Solar cycle

From Wikipedia, the free encyclopedia

400 year sunspot history

"The current prediction for Sunspot Cycle 24 gives a smoothed sunspot number maximum of about 69 in the late Summer of 2013. The smoothed sunspot number reached 68.9 in August 2013 so the official maximum will be at least this high. The smoothed sunspot number has been rising again towards this second peak over the last five months and has now surpassed the level of the first peak (66.9 in February 2012). Many cycles are double peaked but this is the first in which the second peak in sunspot number was larger than the first. We are currently over five years into Cycle 24. The current predicted and observed size makes this the smallest sunspot cycle since Cycle 14 which had a maximum of 64.2 in February of 1906."^[1] The monthly sunspot number was still rising as of March 2014.^[2]

The solar cycle (or solar magnetic activity cycle) is the periodic change in the Sun's activity (including changes in the levels of solar radiation and ejection of solar material) and appearance (visible in changes in the number of sunspots, flares, and other visible manifestations). Solar cycles have an average duration of about 11 years. They have been observed (by changes in the sun's appearance and by changes seen on Earth, such as auroras) for hundreds of years.

Solar variation causes changes in space weather, weather, and climate on Earth. It causes a periodic change in the amount of irradiation from the Sun that is experienced on Earth.

It is one component of solar variation, the other being aperiodic fluctuations.

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Evolution of magnetism on the Sun.

Powered by a hydromagnetic dynamo process, driven by the inductive action of internal solar flows, the solar cycle:

• Structures the Sun's atmosphere, its corona and the wind;
• Modulates the solar irradiance;
• Modulates the flux of short-wavelength solar radiation, from ultraviolet to X-ray;
• Modulates the occurrence frequency of solar flares, coronal mass ejections, and other geoeffective solar eruptive phenomena;
• Indirectly modulates the flux of high-energy galactic cosmic rays entering the solar system.

History

Samuel Heinrich Schwabe (1789–1875). German astronomer, discovered the solar cycle through extended observations of sunspots

Rudolf Wolf (1816–1893), Swiss astronomer, carried out historical reconstruction of solar activity back to the seventeenth century

The solar cycle was discovered in 1843 by Samuel Heinrich Schwabe, who after 17 years of observations noticed a periodic variation in the average number of sunspots seen from year to year on the solar disk.^[3] Rudolf Wolf compiled and studied these and other observations, reconstructing the cycle back to 1745, eventually pushing these reconstructions to the earliest observations of sunspots by Galileo and contemporaries in the early seventeenth century. Starting with Wolf, solar astronomers have found it useful to define a standard sunspot number index, which continues to be used today.

Until recently it was thought that there were 28 cycles in the 309 years between 1699 and 2008, giving an average length of 11.04 years, but recent research has showed that the longest of these (1784–1799) seems actually to have been two cycles,^[4][5] so that the average length is only around 10.66 years. Cycles as short as 9 years and as long as 14 years have been observed, and in the double cycle of 1784-1799 one of the two component cycles had to be less than 8 years in length. Significant variations in amplitude also occur. Solar maximum and solar minimum refer respectively to epochs of maximum and minimum sunspot counts. Individual sunspot cycles are partitioned from one minimum to the next.

Following the numbering scheme established by Wolf, the 1755–1766 cycle is traditionally numbered "1". The period between 1645 and 1715, a time during which very few sunspots were observed, is a real feature, as opposed to an artifact due to missing data.^[6] This epoch is now known as the Maunder minimum, after Edward Walter Maunder, who extensively researched this peculiar event, first noted by Gustav Spörer. In the second half of the nineteenth century it was also noted (independently) by Richard Carrington and by Spörer that as the cycle progresses, sunspots appear first at mid-latitudes, and then closer and closer to the equator until solar minimum is reached. This pattern is best visualized in the form of the so-called butterfly diagram, first constructed by the husband-wife team of E. Walter and Annie Maunder in the early twentieth century (see graph below). Images of the Sun are divided into latitudinal strips, and the monthly-averaged fractional surface of sunspots calculated. This is plotted vertically as a color-coded bar, and the process is repeated month after month to produce this time-latitude diagram.

The sunspot butterfly diagram. This modern version is constructed (and regularly updated) by the solar group at NASA Marshall Space Flight Center.

The physical basis of the solar cycle was elucidated in the early twentieth century by George Ellery Hale and collaborators, who in 1908 showed that sunspots were strongly magnetized (this was the first detection of magnetic fields outside the Earth), and in 1919 went on to show that the magnetic polarity of sunspot pairs:

• Is always the same in a given solar hemisphere throughout a given sunspot cycle;
• Is opposite across hemispheres throughout a cycle;
• Reverses itself in both hemispheres from one sunspot cycle to the next.

Hale's observations revealed that the solar cycle is a magnetic cycle with an average duration of 22 years. However, because very nearly all manifestations of the solar cycle are insensitive to magnetic polarity, it remains common usage to speak of the "11-year solar cycle".

Half a century later, the father-and-son team of Harold Babcock and Horace Babcock showed that the solar surface is magnetized even outside of sunspots; that this weaker magnetic field is to first order a dipole; and that this dipole also undergoes polarity reversals with the same period as the sunspot cycle (see graph below). These various observations established that the solar cycle is a spatiotemporal magnetic process unfolding over the Sun as a whole.

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 (and regularly updated) by the solar group at NASA Marshall Space Flight Center.

Phenomena, measurement, and causes

Spots from two adjacent cycles can co-exist for some time, and since it was discovered that the Sun reverses magnetic polarity from one solar cycle to the next, spots from different cycles can be distinguished by direction of their magnetic field. However, it takes some months before a definite decision can be made as to the true date of solar minimum. One of the principal authorities that determine the date of the solar minimum is SIDC (the Solar Influences Data Analysis Center), which is located in Belgium and works with agencies such as NASA and ESA.
The most important information today comes from SOHO (a project of international cooperation between ESA and NASA), such as the MDI magnetogram, where the solar "surface" magnetic field can be seen.

The basic causes of the solar variability and its cycles are still under debate, with some researchers suggesting a link with the tidal forces due to the gas giants Jupiter and Saturn,^[7][8] or due to the solar inertial motion.^[9][10] Another cause of Sun spots can be solar jet stream "torsional oscillation".

Patterns have been noted in solar cycles. For example, the Waldmeier effect is the phenomenon that cycles with larger maximum amplitudes tend to take less time to reach their maxima than cycles with smaller amplitudes;^[11] there is also a negative correlation between maximum amplitudes and the lengths of earlier cycles, which allows a degree of prediction.^[12]

Effects of the solar cycle

Activity cycles 21, 22 and 23 seen in sunspot number index, TSI, 10.7cm radio flux, and flare index. The vertical scales for each quantity have been adjusted to permit overplotting on the same vertical axis as TSI. Temporal variations of all quantities are tightly locked in phase, but the degree of correlation in amplitudes is variable to some degree.

The Sun's magnetic field structures its atmosphere and outer layers all the way through the corona and into the solar wind. Its spatiotemporal variations lead to a host of phenomena collectively known as solar activity. All of solar activity is strongly modulated by the solar magnetic cycle, since the latter serves as the energy source and dynamical engine for the former.

Surface magnetism

Sunspots may exist anywhere from a few days to a few months, but they eventually decay, and this releases magnetic flux in the solar photosphere. This magnetic field is dispersed and churned by turbulent convection, and solar large-scale flows. These transport mechanisms lead to the accumulation of the magnetized decay products at high solar latitudes, eventually reversing the polarity of the polar fields (notice how the blue and yellow fields reverse in the graph above).

The dipolar component of the solar magnetic field is observed to reverse polarity around the time of solar maximum, and reaches peak strength at the solar minimum. Sunspots, on the other hand, are produced from a strong toroidal (longitudinally directed) magnetic field within the solar interior. Physically, the solar cycle can be thought of as a regenerative loop where the toroidal component produces a poloidal field, which later produces a new toroidal component of sign such as to reverse the polarity of the original toroidal field, which then produces a new poloidal component of reversed polarity, and so on.

Total solar irradiance

The total solar irradiance (TSI) is the amount of solar radiative energy incident on the Earth's upper atmosphere. TSI variations were undetectable until satellite observations began in late 1978. A series of radiometers carried on satellites from the 1970s to the 2000s.^[13] TSI differed from 1360 to 1370 W/m2 across ten satellites. The controversial 1989-1991 “ACRIM gap” between non-overlapping satellites has been interpolated by an ACRIM composite showing +0.037%/decade rise by the ACRIM group, and a PMOD composite with a -0.008%/decade downward trend by the PMOD group.^[14] This 0.045%/decade difference strongly impacts climate models.
Satellite measurements show that solar irradiance varies systematically over the 11-year sunspot cycle,^[15] both in total irradiance and in the relative components of the irradiance (UV Light ratios to Visible Light Ratios). The solar luminosity is about 0.07 percent brighter during solar maximum than during solar minimum. Photospheric magnetism appears to be the primary cause (96%) of 1996-2013 TSI variation.^[16] Observations from spacecraft in the 2000s showed that the ratio of ultraviolet to visible light is much more variable than previously thought.^[17]

The major finding of satellite observations is that TSI varies in phase with the solar magnetic activity cycle^[18] with an amplitude of about 0.1% and an average value of about 1361.5 W/m^2[19] (the "solar constant"). Variations about the average up to −0.3% are caused by large sunspot groups and of +0.05% by large faculae and bright network on a week to 10-day timescale^[20] (see TSI variation graphics).^[21] TSI variations over the several decades of continuous satellite observation show small but detectable trends.^[22][23]

TSI is higher at solar maximum, even though sunspots are darker (cooler) than the average photosphere. This is caused by magnetized structures other than sunspots during solar maxima, such as faculae and active elements of the "bright" network, that are brighter (hotter) than the average photosphere. They collectively overcompensate for the irradiance deficit associated with the cooler but less numerous sunspots. The primary driver of TSI changes on solar rotational and sunspot cycle timescales is the varying photospheric coverage of these radiatively active solar magnetic structures.

Short-wavelength radiation

A solar cycle: a montage of ten years' worth of Yohkoh SXT images, demonstrating the variation in solar activity during a sunspot cycle, from after August 30, 1991, to September 6, 2001. Credit: the Yohkoh mission of ISAS (Japan) and NASA (US).

With a temperature of 5870 kelvins, the photosphere of the Sun emits a very small proportion of radiation in the extreme ultraviolet (EUV) and above. However, hotter upper layers of the Sun's atmosphere (chromosphere and corona) emit more short-wavelength radiation. Since the upper atmosphere is not homogeneous and contains significant magnetic structure, the solar ultraviolet (UV), EUV and X-ray flux varies markedly in the course of the solar cycle.

The photo montage to the left illustrates this variation for soft X-ray, as observed by the Japanese satellite Yohkoh from after August 30, 1991, at the peak of cycle 22, to September 6, 2001, at the peak of cycle 23. Similar cycle-related variations are observed in the flux of solar UV or EUV radiation, as observed, for example, by the SOHO or TRACE satellites.

Even though it only accounts for a minuscule fraction of total solar radiation, the impact of solar UV, EUV and X-ray radiation on the Earth's upper atmosphere is profound. Solar UV flux is a major driver of stratospheric chemistry, and increases in ionizing radiation significantly affect ionosphere-influenced temperature and electrical conductivity.

Solar radio flux

Emission from the Sun at centimetric (radio) wavelength is due primarily to coronal plasma trapped in the magnetic fields overlying active regions.^[24] The F10.7 index is a measure of the solar radio flux per unit frequency at a wavelength of 10.7 cm, near the peak of the observed solar radio emission. F10.7 is often expressed in SFU or solar flux units (1 SFU = 10⁻²² W m⁻² Hz⁻¹). It represents a measure of diffuse, nonradiative heating of the coronal plasma trapped by magnetic fields over active regions. It is an excellent indicator of overall solar activity levels and correlates well with solar UV emissions.

The solar F10.7 index is measured daily at local noon in a bandwidth of 100 MHz centered on 2800 MHz at the Penticton site of the Dominion Radio Astrophysical Observatory (DRAO), Canada. The solar F10.7 cm record extends back to 1947, and is the longest direct record of solar activity available, other than sunspot-related quantities.^[25][26]

Sunspot activity has a major effect on long distance radio communications particularly on the shortwave bands although medium wave and low VHF frequencies are also affected. High levels of sunspot activity lead to improved signal propagation on higher frequency bands, although they also increase the levels of solar noise and ionospheric disturbances. These effects are caused by impact of the increased level of solar radiation on the ionosphere.

It has been suggested that 10.7 cm solar flux could interfere with point-to-point terrestrial communications.^[27]

Geoeffective eruptive phenomena

An overview of three solar cycles shows the relationship between the sunspot cycle, galactic cosmic rays, and the state of our near-space environment.^[28]

The solar magnetic field structures the corona, giving it its characteristic shape visible at times of solar eclipses. Complex coronal magnetic field structures evolve in response to fluid motions at the solar surface, and emergence of magnetic flux produced by dynamo action in the solar interior. For reasons not yet understood in detail, sometimes these structures lose stability, leading to coronal mass ejections into interplanetary space, or flares, caused by sudden localized release of magnetic energy driving copious emission of ultraviolet and X-ray radiation as well as energetic particles. These eruptive phenomena can have a significant impact on Earth's upper atmosphere and space environment, and are the primary drivers of what is now called space weather.

The occurrence frequency of coronal mass ejections and flares is strongly modulated by the solar activity cycle. Flares of any given size are some 50 times more frequent at solar maximum than at minimum. Large coronal mass ejections occur on average a few times a day at solar maximum, down to one every few days at solar minimum. The size of these events themselves does not depend sensitively on the phase of the solar cycle. A good recent case in point are the three large X-class flares having occurred in December 2006, very near solar minimum; one of these (an X9.0 flare on Dec 5) stands as one of the brightest on record.^[29]

Cosmic ray flux

The outward expansion of solar ejecta into interplanetary space provides overdensities of plasma that are efficient at scattering high-energy cosmic rays entering the solar system from elsewhere in the galaxy. Since the frequency of solar eruptive events is strongly modulated by the solar cycle, the degree of cosmic ray scattering in the outer solar system varies in step. As a consequence, the cosmic ray flux in the inner solar system is anticorrelated with the overall level of solar activity. This anticorrelation is clearly detected in cosmic ray flux measurements at the Earth's surface.

A drawing of a sunspot in the Chronicles of John of Worcester.

Some high-energy cosmic rays entering Earth's atmosphere collide hard enough with molecular atmospheric constituents to cause occasionally nuclear spallation reactions. Some of the fission products include radionuclides such as ¹⁴C and ¹⁰Be, which settle down on Earth's surface. Their concentration can be measured in ice cores, allowing a reconstruction of solar activity levels into the distant past.^[30] Such reconstructions indicate that the overall level of solar activity since the middle of the twentieth century stands amongst the highest of the past 10,000 years, and that Maunder minimum-like epochs of suppressed activity, of varying durations have occurred repeatedly over that time span.

Effects on Earth

Terrestrial organisms

The impact of the solar cycle on living organisms has been investigated (see chronobiology). Some researchers claim to have found connections with human health.^[31][32]
The amount of ultraviolet UVB light at 300 nm reaching the Earth varies by as much as 400% over the solar cycle due to variations in the protective ozone layer. In the stratosphere, ozone is continuously regenerated by the splitting of O₂ molecules by ultraviolet light. During a solar minimum, the decrease in ultraviolet light received from the Sun leads to a decrease in the concentration of ozone, allowing increased UVB to penetrate to the Earth's surface.^[33]

Radio communication

Skywave modes of radio communication operate by bending (refracting) radio waves (electromagnetic radiation) through the Ionosphere. During the "peaks" of the solar cycle, the ionosphere becomes increasingly ionized by solar photons and cosmic rays. This affects the path (propagation) of the radio wave in complex ways which can either facilitate or hinder local and long distance communications. Forecasting of skywave modes is of considerable interest to commercial marine and aircraft communications, amateur radio operators, and shortwave broadcasters. 

These users utilize frequencies within the High Frequency or 'HF' radio spectrum which are most affected by these solar and ionospheric variances. Changes in solar output affect the maximum usable frequency, a limit on the highest frequency usable for communications.

Terrestrial climate

Both long-term and short-term variations in solar activity are hypothesized to affect global climate, but it has proven extremely challenging to directly quantify the link between solar variation and the earth’s climate.^[34] The topic continues to be a subject of active study.
Early research attempted to find a correlation between weather and sunspot activity, mostly without notable success.^[35] Later research has concentrated more on correlating solar activity with global temperature. Most recently, research suggests that there may also be regional climate impacts due to the solar cycle. Measurements from the Spectral Irradiance Monitor on NASA’s Solar Radiation and Climate Experiment show that solar UV output is more variable over the course of the solar cycle than scientists had previously thought, resulting in, for example, colder winters in the US and southern Europe and warmer winters in Canada and northern Europe during solar minima.^[36]

There are three suggested mechanisms by which solar variations are hypothesized to have an effect on climate:

• Solar irradiance changes directly affecting the climate ("Radiative forcing").
• Variations in the ultraviolet component. The UV component varies by more than the total, so if UV were for some (as yet unknown) reason having a disproportionate effect, this might cause an effect on climate.
• Effects mediated by changes in cosmic rays (which are affected by the solar wind) such as changes in cloud cover.

The sunspot cycle variation of 0.1% has small but detectable effects on the Earth’s climate.^[37][38][39] Work by Camp and Tung suggests that changes in solar irradiance correlate with a variation of 0.18 K ±0.08 K (0.32 °F ±0.14 °F) in measured average global temperature between the peak and minimum of the 11-year solar cycle.^[40]

The effect of solar variation at time scales longer than a solar cycle is also of interest to climate science. The current scientific consensus is that solar variations do not play a major role in determining present-day global warming,^[34] since the measured magnitude of recent solar variation is much smaller than the forcing due to greenhouse gases,^[41] but the level of understanding of solar impacts is low.^[42]

Effects on spacecraft

The coronal mass ejections ("CME") associated with solar flares produces a radiation flux of high-energy protons, sometimes known as solar cosmic rays. These can cause radiation damage to electronics and solar cells in satellites. The solar proton events also can cause single-event upset (SEU) events on electronics; at the same, the reduced flux of galactic cosmic radiation during solar maximum (see section "Cosmic ray flux" above) will decrease the high-energy component of particle flux.

If astronauts on a space mission are above the shielding effect produced by the Earth's magnetic field, the radiation from a CME would also be dangerous to humans; many future mission designs (e.g., for a Mars Mission) therefore incorporate a radiation-shielded "storm shelter" for astronauts to retreat to during such a radiation event.

In view of the problems in space flight occurring during high solar activity, prediction of the latter becomes more and more important. A particular method that relies on several consecutive cycles was established by Wolfgang Gleißberg.^[43]

Sunspot

From Wikipedia, the free encyclopedia

Sunspot region 2192,^[1] during the partial solar eclipse of 23 October 2014

Detailed view, 13 December 2006

Heliophysics
Phenomena
Coronal mass ejection Geomagnetic storm Solar EMP Solar flare Solar prominence Solar proton event Solar superstorm Sunspot

Sunspots are temporary phenomena on the photosphere of the Sun that appear visibly as dark spots compared to surrounding regions. They correspond to concentrations of magnetic field that inhibit convection and result in reduced surface temperature compared to the surrounding photosphere. Sunspots usually appear as pairs, with each spot having the opposite magnetic polarity of the other.^[2]

Although they are at temperatures of roughly 3,000–4,500 K (2,700–4,200 °C), the contrast with the surrounding material at about 5,780 K (5,500 °C) leaves them clearly visible as dark spots, as the luminous intensity of a heated black body (closely approximated by the photosphere) is proportional to the fourth power of its temperature. If the sunspot were isolated from the surrounding photosphere it would be brighter than the Moon.^[3] Sunspots expand and contract as they move across the surface of the Sun and can be as small as 16 kilometers (10 mi)^[4] and as large as 160,000 kilometers (100,000 mi)^[5] in diameter, making the larger ones visible from Earth without the aid of a telescope.^[6] They may also travel at relative speeds ("proper motions") of a few hundred meters per second when they first emerge onto the solar photosphere.

Manifesting intense magnetic activity, sunspots host secondary 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.^[7]

§History

§Prehistoric evidence

Studies of stratigraphic data have suggested that the solar cycles have been active for hundreds of millions of years, if not longer; measuring varves in precambrian sedimentary rock has revealed repeating peaks in layer thickness, with a pattern repeating approximately every eleven years. 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) could have occurred during years with greater sunspot activity.^[8][9] This would presume annual layering; however, alternate explanations (diurnal) have also been proposed.^[10]

Analysis of tree rings has revealed a detailed picture of past solar cycles: Dendrochronologically dated radiocarbon 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.^[11]

§Early observations

A drawing of a sunspot in the Chronicles of John of Worcester

The earliest surviving record of sunspot observation dates from 364 BC, based on comments by Chinese astronomer Gan De in a star catalogue.^[12] By 28 BC, Chinese astronomers were regularly recording sunspot observations in official imperial records.^[13]

The first clear mention of a sunspot in Western literature, around 300 BC, was by the ancient Greek scholar Theophrastus, student of Plato and Aristotle and successor to the latter.^[14] A more recent sunspot observation was made on 17 March 807 AD by the Benedictine monk Adelmus, who observed a large sunspot that was visible for eight days; however, Adelmus incorrectly concluded he was observing a transit of Mercury.^[15] A large sunspot was also seen at the time of Charlemagne's death in 813 AD.^[16] Sunspot activity in 1129 was described by John of Worcester, and Averroes provided a description of sunspots later in the 12th century;^[17] however, these observations were also misinterpreted as planetary transits, until Galileo gave the correct explanation in 1612.^[18]

§17th and 18th centuries

Sunspots in 1794 Samuel Dunn Map

Sunspots were first observed telescopically in late 1610 by the English astronomer Thomas Harriot and Frisian astronomers Johannes and David Fabricius, who published a description in June 1611. At the latter time, Galileo had been showing sunspots to astronomers in Rome, and Christoph Scheiner had probably been observing the spots for two or three months using an improved helioscope 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. They showed that the Sun rotated, and their comings and goings showed that the Sun changed, contrary to Aristotle (who taught that all celestial bodies were perfect, unchanging spheres).

Rudolf Wolf 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 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.

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 during solar eclipses was noted prior to 1715. The period of low sunspot activity from 1645 to 1717 is known as the "Maunder Minimum".

§19th century

The cyclic variation of the number of sunspots was first observed by Heinrich Schwabe between 1826 and 1843 and led Wolf to make systematic observations starting in 1848. The Wolf number is a measure of individual spots and spot groupings, which correlates to a number of solar observables. Also in 1848, Joseph Henry projected an image of the Sun onto a screen and determined that sunspots were cooler than the surrounding surface.^[19]

After the resumption of sunspot activity, Heinrich Schwabe in 1844 in Astronomische Nachrichten (Astronomical News) reported a periodic change in the number of sunspots.

The Sun emitted an extremely powerful flare on its visible hemisphere on 1 September 1859, leading to what is known as the Carrington Event. It interrupted electrical telegraph service and caused visible aurorae as far south as Havana, Hawaii, and Rome with similar activity in the southern hemisphere.

§20th century

The American solar astronomer George Ellery Hale, as an undergraduate at MIT, invented the spectroheliograph, with which he made the discovery of solar vortices. In 1908, Hale used a modified spectroheliograph to show that the spectra of hydrogen exhibited the Zeeman effect whenever the area of view passed over a sunspot on the solar disc. This was the first indication that sunspots were basically magnetic phenomena, which appeared in pairs that corresponded with two magnetic poles of opposite polarity.^[20] Subsequent work by Hale demonstrated a strong tendency for east-west alignment of magnetic polarities in sunspots, with mirror symmetry across the solar equator; and that the magnetic polarity for sunspots in each hemisphere switched orientation, from one sunspot cycle to the next.^[21] This systematic property of sunspot magnetic fields is now commonly referred to as the "Hale–Nicholson law",^[22] or in many cases simply "Hale's law".

§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 (decade) and based also on observation of infrared spectral lines, have suggested that sunspot activity may again be disappearing, possibly leading to a new minimum.^[23] From 2007 to 2009, sunspot levels were far below average. In 2008, the Sun was spot-free 73 percent of the time, extreme even for a solar minimum. Only 1913 was more pronounced, with 85 percent 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.^[24]

Nasa's 2006 prediction. At 2010/2011, the sunspot count was expected to be at its maximum, but in reality in 2010 it was still at its minimum.

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.^[25][26] 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 unusually low current activity.^[27]

Due to a missing jet stream, fading spots, and slower activity near the poles, independent scientists of the National Solar Observatory (NSO) and the Air Force Research Laboratory (AFRL) predicted in 2011 that the next 11-year solar sunspot cycle, Cycle 25, would be greatly reduced or might not happen at all.^[28]

Cycle 24 is now well underway (as of March 2013^[update]). Measurements indicate that the minimum occurred around December 2008 and the next maximum was predicted to reach a sunspot number of 90 around May 2013.^[29] The monthly mean sunspot number in the northern solar hemisphere peaked in November 2011 but that in the southern hemisphere appears to have peaked in February 2014, giving a peak total monthly mean of 102 in that month. Subsequent months have seen a decline to around 70 by June 2014.^[30] In October of 2014, the sunspot known as AR 12192 was shown to be the largest observed since 1990.^[31] The flare that erupted from this sunspot was classified as an X3.1-class solar storm.^[32]

§Physics

A sunspot viewed close-up in ultraviolet light, taken by the TRACE spacecraft

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, 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 tells us that sunspots are actually 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.

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

Magnetic pressure should tend to remove field concentrations, causing the sunspots to disperse, but sunspot lifetimes are measured in days or even weeks. In 2001, observations from the Solar and Heliospheric Observatory (SOHO) using sound waves traveling below the Sun's photosphere (local helioseismology) were 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 that concentrates the magnetic field.^[33] Sunspots can thus be thought of as self-perpetuating storms, analogous in some ways to terrestrial hurricanes.

Butterfly diagram showing paired Spörer's law behavior

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.

Wolf number 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, who first linked magnetic fields and sunspots in 1908.^[20] Hale suggested that the sunspot cycle period is 22 years, covering two 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.

§Variation

400 year sunspot history

11,000 year sunspot reconstruction

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.^[34] 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.^[11]

Sunspots are caused by solar magnetic fields in the photosphere and the associated centennial variations in magnetic fields in the corona and heliosphere have also been deduced using carbon-14 and beryllium-10 cosmogenic isotopes stored in terrestrial reservoirs such as ice sheets and tree rings^[35] and by using historic observations of geomagnetic storm activity, which bridge the time gap between the end of the usable cosmogenic isotope data and the start of modern spacecraft data.^[36] These variations have been successfully reproduced using models which employ magnetic flux continuity equations and observed sunspot numbers to quantify the emergence of magnetic flux from the top of the solar atmosphere and into the heliosphere,^[37] showing that sunspot observations, geomagnetic activity and cosmogenic isotopes are giving a coherent understanding of solar activity variations.

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. 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).^[38][39] Sunspots were rarely observed during the Maunder Minimum in the second part of the 17th century (approximately from 1645 to 1715).

The 11-year solar cycles are numbered sequentially, starting with the observations made in the 1750s.^[40]

§Observation

The Swedish 1-m Solar Telescope at Roque de los Muchachos Observatory

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 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 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.

§Application

Detail of a sunspot in 2005. The granulation of the Sun's surface can be seen clearly

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. Solar activity (and the sunspot cycle) are frequently discussed in the context of global warming; Jack Eddy noted the apparent correlation between the Maunder Minimum of sunspot occurrence and the Little Ice Age in European winter climate.^[41] Sunspots themselves, in terms of the magnitude of their radiant-energy deficit, have only a weak effect on the terrestrial climate^[42] 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 increase slightly at sunspot maxima, when naively one might expect that sunspots would make it decrease.^[43]

British economist William Stanley Jevons suggested in the 1870s that there is a relationship between sunspots and business cycle crises. Jevons reasoned that sunspots affect Earth's weather, which, in turn, influences crops and, therefore, the economy.^[44]

§Spots on other stars

In 1947, G. E. Kron proposed that starspots were the reason for periodic changes in brightness on red dwarfs.^[7]
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).^[7][45]

§Gallery

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Sunspot NOAA 875.

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A flare from sunspot NOAA 875.

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This visualization tracks the emergence and evolution of a sunspot group as seen starting in early February 2011 and continuing for two weeks. Images are sampled one hour apart. The camera tracks the movement of the solar rotation. At this scale, a 'shimmer' of the solar surface is visible, created by the turnover of convection cells.

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Groups of sunspots grow and die over a matter of days. This is a movie built from images taken by the SDO/HMI instrument over the course of 13 days during the rise of solar cycle 24.

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  Sunspots, September 2011.
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  A view of the coronal structure above a different sunspot seen in October 2010.
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  Sunspot 923 at sunset and in solar scope.
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  Sunset Superior Mirage of sunspot #930.
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  Sunset in Bangladesh, January 2004.