Sunspot

Sunspot

Overview
Sunspots are temporary phenomena on the photosphere
Photosphere
The photosphere of an astronomical object is the region from which externally received light originates. The term itself is derived from Ancient Greek roots, φῶς, φωτός/phos, photos meaning "light" and σφαῖρα/sphaira meaning "sphere", in reference to the fact that it is a spheric surface perceived...

 of the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

 that appear visibly
Visible spectrum
The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light. A typical human eye will respond to wavelengths from about 390 to 750 nm. In terms of...

 as dark spots compared to surrounding regions. They are caused by intense magnetic
Magnetism
Magnetism is a property of materials that respond at an atomic or subatomic level to an applied magnetic field. Ferromagnetism is the strongest and most familiar type of magnetism. It is responsible for the behavior of permanent magnets, which produce their own persistent magnetic fields, as well...

 activity, which inhibits convection
Convection
Convection is the movement of molecules within fluids and rheids. It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids....

 by an effect comparable to the eddy current brake, forming areas of reduced surface temperature. Like magnets, they also have two poles. Although they are at temperatures of roughly 3000 –, the contrast with the surrounding material at about 5,780 K leaves them clearly visible as dark spots, as the intensity of a heated black body
Black body
A black body is an idealized physical body that absorbs all incident electromagnetic radiation. Because of this perfect absorptivity at all wavelengths, a black body is also the best possible emitter of thermal radiation, which it radiates incandescently in a characteristic, continuous spectrum...

 (closely approximated by the photosphere) is a function of temperature to the fourth power.
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Encyclopedia
Sunspots are temporary phenomena on the photosphere
Photosphere
The photosphere of an astronomical object is the region from which externally received light originates. The term itself is derived from Ancient Greek roots, φῶς, φωτός/phos, photos meaning "light" and σφαῖρα/sphaira meaning "sphere", in reference to the fact that it is a spheric surface perceived...

 of the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

 that appear visibly
Visible spectrum
The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light. A typical human eye will respond to wavelengths from about 390 to 750 nm. In terms of...

 as dark spots compared to surrounding regions. They are caused by intense magnetic
Magnetism
Magnetism is a property of materials that respond at an atomic or subatomic level to an applied magnetic field. Ferromagnetism is the strongest and most familiar type of magnetism. It is responsible for the behavior of permanent magnets, which produce their own persistent magnetic fields, as well...

 activity, which inhibits convection
Convection
Convection is the movement of molecules within fluids and rheids. It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids....

 by an effect comparable to the eddy current brake, forming areas of reduced surface temperature. Like magnets, they also have two poles. Although they are at temperatures of roughly 3000 –, the contrast with the surrounding material at about 5,780 K leaves them clearly visible as dark spots, as the intensity of a heated black body
Black body
A black body is an idealized physical body that absorbs all incident electromagnetic radiation. Because of this perfect absorptivity at all wavelengths, a black body is also the best possible emitter of thermal radiation, which it radiates incandescently in a characteristic, continuous spectrum...

 (closely approximated by the photosphere) is a function of temperature to the fourth power. If the sunspot were isolated from the surrounding photosphere it would be brighter than an electric arc
Electric arc
An electric arc is an electrical breakdown of a gas which produces an ongoing plasma discharge, resulting from a current flowing through normally nonconductive media such as air. A synonym is arc discharge. An arc discharge is characterized by a lower voltage than a glow discharge, and relies on...

. Sunspots expand and contract as they move across the surface of the Sun and can be as large as 80000 kilometres (49,709.8 mi) in diameter, making the larger ones visible from Earth without the aid of a telescope
Telescope
A telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation . The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s , using glass lenses...

.
They may also travel at relative speeds ("proper motions") of a few hundred m/s when they first emerge onto the solar photosphere
Photosphere
The photosphere of an astronomical object is the region from which externally received light originates. The term itself is derived from Ancient Greek roots, φῶς, φωτός/phos, photos meaning "light" and σφαῖρα/sphaira meaning "sphere", in reference to the fact that it is a spheric surface perceived...

.

Manifesting intense magnetic activity, sunspots host secondary phenomena such as coronal loop
Coronal loop
Coronal loops form the basic structure of the lower corona and transition region of the Sun. These highly structured and elegant loops are a direct consequence of the twisted solar magnetic flux within the solar body. The population of coronal loops can be directly linked with the solar cycle; it...

s (prominences) and reconnection
Magnetic reconnection
Magnetic reconnection is a physical process in highly conducting plasmas in which the magnetic topology is rearranged and magnetic energy is converted to kinetic energy, thermal energy, and particle acceleration...

 events. Most solar flares and coronal mass ejections originate in magnetically active regions around visible sunspot groupings. Similar phenomena indirectly observed on star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...

s are commonly called starspot
Starspot
Starspots are equivalent to sunspots but located on other stars. Spots the size of sunspots are very hard to detect since they are too small to cause fluctuations in brightness...

s and both light and dark spots have been measured.

Sunspot variation




Sunspot populations quickly rise and more slowly fall on an irregular cycle of 11 years, although significant variations in the number of sunspots attending the 11-year period are known over longer spans of time. For example, from 1900 to the 1960s the solar maxima trend of sunspot count has been upward; from the 1960s to the present, it has diminished somewhat. Over the last decades the Sun has had a markedly high average level of sunspot activity; it was last similarly active over 8,000 years ago.

The number of sunspots correlates with the intensity of solar radiation over the period since 1979, when satellite measurements of absolute radiative flux became available. Since sunspots are darker than the surrounding photosphere it might be expected that more sunspots would lead to less solar radiation and a decreased solar constant
Solar constant
The solar constant, a measure of flux density, is the amount of incoming solar electromagnetic radiation per unit area that would be incident on a plane perpendicular to the rays, at a distance of one astronomical unit...

. However, the surrounding margins of sunspots are brighter than the average, and so are hotter; overall, more sunspots increase the Sun's solar constant or brightness. The variation caused by the sunspot cycle to solar output is relatively small, on the order of 0.1% of the solar constant (a peak-to-trough range of 1.3 W.m−2 compared to 1366 W.m−2 for the average solar constant). Sunspots were rarely observed during the Maunder Minimum
Maunder Minimum
The Maunder Minimum is the name used for the period roughly spanning 1645 to 1715 when sunspots became exceedingly rare, as noted by solar observers of the time....

 in the second part of the 17th century (approximately from 1645 to 1715). This coincides with the middle (and coldest) part of a period of cooling known as the Little Ice Age
Little Ice Age
The Little Ice Age was a period of cooling that occurred after the Medieval Warm Period . While not a true ice age, the term was introduced into the scientific literature by François E. Matthes in 1939...

.

Prehistoric evidence


Studies of stratigraphic
Stratigraphy
Stratigraphy, a branch of geology, studies rock layers and layering . It is primarily used in the study of sedimentary and layered volcanic rocks....

 data have suggested that the solar cycles have been active for hundreds of millions of years, if not longer; measuring varves in precambrian
Precambrian
The Precambrian is the name which describes the large span of time in Earth's history before the current Phanerozoic Eon, and is a Supereon divided into several eons of the geologic time scale...

 sedimentary rock has revealed repeating peaks in layer thickness, with a pattern approximately repeating every eleven layers. It is possible that the early atmosphere on Earth was more sensitive to changes in solar radiation than today, so that greater glacial melting (and thicker sediment deposits) might have occurred during years with greater sunspot activity.

This would presume annual layering, although alternate explanations (diurnal) now seem more probable.
Analysis of tree rings has revealed a detailed picture of past solar cycles: Dendrochronologically-dated
Dendrochronology
Dendrochronology or tree-ring dating is the scientific method of dating based on the analysis of patterns of tree-rings. Dendrochronology can date the time at which tree rings were formed, in many types of wood, to the exact calendar year...

 radiocarbon
Carbon-14
Carbon-14, 14C, or radiocarbon, is a radioactive isotope of carbon with a nucleus containing 6 protons and 8 neutrons. Its presence in organic materials is the basis of the radiocarbon dating method pioneered by Willard Libby and colleagues , to date archaeological, geological, and hydrogeological...

 concentrations have allowed for a reconstruction of sunspot activity dating back 11,400 years, far beyond the four centuries of available reliable records from direct solar observation.

Early observations


The earliest surviving record of sunspot observation dates from the 364 BC
364 BC
Year 364 BC was a year of the pre-Julian Roman calendar. At the time, it was known as the Year of the Consulship of Peticus and Calvus...

, based on comments by Chinese astronomer
Timeline of Chinese astronomy
This is a timeline of Chinese records and investigations in astronomy.*2137 BC - October 22, Chinese book Classic of History; records the earliest known solar eclipse....

 Gan De in a star catalogue. By 28 BC
28 BC
Year 28 BC was either a common year starting on Saturday, Sunday or Monday or a leap year starting on Saturday or Sunday of the Julian calendar and a common year starting on Saturday of the Proleptic Julian calendar...

, Chinese astronomers were regularly recording sunspot observations in official imperial records. The first clear mention of a sunspot in Western literature, around 300 BC, was by Theophrastus
Theophrastus
Theophrastus , a Greek native of Eresos in Lesbos, was the successor to Aristotle in the Peripatetic school. He came to Athens at a young age, and initially studied in Plato's school. After Plato's death he attached himself to Aristotle. Aristotle bequeathed to Theophrastus his writings, and...

, student of Plato
Plato
Plato , was a Classical Greek philosopher, mathematician, student of Socrates, writer of philosophical dialogues, and founder of the Academy in Athens, the first institution of higher learning in the Western world. Along with his mentor, Socrates, and his student, Aristotle, Plato helped to lay the...

 and Aristotle
Aristotle
Aristotle was a Greek philosopher and polymath, a student of Plato and teacher of Alexander the Great. His writings cover many subjects, including physics, metaphysics, poetry, theater, music, logic, rhetoric, linguistics, politics, government, ethics, biology, and zoology...

 and successor to the latter. A more recent sunspot observation was made on 17 March 807 AD by the Benedictine
Benedictine
Benedictine refers to the spirituality and consecrated life in accordance with the Rule of St Benedict, written by Benedict of Nursia in the sixth century for the cenobitic communities he founded in central Italy. The most notable of these is Monte Cassino, the first monastery founded by Benedict...

 monk Adelmus, who observed a large sunspot that was visible for eight days; however, Adelmus incorrectly concluded he was observing a transit of Mercury
Mercury (planet)
Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 87.969 Earth days. The orbit of Mercury has the highest eccentricity of all the Solar System planets, and it has the smallest axial tilt. It completes three rotations about its axis for every two orbits...

. A large sunspot was also seen at the time of Charlemagne
Charlemagne
Charlemagne was King of the Franks from 768 and Emperor of the Romans from 800 to his death in 814. He expanded the Frankish kingdom into an empire that incorporated much of Western and Central Europe. During his reign, he conquered Italy and was crowned by Pope Leo III on 25 December 800...

's death in 813 AD. Sunspot activity in 1129 was described by John of Worcester
John of Worcester
John of Worcester was an English monk and chronicler. He is usually held to be the author of the Chronicon ex chronicis.-Chronicon ex chronicis:...

, and Averroes
Averroes
' , better known just as Ibn Rushd , and in European literature as Averroes , was a Muslim polymath; a master of Aristotelian philosophy, Islamic philosophy, Islamic theology, Maliki law and jurisprudence, logic, psychology, politics, Arabic music theory, and the sciences of medicine, astronomy,...

 provided a description of sunspots later in the 12th century; however, these observations were also misinterpreted as planetary transits, until Galileo
Galileo Galilei
Galileo Galilei , was an Italian physicist, mathematician, astronomer, and philosopher who played a major role in the Scientific Revolution. His achievements include improvements to the telescope and consequent astronomical observations and support for Copernicanism...

 gave the correct explanation in 1612.

17th & 18th centuries


Sunspots were first observed telescopically in late 1610 by the English astronomer Thomas Harriot
Thomas Harriot
Thomas Harriot was an English astronomer, mathematician, ethnographer, and translator. Some sources give his surname as Harriott or Hariot or Heriot. He is sometimes credited with the introduction of the potato to Great Britain and Ireland...

 and Frisian
Frisians
The Frisians are a Germanic ethnic group native to the coastal parts of the Netherlands and Germany. They are concentrated in the Dutch provinces of Friesland and Groningen and, in Germany, East Frisia and North Frisia, that was a part of Denmark until 1864. They inhabit an area known as Frisia...

 astronomers Johannes
Johannes Fabricius
Johann Fabricius , eldest son of David Fabricius , was a Frisian/German astronomer and a discoverer of sunspots , independently of Galileo Galilei.-Biography:...

 and David Fabricius
David Fabricius
David Fabricius , was a German theologian who made two major discoveries in the early days of telescopic astronomy, jointly with his eldest son, Johannes Fabricius ....

, who published a description in June 1611. At the latter time, Galileo had been showing sunspots to astronomers in Rome, and Christoph Scheiner
Christoph Scheiner
Christoph Scheiner SJ was a Jesuit priest, physicist and astronomer in Ingolstadt....

 had probably been observing the spots for two or three months using an improved helioscope
Helioscope
A helioscope is an instrument used in observing the sun.The helioscope was first used by Benedetto Castelli and refined by Galileo...

 of his own design. The ensuing priority dispute between Galileo and Scheiner, neither of whom knew of the Fabricius' work, was thus as pointless as it was bitter.

Sunspots had some importance in the debate over the nature of the Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...

. They showed that the Sun rotated, and their comings and goings showed that the Sun changed, contrary to Aristotle
Aristotle
Aristotle was a Greek philosopher and polymath, a student of Plato and teacher of Alexander the Great. His writings cover many subjects, including physics, metaphysics, poetry, theater, music, logic, rhetoric, linguistics, politics, government, ethics, biology, and zoology...

 (who taught that all celestial bodies were perfect, unchanging spheres). The details of their apparent motion could not be readily explained except in the heliocentric
Heliocentrism
Heliocentrism, or heliocentricism, is the astronomical model in which the Earth and planets revolve around a stationary Sun at the center of the universe. The word comes from the Greek . Historically, heliocentrism was opposed to geocentrism, which placed the Earth at the center...

 system of Copernicus
Nicolaus Copernicus
Nicolaus Copernicus was a Renaissance astronomer and the first person to formulate a comprehensive heliocentric cosmology which displaced the Earth from the center of the universe....

.

Rudolf Wolf
Rudolf Wolf
Johann Rudolf Wolf was a Swiss astronomer and mathematician best known for his research on sunspots.Wolf was born in Fällanden, near Zurich. He studied at the universities of Zurich, Vienna, and Berlin. Encke was one of his teachers. Wolf became professor of astronomy at the University of Bern in...

 studied the historical record in an attempt to establish a database on past cyclic variations. His database extended only to 1700, although the technology and techniques for careful solar observations were first available in 1610. Gustav Spörer
Gustav Spörer
Friederich Wilhelm Gustav Spörer was a German astronomer.He is noted for his studies of sunspots and sunspot cycles. In this regard he is often mentioned together with Edward Maunder. Spörer was the first to note a prolonged period of low sunspot activity from 1645 to 1715...

 later suggested a 70-year period before 1716 in which sunspots were rarely observed as the reason for Wolf's inability to extend the cycles into the 17th century. The economist William Stanley Jevons
William Stanley Jevons
William Stanley Jevons was a British economist and logician.Irving Fisher described his book The Theory of Political Economy as beginning the mathematical method in economics. It made the case that economics as a science concerned with quantities is necessarily mathematical...

 suggested that there is a relationship between sunspots and business cycle crises. He reasoned that sunspots affect Earth's weather, which, in turn, influences crops and, therefore, the economy.

Sunspots were rarely recorded during the second part of 17th century. Later analysis revealed the problem not to be a lack of observational data but included references to negative observations. Building upon Spörer's earlier work, Edward Maunder suggested that the Sun had changed from a period in which sunspots all but disappeared from the solar surface to a renewal of sunspot cycles starting in about 1700. Adding to this understanding of the absence of solar cycles were observations of aurorae, which were absent at the same time. Even the lack of a solar corona
Corona
A corona is a type of plasma "atmosphere" of the Sun or other celestial body, extending millions of kilometers into space, most easily seen during a total solar eclipse, but also observable in a coronagraph...

 during solar eclipse
Solar eclipse
As seen from the Earth, a solar eclipse occurs when the Moon passes between the Sun and the Earth, and the Moon fully or partially blocks the Sun as viewed from a location on Earth. This can happen only during a new moon, when the Sun and the Moon are in conjunction as seen from Earth. At least...

s was noted prior to 1715. The period of low sunspot activity from 1645 to 1717 is known as the Maunder Minimum
Maunder Minimum
The Maunder Minimum is the name used for the period roughly spanning 1645 to 1715 when sunspots became exceedingly rare, as noted by solar observers of the time....

.

19th century


The cyclic variation of the number of sunspots was first observed by Heinrich Schwabe
Heinrich Schwabe
Samuel Heinrich Schwabe a German astronomer remembered for his work on sunspots.Schwabe was born at Dessau. At first an apothecary, he turned his attention to astronomy, and in 1826 commenced his observations on sunspots. Schwabe was trying to discover a new planet inside the orbit of Mercury...

 between 1826 and 1843 and led Wolf to make systematic observations starting in 1848. The Wolf number
Wolf number
The Wolf number is a quantity that measures the number of sunspots and groups of sunspots present on the surface of the sun....

 is a measure of individual spots and spot groupings, which correlates to a number of solar observables. Also in 1848, Joseph Henry
Joseph Henry
Joseph Henry was an American scientist who served as the first Secretary of the Smithsonian Institution, as well as a founding member of the National Institute for the Promotion of Science, a precursor of the Smithsonian Institution. During his lifetime, he was highly regarded...

 projected an image of the Sun onto a screen and determined that sunspots were cooler than the surrounding surface.

After the resumption of sunspot activity, Heinrich Schwabe
Heinrich Schwabe
Samuel Heinrich Schwabe a German astronomer remembered for his work on sunspots.Schwabe was born at Dessau. At first an apothecary, he turned his attention to astronomy, and in 1826 commenced his observations on sunspots. Schwabe was trying to discover a new planet inside the orbit of Mercury...

 in 1844 in Astronomische Nachrichten
Astronomische Nachrichten
Astronomische Nachrichten , one of the first international journals in the field of astronomy, was founded in 1821 by the German astronomer Heinrich Christian Schumacher. It claims to be the oldest astronomical journal in the world that is still being published...

(Astronomical News) reported a periodic change in the number of sunspots.

The Sun emitted an extremely powerful flare
Solar flare
A solar flare is a sudden brightening observed over the Sun surface or the solar limb, which is interpreted as a large energy release of up to 6 × 1025 joules of energy . The flare ejects clouds of electrons, ions, and atoms through the corona into space. These clouds typically reach Earth a day...

 on its visible hemisphere on 1 September 1859, leading to what is known as the Carrington Event
Solar storm of 1859
The solar storm of 1859, also known as the Solar Superstorm, or the Carrington Event, which occurred during solar cycle 10, was the most powerful solar storm in recorded history, and the largest flare, observed by Richard Christopher Carrington, became known as the Carrington Super...

. It interrupted electrical telegraph service and caused visible aurorae as far south as Havana
Havana
Havana is the capital city, province, major port, and leading commercial centre of Cuba. The city proper has a population of 2.1 million inhabitants, and it spans a total of — making it the largest city in the Caribbean region, and the most populous...

, Hawaii
Hawaii
Hawaii is the newest of the 50 U.S. states , and is the only U.S. state made up entirely of islands. It is the northernmost island group in Polynesia, occupying most of an archipelago in the central Pacific Ocean, southwest of the continental United States, southeast of Japan, and northeast of...

, and Rome
Rome
Rome is the capital of Italy and the country's largest and most populated city and comune, with over 2.7 million residents in . The city is located in the central-western portion of the Italian Peninsula, on the Tiber River within the Lazio region of Italy.Rome's history spans two and a half...

 with similar activity in the southern hemisphere.

21st century


The most powerful flare observed by satellite instrumentation began on 4 November 2003 at 19:29 UTC, and saturated instruments for 11 minutes. Region 486 has been estimated to have produced an X-ray flux of X28. Holographic and visual observations indicate significant activity continued on the far side of the Sun.

Measurements made in the latter part of the 2000s and based also on observation of infrared spectral lines, have suggested that sunspot activity may again be disappearing, possibly leading to a new minimum. From 2007-2009, sunspot levels were far below average. In 2008, the Sun was spot-free 73 per cent of the time, extreme even for a solar minimum. Only 1913 was more pronounced, with 85 per cent of that year clear. The Sun continued to languish through mid-December 2009, when the largest group of sunspots to emerge for several years appeared. Even then, sunspot levels remained well below normal.

In 2006, NASA made a prediction for the next sunspot maximum, being between 150 and 200 around the year 2011 (30-50% stronger than cycle 23), followed by a weak maximum at around 2022. The prediction did not come true. Instead, the sunspot cycle in 2010 was still at its minimum, where it should have been near its maximum, which shows the Sun's current unusual low activity.

Due to a missing jet stream, fading spots, and slower activity near the poles independent scientists of the National Solar Observatory
National Solar Observatory
The mission of the National Solar Observatory is to advance knowledge of the Sun, both as an astronomical object and as the dominant external influence on Earth, by providing forefront observational opportunities to the research community...

 (NSO) and the Air Force Research Laboratory
Air Force Research Laboratory
The Air Force Research Laboratory is a scientific research organization operated by the United States Air Force Materiel Command dedicated to leading the discovery, development, and integration of affordable aerospace warfighting technologies; planning and executing the Air Force science and...

 (AFRL) now (2011) predict that the next 11-year solar sunspot cycle, Cycle 25, will be greatly reduced or may not happen at all.

Physics



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
Convection zone
The convection zone of a star is the range of radii in which energy is transported primarily by convection. In the radiation zone, energy is transported by radiation...

 that get "wound up" by differential rotation
Differential rotation
Differential rotation is seen when different parts of a rotating object move with different angular velocities at different latitudes and/or depths of the body and/or in time. This indicates that the object is not solid. In fluid objects, such as accretion disks, this leads to shearing...

. If the stress on the tubes reaches a certain limit, they curl up like a rubber band and puncture the Sun's surface. Convection is inhibited at the puncture points; the energy flux from the Sun's interior decreases; and with it surface temperature.

The Wilson effect
Wilson effect
In 1769 a Scottish astronomer named Alexander Wilson, working at the Macfarlane Observatory, noticed that the shape of sunspots noticeably flattened as they approached the Sun's limb due to the solar rotation. These observations showed that sunspots were features on the solar surface, as opposed to...

 tells us that sunspots are actually depressions on the Sun's surface. Observations using the Zeeman effect
Zeeman effect
The Zeeman effect is the splitting of a spectral line into several components in the presence of a static magnetic field. It is analogous to the Stark effect, the splitting of a spectral line into several components in the presence of an electric field...

 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.

The sunspot itself can be divided into two parts:
  • The central umbra, which is the darkest part, where the magnetic field is approximately vertical (normal
    Surface normal
    A surface normal, or simply normal, to a flat surface is a vector that is perpendicular to that surface. A normal to a non-flat surface at a point P on the surface is a vector perpendicular to the tangent plane to that surface at P. The word "normal" is also used as an adjective: a line normal to a...

     to the Sun's surface).
  • The surrounding penumbra, which is lighter, where the magnetic field is more inclined.


Magnetic pressure
Magnetic pressure
Magnetic pressure is an energy density associated with the magnetic field. It is identical to any other physical pressure except that it is carried by the magnetic field rather than kinetic energy of the gas molecules. Interplay between magnetic pressure and ordinary gas pressure is important to...

 should tend to remove field concentrations, causing the sunspots to disperse, but sunspot lifetimes are measured in days or even weeks. Recent observations from the Solar and Heliospheric Observatory
Solar and Heliospheric Observatory
The Solar and Heliospheric Observatory is a spacecraft built by a European industrial consortium led by Matra Marconi Space that was launched on a Lockheed Martin Atlas IIAS launch vehicle on December 2, 1995 to study the Sun, and has discovered over 2100 comets. It began normal operations in May...

 (SOHO) using sound waves traveling below the Sun's photosphere (local helioseismology
Helioseismology
Helioseismology is the study of the propagation of wave oscillations, particularly acoustic pressure waves, in the Sun. Unlike seismic waves on Earth, solar waves have practically no shear component . Solar pressure waves are believed to be generated by the turbulence in the convection zone near...

) have been used to develop a three-dimensional image of the internal structure below sunspots; these observations show that there is a powerful downdraft underneath each sunspot, forming a rotating vortex
Vortex
A vortex is a spinning, often turbulent,flow of fluid. Any spiral motion with closed streamlines is vortex flow. The motion of the fluid swirling rapidly around a center is called a vortex...

 that concentrates the magnetic field. Sunspots can thus be thought of as self-perpetuating storms, analogous in some ways to terrestrial hurricanes.

Sunspot activity cycles about every eleven years. The point of highest sunspot activity during this cycle is known as Solar Maximum, and the point of lowest activity is Solar Minimum. Early in the cycle, sunspots appear in the higher latitudes and then move towards the equator as the cycle approaches maximum: this is called Spörer's law
Spörer's law
Spörer's law predicts the variation of sunspot latitudes during a solar cycle. It was discovered by the English astronomer Richard Christopher Carrington around 1861. Carrington's work was refined by the German astronomer Gustav Spörer....

.

Wolf number
Wolf number
The Wolf number is a quantity that measures the number of sunspots and groups of sunspots present on the surface of the sun....

 sunspot index displays various periods, the most prominent of which is at about 11 years in the mean. This period is also observed in most other expressions of solar activity and is deeply linked to a variation in the solar magnetic field that changes polarity with this period, too.

The modern understanding of sunspots starts with George Ellery Hale
George Ellery Hale
George Ellery Hale was an American solar astronomer.-Biography:Hale was born in Chicago, Illinois. He was educated at MIT, at the Observatory of Harvard College, , and at Berlin . As an undergraduate at MIT, he is known for inventing the spectroheliograph, with which he made his discovery of...

, who first linked magnetic fields and sunspots. Hale suggested that the sunspot cycle period is 22 years, covering two polar reversals of the solar magnetic dipole
Dipole
In physics, there are several kinds of dipoles:*An electric dipole is a separation of positive and negative charges. The simplest example of this is a pair of electric charges of equal magnitude but opposite sign, separated by some distance. A permanent electric dipole is called an electret.*A...

 field. Horace W. Babcock
Horace W. Babcock
Horace Welcome Babcock was an American astronomer. He was the son of Harold D. Babcock.He invented and built a number of astronomical instruments, and in 1953 was the first to propose the idea of adaptive optics. He specialized in spectroscopy and the study of magnetic fields of stars...

 later proposed a qualitative model for the dynamics of the solar outer layers. The Babcock Model
Babcock Model
The Babcock Model describes a mechanism which can explain magnetic and sunspot patterns observed on the Sun.A modern understanding of sunspots starts with George Ellery Hale, in which magnetic fields and sunspots are linked. Hale suggested that the sunspot cycle period is 22 years, covering two...

 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 observation



Sunspots are observed with land-based and Earth-orbiting solar telescope
Solar telescope
A solar telescope is a special purpose telescope used to observe the Sun. Solar telescopes usually detect light with wavelengths in, or not far outside, the visible spectrum.-Professional solar telescopes:...

s. These telescopes use filtration and projection techniques for direct observation, in addition to various types of filtered cameras. Specialized tools such as spectroscopes and spectrohelioscope
Spectrohelioscope
A spectrohelioscope is a type of solar telescope designed by George Ellery Hale in 1924 to allow the Sun to be viewed in a selected wavelength of light...

s 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 vision, amateur observation of sunspots is generally conducted indirectly 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 as well as aluminum coated
Vacuum deposition
Vacuum deposition is a family of processes used to deposit layers atom-by-atom or molecule-by-molecule at sub-atmospheric pressure on a solid surface. The layers may be as thin as one atom to millimeters thick . There may be multiple layers of different materials...

 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.

Amateur observers should not use filters, no matter how dark, unless they are specifically intended for solar viewing. Other filters may not provide appropriate protection in the non-visible frequency range and eye damage could result. When using a telescope or binoculars for direct viewing with a filter, follow the manufacturers' guidelines closely. These will usually require the filter to be placed at the objective (far) end of the instrument, since the instrument concentrates heat as well as light and may damage an eyepiece filter and cause immediate eye damage. Securely fasten the filter and ensure that attached auxiliary devices such as spotter scopes have hoods in case they concentrate heat dangerously. Direct observation of the Sun's surface through an optical device is inherently dangerous, and requires both knowledge and close attention to safety precautions.

Application



Due to its link to other kinds of solar activity
Solar variation
Solar variation is the change in the amount of radiation emitted by the Sun and in its spectral distribution over years to millennia. These variations have periodic components, the main one being the approximately 11-year solar cycle . The changes also have aperiodic fluctuations...

, sunspot occurrence can be used to help predict space weather
Space weather
Space weather is the concept of changing environmental conditions in near-Earth space or thespace from the Sun's atmosphere to the Earth's atmosphere. It is distinct from the concept ofweather within the Earth's planetary atmosphere...

, the state of the ionosphere
Ionosphere
The ionosphere is a part of the upper atmosphere, comprising portions of the mesosphere, thermosphere and exosphere, distinguished because it is ionized by solar radiation. It plays an important part in atmospheric electricity and forms the inner edge of the magnetosphere...

, and hence the conditions of short-wave radio propagation
Radio propagation
Radio propagation is the behavior of radio waves when they are transmitted, or propagated from one point on the Earth to another, or into various parts of the atmosphere...

 or satellite communications.
Solar activity (and the sunspot cycle) are frequently discussed in the context of global warming
Global warming
Global warming refers to the rising average temperature of Earth's atmosphere and oceans and its projected continuation. In the last 100 years, Earth's average surface temperature increased by about with about two thirds of the increase occurring over just the last three decades...

; Jack Eddy
Jack Eddy
John Allen "Jack" Eddy was an American astronomer who published professionally under the name John A. Eddy but much of the content referencing him can be found under his nickname Jack which he preferred to use. In 1976 Dr. Eddy published a landmark paper in Science titled "The Maunder Minimum"...

 noted the apparent correlation between the Maunder Minimum
Maunder Minimum
The Maunder Minimum is the name used for the period roughly spanning 1645 to 1715 when sunspots became exceedingly rare, as noted by solar observers of the time....

 of sunspot occurrence and the Little Ice Age
Little Ice Age
The Little Ice Age was a period of cooling that occurred after the Medieval Warm Period . While not a true ice age, the term was introduced into the scientific literature by François E. Matthes in 1939...

 in European climate.
Sunspots themselves, in terms of the magnitude of their radiant-energy deficit, have only a weak effect on the terrestrial climate in a direct sense.
On longer time scales, such as the solar cycle, other magnetic phenomena (faculae and the chromospheric network) do correlate with sunspot occurrence. It is these other features that make the solar constant
Solar constant
The solar constant, a measure of flux density, is the amount of incoming solar electromagnetic radiation per unit area that would be incident on a plane perpendicular to the rays, at a distance of one astronomical unit...

 increase slightly at sunspot maxima, when naïvely one might expect that sunspots would make it decrease

Spots on other stars


In 1947, G. E. Kron proposed that starspot
Starspot
Starspots are equivalent to sunspots but located on other stars. Spots the size of sunspots are very hard to detect since they are too small to cause fluctuations in brightness...

s 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
Photometry (astronomy)
Photometry is a technique of astronomy concerned with measuring the flux, or intensity of an astronomical object's electromagnetic radiation...

 showed starspot growth and decay and showed cyclic behavior similar to the Sun's; spectroscopy
Spectroscopy
Spectroscopy is the study of the interaction between matter and radiated energy. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, e.g., by a prism. Later the concept was expanded greatly to comprise any interaction with radiative...

 examined the structure of starspot regions by analyzing variations in spectral line splitting due to the Zeeman Effect; Doppler imaging
Doppler imaging
Inhomogeneous structures on stellar surfaces, i.e. temperature differences, chemical composition or magnetic fields, create characteristic distortions in the spectral lines due to the Doppler effect. These distortions will move across spectral line profiles due to the stellar rotation...

 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 3500 kelvins (3,226.9 °C), together with a warm spot of 4800 kelvins (4,526.9 °C).

See also

  • Wolf number
    Wolf number
    The Wolf number is a quantity that measures the number of sunspots and groups of sunspots present on the surface of the sun....

     — a quantity that measures the number of sunspots
  • Spörer's law
    Spörer's law
    Spörer's law predicts the variation of sunspot latitudes during a solar cycle. It was discovered by the English astronomer Richard Christopher Carrington around 1861. Carrington's work was refined by the German astronomer Gustav Spörer....

     — predicts the variation of sunspot latitudes during a solar cycle
  • Joy's Law
    Joy's Law (astronomy)
    In astronomy, Joy's Law describes sunspots and states that active sunspot regions tend to be "tilted" with the leading spot closer to the equator than the following spots. It is named after A. H. Joy.-External links:* .*...

  • List of solar cycles
  • Solar cycle
    Solar cycle
    The solar cycle, or the solar magnetic activity cycle, is a periodic change in the amount of irradiation from the Sun that is experienced on Earth. It has a period of about 11 years, and is one component of solar variation, the other being aperiodic fluctuations. Solar variation causes changes in...

  • Solar Rotation
    Solar rotation
    Solar rotation is able to vary with latitude because the Sun is composed of a gaseous plasma. The rate of rotation is observed to be fastest at the equator , and to decrease as latitude increases...

  • Space weather
    Space weather
    Space weather is the concept of changing environmental conditions in near-Earth space or thespace from the Sun's atmosphere to the Earth's atmosphere. It is distinct from the concept ofweather within the Earth's planetary atmosphere...

  • Radio propagation
    Radio propagation
    Radio propagation is the behavior of radio waves when they are transmitted, or propagated from one point on the Earth to another, or into various parts of the atmosphere...


External links



Sunspot data

      • International Sunspot Number—sunspot maximum and minimum 1610–present; annual numbers 1700–present; monthly numbers 1749–present; daily values 1818–present; and sunspot numbers by north and south hemisphere. The McNish-Lincoln sunspot prediction is also included.
      • American sunspot numbers 1945–present
      • Ancient sunspot data 165 BC to 1684 AD
      • Group Sunspot Numbers (Doug Hoyt re-evaluation) 1610-1995