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Sunspots

The solar cycle

Sunspots and weather

Why sunspots are dark

Causes

Resources

Sunspots are relatively dark, temporary spots that appear on the Sun (or, sun) from time to time. When such features appear on other stars, they are called starspots. The largest of these spots are visible to the naked eye, having even been noted by Chinese astronomers since antiquity. Their first mention in Western literature is in The Starry Messenger (1610) by Italian astronomer Galileo Galilei (15641642). Sunspot activitythe number of spots on the Sun at any one timevaries with a period of about eleven years. This corresponds to the period of an overall solar-activity cycle whose other features include solar flares and prominences. The largest flare ever recorded was on November 3, 2003. As of October 2006, the Sun was near its minimum in sunspot activity, with the minimum expected sometime in 2007.

The solar cycle

At the beginning of an active period in the solar cycle, a few sunspots appear at the higher latitudes (i.e., near the poles). These are more or less stationary on the Suns surface, but appear to astronomers on the Earth to move because of the Suns axial rotation. Large spots which may be large enough to sink many Earths inmay last for one or several solar-rotation periods of about a month each. As the solar cycle progresses, the number of spots increases and they tend to disappear at higher latitudes and appear at lower latitudes (i.e., nearer the equator). The end of the cycle is marked by a marked drop in the number of low-latitude sunspots, which is followed by the beginning of the next cycle as spots begin to appear again at high latitudes.

Sunspots, which usually occur in pairs aligned with the direction of the Suns spin, correspond to places where intense magnetic fields emerge from or reenter the solar surface. A bar magnet possesses a looping magnetic field connecting one end of the bar to the other, as revealed by iron filings scattered over a sheet of paper placed above the magnet. In this same manner, sunspot pairs possess a magnetic field that links them and along which charged particles align their motion. Scientists label the ends of a bar magnet

north or south magnetic poles depending on their properties; similarly, each member of a sunspot pair corresponds to either a north or south magnetic pole. During a given solar cycle, the member of a sunspot pair that leads (i.e., is located toward the direction of the Suns rotation) usually has the same magnetic polarity in a pair formed in a particular hemisphere. The order of polarity is reversed for sunspot pairs formed in the opposite hemisphere. The magnetic orientation spot pairs is preserved throughout one entire eleven-year solar cycle; however, during the next cycle the order of leading and trailing polarities is reversed in both hemispheres. Thus, the sunspot cycle actually consists of two eleven-year cycles, since two cycles must pass before conditions are duplicated and the pattern can begin to repeat.

Sunspots and weather

Sunspot activity may be subtly linked to the Earths weather. Suggestive correlations between solar activity, global temperature, and rainfall have been observed, and analysis of tree-ring data spanning centuries seems to show the presence of an eleven-year cycle. There is also geological evidence that the solar cycle may have been affecting terrestrial weather since Precambrian times. However, all these data have been disputed on statistical grounds, and there is presently no consensus among scientists as to whether sunspots actually affect the Earths weather or not, or if so, how. The energy output of the Sun varies very little over the solar cycle (i.e., by about 0.1%), and some scientists doubt whether such slight changes can really affect the troposphere (lower atmosphere) of the Earth, where precipitation occurs. A possible mechanism for amplifying the effects of the solar cycle on tropospheric weather is its influence on the stratosphere (the region of the atmosphere from an altitude of about 10 mi [16 km] to about 30 mi [50 km]). The stratosphere is home to the ozone layer, a diffuse shield of triatomic oxygen (O3) that is an efficient absorber of ultraviolet radiation. Since the Suns ultraviolet output varies ten times more over the solar cycle than its overall radiation output, it is plausibleand has been confirmed by observationthat the temperature (and thus volume) of the stratosphere will vary significantly with the solar cycle. (Those scientists involved in the launching and maintenance of artificial Earth satellites are acutely aware that the upper layers of the Earths atmosphere respond to solar activity by expanding and, thereby, inflicting increased drag on satellites in low orbits.) However, the troposphere is many thousands of times more massive than the stratosphere. Thus, scientists continue to investigate the question of whether temperatures in the frail film of the stratosphere can measurably affect surface weather.

One suggestion of a sunspot-weather link comes from historical records. There was a curious period of about 70 yearsa period of extreme solar inactivity that occurred between 1645 and 1710shortly after Galileos discovery of sunspots when few were observed. This era is called the Maunder minimum after English astronomer Edward Walter Maunder (18511928) who first noted its existence (along with German astronomer Friederich Wilhelm Gustav Sporer [18221895]). Other phenomena such as the aurora borealis (northern lights) that are associated with solar activity are also missing from European records during this period. The interval is also associated with what has long thought to have been a time of unusually severe winters in both Europe and North America that is sometimes termed the Little Ice Age. However, there is now doubt as to whether the Little Ace Age ever happened at all, at least on a global level. Most of the evidence for its occurrence is anecdotal, and most comes from Western Europe. It is unlikely that historical evidence of this kind will resolve the scientific dispute over whether the solar cycle (or anomalies therein) significantly affect terrestrial weather.

Why sunspots are dark

The strong magnetic field in a sunspot, which is several thousand times stronger than that at the surface of the Earth, accounts for the relative dimness of the spot. The hot atmosphere of the Sun contains a significant number of atoms having a net positive charge resulting from collisions between them (i.e., they are ionized, having each lost one or more electrons). Moving charged particles tend to spiral along magnetic field lines. The magnetic field lines passing vertically through a sunspot, therefore, tend to suppress the convection (heat-driven vertical circulation) that usually transports heat to the Suns surface from its depths. Convection requires horizontal motion so that material can reverse its direction, but the strong vertical magnetic field in the sunspot hampers cross-field (horizontal) motion of the electrically charged gas of the solar atmosphere. With less heat being supplied from below, a sunspot cools to about 3,000 K (versus about 5,800 K over the Suns normal surface, where K = Kelvin). Because of its lower temperature, the spots interior is relatively darkabout 40% dimmer than the rest of the Suns surface. Close inspection of a sunspot shows it to have a dark central region called the umbra (Latin for shadow) surrounded by a lighter, radially structured region called the penumbra (almost-shadow). These regions are created by structural differences in the magnetic field responsible for the sunspot. Further, many sunspots are surrounded by bright rings. It is likely that the heat energy, which cannot convect upward into the sunspot, leaks up instead around its edges, superheating the material there.

Causes

in the second half of the twentieth century a mechanism of sunspot formation was proposed, which accounts for much of their observed behavior. To begin with, the Sun does not rotate as a rigid body. The polar regions rotate somewhat more slowly than the equator. (The reason for this occurrence is still not known.) Because the solar material is electrically charged, the Suns overall magnetic field is dragged along with the solar rotation. Because the solar rotation is faster at the equator, the field will be dragged faster at the equator than at the poles. Although the overall magnetic field of the Sun is weak (i.e., similar to that of the Earth), this differential rotation both

KEY TERMS

Maunder minimum A period of time from about 1650 to 1725 when the Sun did not have sunspots visible with the instruments of the time. This interval is correlated with a period of unusually severe winters in Europe and North America known as the Little Ice Age.

Penumbra The lighter, outer region of a sunspot.

Umbra The dark, central region of a sunspot.

distorts and intensifies it over time. The faster-rotating regions of the equator drag the local magnetic field so that the field lines are drawn out into long, thin tubes. The more these tubes are stretched, the more intense the magnetic field within them becomes. As the magnetic tube breaks the surface of the Sun (and returns into it, as all magnetic field lines form closed loops), it forms two spot-like structures. As the field direction is out of the solar surface at one spot and into it at the other, one of these spots will act as a north magnetic pole and the other will act as a south magnetic pole. The global nature of the general solar field is what guarantees that the stretched magnetic tubes will yield leading spots with opposite polarities in opposite hemispheres. A reversal of the Suns general field would account for the observed periodic reversal of this order; however, there is no compelling explanation of why the general field should reverse after each eleven-year solar cycle. Nevertheless, this relatively simple model does provide a beginning basis for understanding sunspots.

The Solar and Heliospheric Observatory (SOHO), launched in 1996, has been studying the outer layer of the Sun for ten years. As a joint venture of the European Space Agency (ESA) and the National Aeronautics and Space Administration (NASA), SOHO is using sound waves, which travel through the photosphere of the Sun, to construct an image of the internal structure that resides below sunspots. As of 2006, the SOHO mission had dramatically increased scientific knowledge of sunspots and of the Sun in general.

Resources

BOOKS

Bhatnagar, Aravind. Fundamentals of Solar Astronomy. Hackensack, NJ: World Scientific, 2005.

Brody, Judit. The Enigma of Sunspots: A Story of Discovery and Scientific Revolution. Edinburgh, UK: Floris Books, 2002.

de Pater, Imke, and Jack J. Lissauer. Planetary Sciences. Cambridge, UK: Cambridge University Press, 2001.

Hill, Steele, and Michael Carlowicz. The Sun. New York: Abrams, 2006.

Mendillo, Michael, Andrew Nagy, and J.H. Waite, eds. Atmospheres in the Solar System: Comparative Aeronomy. Washington, DC: American Geophysical Union, 2002.

Soon, Willie. The Maunder Minimum and the Variable Sun-Earth Connection. River Edge, NJ: World Scientific, 2003.

George W. Collins, II

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Sunspots

Sunspots are relatively dark, temporary spots that appear on the Sun from time to time. The largest of these spots are visible to the naked eye and have been noted by Chinese astronomers since antiquity, but their first mention in Western literature is in The Starry Messenger (1610) by Italian astronomer Galileo Galilei (1564–1642). Sunspot activity—the number of spots on the Sun at any one time—varies with a period of 11–13 years. This corresponds to the period of an overall solar-activity cycle whose other features include solar flares and prominences.


The solar cycle

At the beginning of an active period in the solar cycle, a few sunspots appear at the higher latitudes (i.e., near the poles). These are more or less stationary on the Sun's surface, but appear to us to move because of the Sun's axial rotation . Large spots—which may be large enough to sink many Earths in—may last for one or several solar-rotation periods of about a month each. As the solar cycle progresses, the number of spots increases and they tend to disappear at higher latitudes and appear at lower latitudes (i.e., nearer the equator). The end of the cycle is marked by a marked drop in the number of low-latitude sunspots, which is followed by the beginning of the next cycle as spots begin to appear again at high latitudes.

Sunspots, which usually occur in pairs aligned with the direction of the Sun's spin, correspond to places where intense magnetic fields emerge from or reenter the solar surface. Just as a bar magnet possesses a looping magnetic field connecting one end of the bar to the other, as revealed by iron filings scattered over a sheet of paper placed above the magnet, sunspot pairs possess a magnetic field that links them and along which charged particles align their motion . Scientists label the ends of a bar magnet "north" or "south" magnetic poles depending on their properties; similarly, each member of a sunspot pair corresponds to either a north or south magnetic pole. During a given solar cycle, the member of a sunspot pair that leads (i.e., is located toward the direction of the Sun's rotation) usually has the same magnetic polarity in a pair formed in a particular hemisphere. The order of polarity is reversed for sunspot pairs formed in the opposite hemisphere. The magnetic orientation spot pairs is preserved throughout one entire 11–13 year solar cycle; however, during the next cycle the order of leading and trailing polarities is reversed in both hemispheres. Thus the sunspot cycle actually consists of two 11–13 year cycles, since two cycles must pass before conditions are duplicated and the pattern can begin to repeat.

Sunspots and weather

Sunspot activity may be subtly linked to the earth's weather . Suggestive correlations between solar activity, global temperature , and rainfall have been observed, and analysis of tree-ring data spanning centuries seems to show the presence of an 11–13 year cycle. There is also geological evidence that the solar cycle may have been affecting terrestrial weather since Precambrian times. However, all these data have been disputed on statistical grounds, and there presently no consensus among scientists as to whether sunspots actually affect the earth's weather or not, or if so, how. The energy output of the Sun varies very little over the solar cycle (i.e., by about 0.1%), and some scientists doubt whether such slight changes can really affect the troposphere (lower atmosphere) of the earth , where precipitation occurs. A possible mechanism for amplifying the effects of the solar cycle on tropospheric weather is its influence on the stratosphere (the region of the atmosphere from an altitude of about 10 mi [16 km] to about 30 mi [50 km]). The stratosphere is home to the ozone layer, a diffuse shield of triatomic oxygen (O3) that is an efficient absorber of ultraviolet radiation . Since the Sun's ultraviolet output varies 10 times more over the solar cycle than its overall radiation output, it is plausible—and has been confirmed by observation—that the temperature (and thus volume ) of the stratosphere will vary significantly with the solar cycle. (Those involved in the launching and maintenance of Earth satellites are acutely aware that the upper layers of the earth's atmosphere respond to solar activity by expanding and thereby inflicting increased drag on satellites in low orbits.) However, the troposphere is many thousands of times more massive than the stratosphere, and scientists continue to investigate the question of whether temperatures in the frail film of the stratosphere can measurably affect surface weather.

One suggestion of a sunspot-weather link comes from historical records. There was a curious period of about 75 years shortly after Galileo's discovery of sunspots when few were observed. This era is called the Maunder minimum after the astronomer who first noted its existence. Other phenomena such as the aurora borealis (northern lights) that are associated with solar activity are also missing from European records during this period. The interval is also associated with what has long thought to have been time of unusually severe winters in both Europe and the North America that is sometimes termed the Little ice Age. However, there is now doubt as to whether the Little Ace Age ever happened at all, at least on a global level. Most of the evidence for its occurrence is anecdotal, and most comes from Western Europe. It is unlikely that historical evidence of this kind will resolve the scientific dispute over whether the solar cycle (or anomalies therein) significantly affect terrestrial weather.


Why sunspots are dark

The strong magnetic field in a sunspot, which is several thousand times stronger than that at the surface of the earth, accounts for the relative dimness of the spot. The hot atmosphere of the Sun contains a significant number of atoms having a net positive charge resulting from collisions between them (i.e., they are ionized, having each lost one or more electrons). Moving charged particles tend to spiral along magnetic field lines. The magnetic field lines passing vertically through a sunspot therefore tend to suppress the convection (heat-driven vertical circulation) that usually transports heat to the Sun's surface from its depths; convection requires horizontal motion so that material can reverse its direction, but the strong vertical magnetic field in the sunspot hampers cross-field (horizontal) motion of the electrically charged gas of the solar atmosphere. With less heat being supplied from below, a sunspot cools to about 3,000K (versus about 5,800K over the Sun's normal surface). Because of its lower temperature, the spot's interior is relatively dark—about 40% dimmer than the rest of the Sun's surface. Close inspection of a sunspot shows it to have a dark central region called the umbra (Latin for "shadow") surrounded by a lighter, radially structured region called the penumbra ("almost-shadow"). These regions are created by structural differences in the magnetic field responsible for the sunspot. Further, many sunspots are surrounded by bright rings. It is likely that the heat energy which cannot convect upward into the sunspot leaks up instead around its edges, superheating the material there.

Causes

In the second half of the twentieth century a mechanism of sunspot formation was proposed which accounts for much of their observed behavior. To begin with, the Sun does not rotate as a rigid body; the polar regions rotate somewhat more slowly than the equator. (The reason for this is still not known.) Because the solar material is electrically charged, the Sun's overall magnetic field is dragged along with the solar rotation; because the solar rotation is faster at the equator, the field will be dragged faster at the equator than at the poles. Although the overall magnetic field of the Sun is weak (i.e., similar to that of the earth), this differential rotation both distorts and intensifies it over time. The faster-rotating regions of the equator drag the local magnetic field so that the field lines are drawn out into long, thin tubes; the more these tubes are stretched, the more intense the magnetic field within them becomes. As the magnetic tube breaks the surface of the Sun (and returns into it, as all magnetic field lines form closed loops), it forms two spot-like structures. As the field direction is out of the solar surface at one spot and into it at the other, one of these spots will have act as north magnetic pole and the other will act as a south magnetic pole. The global nature of the general solar field is what guarantees that the stretched magnetic tubes will yield leading spots with opposite polarities in opposite hemispheres. A reversal of the Sun's general field between 11–13 year cycles would account for the observed periodic reversal of this order; however, there is no compelling explanation of why the general field should reverse after each 11–13 year solar cycle. Nevertheless, this relatively simple model does provide a beginning basis for understanding sunspots.


Resources

books

de Pater, Imke, and Jack J. Lissauer. Planetary Sciences. Cambridge, UK: Cambridge University Press, 2001.

Prialnik, D. An Introduction to the Theory of Stellar Structure and Evolution. Cambridge: Cambridge University Press, 2000.


periodicals

Kerr, Richard A. "Link Between Sunspots, Stratosphere Buoyed." Science 5412 (April 9, 1999): 234-235.

Rast, M.P., et al. "Bright Rings around Sunspots." Nature 401 (October 14, 1999): 678-679.

Solanki, S.K., M. Schüssler, and M. Fligge. "Evolution of the Sun's Large-scale Magnetic Field Since the Maunder Minimum." Nature 408 (November 23, 2000): 445-447.


George W. Collins

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Maunder minimum

—A period of time from about 1650–1725 when the Sun did not have sunspots visible with the instruments of the time. This interval is correlated with a period of unusually severe winters in Europe and North America known as the Little Ice Age.

Penumbra

—The lighter, outer region of a sunspot.

Umbra

—The dark, central region of a sunspot.

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sunspot Region in the Sun's photosphere which is cooler than its surroundings and appears darker. Sunspots vary in size from c.1000 to 50,000km (600 to 30,000mi), and occasionally up to c.200,000km (125,000mi). Their duration varies from a few hours to a few weeks, or months for the very biggest. Sunspots occur where there is a local strengthening of the Sun's magnetic field.

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sun·spot / ˈsənˌspät/ • n. Astron. a spot or patch appearing from time to time on the sun's surface, appearing dark by contrast with its surroundings.

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