Sunspot

Sunspots imaged on 2004-06-22

Sunspots are temporary phenomena on the surface of the Sun (the photosphere) that appear visibly as dark spots compared to surrounding regions. They are caused by intense magnetic activity, which inhibits convection, forming areas of reduced surface temperature. Although they are at temperatures of roughly 3,000–4,500 K (4,940–7,640 °F), 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 (closely approximated by the photosphere) is a function of T (temperature) to the fourth power. If the sunspot were isolated from the surrounding photosphere it would be brighter than an electric arc. Sunspots expand and contract as they move across the surface of the sun and can be as large as 80,000 kilometers (49,710 mi) in diameter, making the larger ones visible from Earth without the aid of a telescope.[1]

Manifesting intense magnetic activity, sunspots host secondary phenomena such as coronal loops 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 are commonly called starspots and both light and dark spots have been measured.[2]

Contents

Sunspot variation

Line graph showing Maunder and Dalton minima, and the Modern Maximum
400 year sunspot history
Line graph showing a downward trend over 2000 B.C.-1600 A.D. followed by the recent 400 year uptrend
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.[3] 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.[4]

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).[5][6] Sunspots were rarely observed during the Maunder Minimum 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.

There may be a resonant gravitational link between a photospheric tidal force exerted by the planets and the sunspot cycle.[7][8]

History

Prehistoric Evidence

Studies of stratigraphic data suggest 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, at roughly eleven-year intervals. It is thought that the early atmosphere on earth was more sensitive to changes in solar radiation than today, and greater glacial melting (and thicker sediment deposits) occurred during years with greater sunspot activity.[9] [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]

Black and white drawing showing Latin script surrounding two concentric circles with two black dots inside the inner circle
A drawing of a sunspot in the Chronicles of John of Worcester

Early Observations

The earliest surviving record of sunspot observation dates from the 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 was on 17 March, 807 AD by the Benedictine monk Adelmus, who observed a large sunspot which was visible for eight days; however, Adelmus incorrectly concluded he was observing a transit of Mercury.[14] A large sunspot was also seen at the time of Charlemagne's death in 813 AD. Sunspot activity in 1129 was described by John of Worcester, and Averroes provided a description of sunspots in the 12th century;[15] however, these observations were also misinterpreted as planetary transits, until Galileo gave the correct explanation in 1612.

17th & 18th centuries

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

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 seventeenth century. The economist William Stanley Jevons 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.[16]

Edward Maunder later 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 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 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.

Sunspots were rarely observed during the second part of 17th century in the time called the Maunder Minimum (1645–1717).

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.[17]

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 toward Earth on 1 September 1859, known as the Carrington Event. It interrupted electrical telegraph service and caused visible Aurora as far south as Havana, Hawaii, and Rome 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.

Recent measurements, based also on observation of infra-red spectral lines, have suggested that sunspot activity may again be disappearing, possibly leading to a new minimum.[18] 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.[19]

Nasa's 2006 prediction. At 2010/2011 the sunspot count was expected to be at its maximum, but in reality in 2010 it is still at it's 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.[20] [21] The prediction did not come true. Instead, the sunspot cycle in 2010 is still at its minimum, where it should have been near its maximum, showing the suns unusual low activity.

Physics

Photo showing irregular black, red, and yellow areas on curved surface with thin, curved red lines projecting upwards from the surface
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:

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 traveling 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.

Point chart showing sunspot area as percent of the total area at various latitudes, above grouped bar chart showing average daily sunspot area as�% of visible hemisphere
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, which links magnetic fields and sunspots. 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.

Sunspot observation

Photo of six-story building with fenced balcony containing large telescope
The 1 m Swedish 1-m Solar Telescope at Roque de los Muchachos Observatory, La Palma in the Canary Islands

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.

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

Photo of black spots surrounded by grey areas on mottled white surface.
A large group of sunspots in 2004. The grey area around the spots can be seen very clearly, as well as the granulation of the sun's surface

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

Don Easterbrook, a Professor Emeritus of geology at Western Washington University, claims that there is a cause-and-effect relationship between sunspot activity and measured changes in global temperatures on Earth.

The 2007 British documentary The Great Global Warming Swindle proposes that sunspots and solar activity are the main cause of global warming and not carbon dioxide greenhouse gas emissions.

Starspots on other stars

In 1947 G. E. Kron proposed that spots were the reason for periodic changes in brightness on red dwarfs.[2] 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 kelvins, together with a warm spot of 4,800 kelvins.[2][22]

Gallery

Sunspot 923 at sunset and in solar scope

Sunset Superior Mirage of sunspot #930

Sunset in Bangladesh, January 2004

See also

References

  1. ↑ [1]
  2. ↑ 2.0 2.1 2.2 press release 990610, K. G. Strassmeier, 1999-06-10, University of Vienna, "starspots vary on the same (short) time scales as Sunspots do", "HD 12545 had a warm spot (350 K above photospheric temperature; the white area in the picture)"
  3. ↑ Sunspot index graphics, Solar Influences Data Analysis Center (retrieved 27 September 2007).
  4. ↑ Solanki, S.K.; Usoskin, I.G., Kromer, B., Schüssler, M. and Beer, J. (2004). "Unusual activity of the Sun during recent decades compared to the previous 11,000 years". Nature 431 (7012): 1084–1087. doi:10.1038/nature02995. PMID 15510145. http://www.ncdc.noaa.gov/paleo/pubs/solanki2004/solanki2004.html. 
  5. ↑ "Solar Forcing of Climate". Climate Change 2001: Working Group I: The Scientific Basis. http://www.grida.no/climate/ipcc_tar/wg1/244.htm. Retrieved 2005-03-10. 
  6. ↑ Weart, Spencer (2006). "The Discovery of Global Warming". In Weart, Spencer. American Institute of Physics. http://www.aip.org/history/climate/index.html. Retrieved 2007-04-14. 
  7. ↑ Wainwright, G. (2004). "Jupiter's influence". New Scientist. http://www.newscientist.com/article/mg18124394.100-jupiters-influence.html. Retrieved 27 September 2007. 
  8. ↑ Wilson, Ian. Do periodic peaks in the planetary tidal forces acting upon the Sun influence the sunspot cycle?. http://heliogenic.blogspot.com/2008/12/new-paper-from-ian-wilson.html. Retrieved October 07 2009. 
  9. ↑ Williams, G.E. (1985). "Solar affinity of sedimentary cycles in the late Precambrian Elatina Formation". Australian Journal of Physics 38: 1027–1043. http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1985AuJPh..38.1027W&data_type=PDF_HIGH&whole_paper=YES&type=PRINTER&filetype=.pdf. Retrieved 2010-07-14. 
  10. ↑ Information, Reed Business (1981). "Digging down under for sunspots". New Scientist 91: 147. http://books.google.com/?id=4CpdD9YdMeoC&pg=PA147&lpg=PA147&dq=williams+sunspot+varve&q=williams%20sunspot%20varve. Retrieved 2010-07-14. 
  11. ↑ Solanki, S.K.; Usoskin, I.G., Kromer, B., Schüssler, M. and Beer, J. (2004). "Unusual activity of the Sun during recent decades compared to the previous 11,000 years". Nature 431 (7012): 1084–1087. doi:10.1038/nature02995. PMID 15510145. http://www.ncdc.noaa.gov/paleo/pubs/solanki2004/solanki2004.html. Retrieved 2010-07-14. 
  12. ↑ "Early Astronomy and the Beginnings of a Mathematical Science". 2007. http://nrich.maths.org/6843. Retrieved 2010-07-14. 
  13. ↑ "The Observation of Sunspots". UNESCO Courier. 1988. http://findarticles.com/p/articles/mi_m1310/is_1988_Oct/ai_6955852/. Retrieved 2010-07-14. 
  14. ↑ Wilson, E. R. (1917). "A Few Pre-Copernican Astronomers". Popular Astronomy 25: 88–101. Bibcode: 1917PA.....25...88W. http://adsabs.harvard.edu/full/1917PA.....25...88W. Retrieved 2010-03-07.  See p. 93.
  15. ↑ Ead, Hamed A.. "Averroes As A Physician". University of Cairo. http://levity.com/alchemy/islam21.html. 
  16. ↑ Carlos Garcia-Mata C, Shaffner FI (November, 1934). "Solar and economic relationships: a preliminary report". The Quarterly Journal of Economics (The MIT Press) 49 (1): 1–51. doi:10.2307/1883875. http://www.jstor.org/stable/1883875. 
  17. ↑ Hellemans, Alexander; Bryan Bunch (1988). The Timetables of Science. New York, New York: Simon and Schuster. p. 317. ISBN 0671621300. 
  18. ↑ Phillips, Dr. Tony (09.03.2009). "Are Sunspots Disappearing?". http://science.nasa.gov/headlines/y2009/03sep_sunspots.htm. 
  19. ↑ Clark, Stuart (June 14 , 2010). "What's wrong with the sun?". http://www.newscientist.com/article/mg20627640.800-whats-wrong-with-the-sun.html. 
  20. ↑ Phillips, Tony (May 10, 2006). "Long Range Solar Forecast: Solar Cycle 25 peaking around 2022 could be one of the weakest in centuries.". http://science.nasa.gov/science-news/science-at-nasa/2006/10may_longrange/. 
  21. ↑ Dikpati, Mausumi (March 6, 2006). "NCAR News Release: Scientists Issue Unprecedented Forecast of Next Sunspot Cycle.". http://www.ucar.edu/news/releases/2006/sunspot.shtml. 
  22. ↑ derived images showing rotation of cool and warm starspots

External links

Sunspot data

The Sun
Internal structure
Core · Radiation zone · Convection zone
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Atmosphere
Supergranulation · Granule · Faculae · Sunspot
Chromosphere
Plage · Spicule · Moreton wave
Transition region · Coronal hole · Coronal loop · Coronal mass ejection · Prominence · Helmet streamer
Variation
Solar cycle · Solar maximum · Solar minimum · Wolf number · Flare · Helioseismology
Heliosphere
Solar wind (Current sheet) · Termination shock · Heliosheath · Heliopause · Bow shock
Related topics
Solar System · Solar dynamo · Solar telescope · Eclipse · Radiation
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