8,000 years ago.
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).
Sunspots are observed with land-based and Earth-orbiting solar telescopes. These telescopes use filtration and projection techniques for direct observation, in addition to various types of filtered cameras. Specialized tools such as spectroscopes and spectrohelioscopes are used to examine sunspots and sunspot areas. Artificial eclipses allow viewing of the circumference of the Sun as sunspots rotate through the horizon.
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 filter glass, 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.
Due to its link to other kinds of solar activity, sunspot occurrence can be used to help predict space weather, the state of the ionosphere, and hence the conditions of short-wave radio propagation or satellite communications. High sunspot activity is celebrated by members of the amateur radio community as a harbinger of excellent ionospheric propagation conditions that greatly increase radio range in the HF bands. During sunspot peaks, worldwide radio communication can be possible on frequencies as high as the 6-meter VHF band. Solar activity (and the solar cycle) have been implicated in global warming, originally the role of the Maunder Minimum of sunspot occurrence in the Little Ice Age in European winter climate. Sunspots themselves, in terms of the magnitude of their radiant-energy deficit, have a weak effect on solar flux however the total solar flux increases as "At solar maximum the Sun is some 0.1% brighter than its solar-minimum level". On longer time scales, such as the solar cycle, other magnetic phenomena (faculae and the chromospheric network) correlate with sunspot occurrence.
In 1947, G. E. Kron proposed that starspots were the reason for periodic changes in brightness on red dwarfs. Since the mid-1990s, starspot observations have been made using increasingly powerful techniques yielding more and more detail: photometry showed starspot growth and decay and showed cyclic behavior similar to the Sun's; spectroscopy examined the structure of starspot regions by analyzing variations in spectral line splitting due to the Zeeman effect; Doppler imaging showed differential rotation of spots for several stars and distributions different from the Sun's; spectral line analysis measured the temperature range of spots and the stellar surfaces. For example, in 1999, Strassmeier reported the largest cool starspot ever seen rotating the giant K0 star XX Triangulum (HD 12545) with a temperature of 3,500 K (3,230 °C), together with a warm spot of 4,800 K (4,530 °C).
starspots vary on the same (short) time scales as Sunspots do ... HD 12545 had a warm spot (350 K above photospheric temperature; the white area in the picture)