sunspot

 

dark, usually irregularly shaped spots on the sun's surface that are actually solar magnetic storms. The Chinese recorded dark features on the sun seen with the naked eye in 28 B.C. Other observers including Kepler suspected that these events might be transits of Mercury or Venus. Galileo observed them systematically for several weeks before concluding that they had to be events taking place on the solar surface. The temperature of the spots is lower than that of the surrounding photosphere; thus the spots are darker. All but the smallest show a dark central portion (the umbra) with a lighter outer area (the penumbra). Studies of the spectra of sunspots show evidence of the Zeeman effect, indicating the presence of a large magnetic field. In addition, measurements of the Doppler effect in the spectral lines show that there is a vortex motion in sunspots similar to that of a tornado on earth. The lower temperature of the gases constituting a sunspot results from the lower pressure due to the strong magnetic field. Sunspots appear usually only between latitudes from 5° to 35° north and south of the sun's equator. Sunspots are not permanent since the sun's surface is gaseous. Because the sun rotates on its axis, a sunspot cannot be observed continuously for more than about two weeks. In 1826 amateur astronomer Heinrich Schwabe began a series of solar observations (in hopes of finding planet Vulcan). By 1843 he had collected enough data to announce the existence of the sunspot cycle. An 11-year cycle from one period of maximum activity to the next is usually observed. However, a period during which most sunspots have one magnetic polarity is followed by another period during which most have the opposite magnetic polarity; thus, the cycle actually covers 22 years. During each 11-year period sunspots appear first at higher latitudes and later at latitudes closer to the solar equator as the period progresses. The spots often form in pairs or groups, with a large, long-lived leader spot matched with one or more smaller spots of opposite magnetic polarity. A number of phenomena are associated with sunspots. Sunspot activity produces various disturbances on earth—these include magnetic storms which manifest themselves as aurorae, interference with radio reception and electric power grids, and disturbances of the magnetic compass. Periods in which an increase in sunspots is observed are called active periods. Reviewing historical records in 1890, E. Maunder noticed that sunspot counts fell drastically between 1645 and 1715. In 1976 J. Eddy correlated Maunder's data with a low frequency of aurorae and the reduced sizes of annual tree rings. This “Maunder Minimum” may have played a role in the unusually low temperatures in the northern hemisphere during this period, which is known as the Little Ice Age.

A sunspot is a region on the Sun's surface (photosphere) that is marked by a lower temperature than its surroundings and has intense magnetic activity, which inhibits convection, forming areas of low surface temperature. Although they are blindingly bright at temperatures of roughly 4000-4500 K, the contrast with the surrounding material at about 5800 K leaves them clearly visible as dark spots. If they were isolated from the surrounding photosphere they would be brighter than an electric arc. A minimum in the eleven-year sunspot cycle is predicted for 2007 [1]. Sunspots are often related to intense magnetic activity such as coronal loops and reconnection. Most solar flares and coronal mass ejections originate in magnetically active regions around sunspot groupings. Similar phenomena observed on stars other than the Sun are commonly called starspots.

Sunspot variation

Main article: Solar variation

400 year sunspot history

11,000 year sunspot reconstruction

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

Sunspot numbers rise and fall with an irregular cycle with a length of approximately 11 years. In addition to this, there are variations over longer periods. The recent trend is upward from 1900 to the 1960s, then somewhat downward^[1]. The Sun was last similarly active over 8,000 years ago. The number of sunspots has been found to correlate with the intensity of solar radiation over the period (since 1979) when satellite measurements of radiation are available. Since sunspots are dark it might be expected that more sunspots lead to less solar radiation and a decreased solar constant. However, the surrounding areas are brighter and the overall effect is that more sunspots means a brighter sun. The variation caused by the sunspot cycle to solar output is relatively small, of 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)^[2][3]. This range is slightly smaller than the change in radiative forcing caused by the increase in atmospheric CO₂ since the 18th century^[4]. During the Maunder Minimum in the 17th Century there were hardly any sunspots at all. This coincides with a period of cooling known as the Little Ice Age. It has been speculated that there may be a resonant gravitational link between a photospheric tidal force from the planets, the dominant component by summing gravitational tidal force (75%) being Jupiter's with an 11 year cycle^[5].

History

Apparent references to sunspots were made by Chinese astronomers in 28 BC (Hanshu, 27), who probably could see the largest spot groups when the sun's glare was filtered by wind-borne dust from the various central Asian deserts. A large sunspot was also seen in the time of Charlemagne and sunspot activity in 1129 was described by John of Worcester. However, these observations were misinterpreted until Galileo gave the correct explanation in 1612.

They 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. 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 the teaching of Aristotle. The details of their apparent motion could not be readily explained except in the heliocentric system of Copernicus.

The cyclic variation of the number of sunspots was first observed by Heinrich Schwabe between 1826 and 1843 and led Rudolf Wolf to make systematic observations starting in 1848. The Wolf number is an expression of individual spots and spot groupings, which has demonstrated success in its correlation 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.^[6]

Wolf also studied the historical record in an attempt to establish a database on cyclic variations of the past. He established a cycle database to only 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 seventeenth century. The economist William Stanley Jevons suggested that there is a relationship between sunspots and crises in business cycles. He reasoned that sunspots affect earth's weather, which, in turn, influences crop yields and, therefore, the economy.

Edward Maunder would later suggest a period over which the Sun had changed modality from a period in which sunspots all but disappeared from the solar surface, followed by the appearance of sunspot cycles starting in 1700. Careful studies revealed the problem not to be a lack of observational data but included references to negative observations. Adding to this understanding of the absence of solar activity cycles were observations of aurorae, which were also absent at the same time. Even the lack of a solar corona during solar eclipses was noted prior to 1715.

Sunspot research was dormant for much of the 17th and early 18th centuries because of the Maunder Minimum, during which no sunspots were visible for some years; but after the resumption of sunspot activity, Heinrich Schwabe in 1843 reported a periodic change in the number of sunspots.

Significant events

An extremely powerful flare was emitted toward Earth on 1 September 1859. It interrupted telegraph service and caused visible Aurora Borealis as far south as Havana, Hawaii, and Rome with similar activity in the southern hemisphere.

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.

Physics

Although the details of sunspot generation are still somewhat a matter of research, it is quite clear that sunspots are the visible counterparts of magnetic flux tubes in the convective zone of the sun that get "wound up" by differential rotation. If the stress on the flux tubes reaches a certain limit, they curl up quite like a rubber band and puncture the sun's surface. At the puncture points convection is inhibited, the energy flux from the sun's interior decreases, and with it the surface temperature.

The Wilson effect tells us that sunspots are actually depressions on the sun's surface. This model is supported by observations using the Zeeman effect that 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
• The surrounding penumbra, which is lighter, where the magnetic field lines are more inclined.

Magnetic field lines would ordinarily repel each other, causing sunspots to disperse rapidly, but sunspot lifetime is about two weeks. Recent observations from the Solar and Heliospheric Observatory (SOHO) using sound waves travelling through the Sun's photosphere to develop a detailed image of the internal structure below sunspots show that there is a powerful downdraft underneath each sunspot, forming a rotating vortex that concentrates magnetic field lines. Sunspots are self-perpetuating storms, similar 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. At the start of a cycle, sunspots tend to appear in the higher latitudes and then move towards the equator as the cycle approaches maximum: this is called Spörer's law.

Today it is known that there are various periods in the Wolf number sunspot index, 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.

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 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 the behavior described by Spörer's law, as well as other effects, as being due to magnetic fields which are twisted by the Sun's rotation.

Observing sunspots

Looking directly at the Sun with the naked eye, through binoculars or a telescope is extremely dangerous. The central part of the retina, known as the macula, is highly sensitive and cells in this area can be very easily burned since they absorb a maximum quantity of radiation. Retinal burns produce no pain so there is no warning at the time burns occur and, once damaged, these cells never regenerate. Never leave viewing apparatus unattended. No one must ever accidentally look through the eyepiece while any unfiltered instrument is pointed towards the Sun.

The safest way to observe sunspots with the unaided eye is by looking directly through a small plate of very dark filter glass, such as a #14 welder's glass as normally used in a welding helmet. If using a telescope the magnified image can be projected, without filtration, on to a white screen and viewed indirectly to allow more detailed observations of the solar surface. 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) can be fitted over the front of a telescope and will provide safe direct observation through the eyepiece when used consistently with the manufacturers directions.

Application

Due to their link to other kinds of solar activity, sunspots can be used to predict the space weather and with it the state of the ionosphere. Thus, sunspots can help predict conditions of short-wave radio propagation or satellite communications.

See also

• List of Solar Cycles
• Solar Rotation

References

External links

• Solar Cycle 24 and VHF Aurora Website (www.solarcycle24.com)
• Belgium World Data Center for the sunspot index
• High resolution sunspot image
• Sunspot images in high-res Impressive collection of sunspot images
• http://www.tvweather.com/awpage/history_of_the_atmosphere.htm
• NOAA Solar Cycle Progression: Current solar cycle.
  • Current conditions: Space weather
• Lockheed Martin Solar and Astrophysics Lab

Sunspot data

• 11,000 Year Sunspot Number Reconstruction. Global Change Master Directory. Retrieved on 11 March 2005.
  • Unusual activity of the Sun during recent decades compared to the previous 11,000 years. WDC for Paleoclimatology. Retrieved on 11 March 2005.
• Sunspot Numbers from Ancient Times to Present from NOAA/NGDC. Global Change Master Directory. Retrieved on 11 March 2005.
  • SUNSPOT NUMBERS. NOAA NGDC Solar Data Services. Retrieved on 11 March 2005.
    • 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 1944-present
    • Ancient sunspot data 165 BC to 1684 AD
    • Group Sunspot Numbers (Doug Hoyt re-evaluation) 1610-1995

The Sun
Structure	Solar core - Radiation zone - Convection zone
Atmosphere	Photosphere - Chromosphere - Transition region - Corona
Extended structure	Termination Shock - Heliosphere - Heliopause - Heliosheath - Bow Shock
Solar phenomena	Sunspots - Faculae - Granules - Supergranulation - Solar Wind - Spicules - Coronal loops - Solar Flares - Solar Prominences - Coronal Mass Ejections - Moreton Waves - Coronal Holes
Other	Solar System - Solar Variation - Solar Dynamo - Heliospheric Current Sheet - Solar Radiation - Solar Eclipse - Spectral Class: G2

Donate to Wikimedia