Hellenica World


Sunspots imaged by the MDI instrument on NASA’s Solar and Heliospheric Observatory (SOHO) on 30 March 2001. It was the source of numerous flares and coronal mass ejections, including one of the largest flares recorded in 25 years on 2 April 2001.

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. You can only see them if you use sun-protective lenses. 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 may have taken place in late 2007 [1] and while the observation of a reverse polarity sunspot [1] on 4 January 2008 the 1st Cycle 24 sunspot was sighted, no additional sunspots have yet been seen in this cycle. The definition of a new sunspot cycle is when the averages number of sunspots of the new cycle outnumber those of the old cycle. Forecasts expected cycle 24 to start at least a year ago, but new estimates could place the official cycle 24 start in the beginning of 2009. 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 and both light (warm) and dark (cool) spots are seen.[2]

Sunspot variation

Main article: Solar variation

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[3]. 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, 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)[4][5]. This range is slightly smaller than the change in radiative forcing caused by the increase in atmospheric CO2 since the 18th century[6]. 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[7].


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 at the time of Charlemagne's death in 813 A.D. 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.[8]

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. Since 1981, the Royal Observatory of Belgium keeps track of sunspots as the World data center for the Sunspot Index.

Radio communications interference

Solar flares also create a wide spectrum of radio noise; at VHF (and under unusual conditions at HF) this noise may interfere directly with a wanted signal. The frequency with which a radio operator experiences solar flare effects will vary with the approximately 11-year sunspot cycle; more effects occur during solar maximum (when flare occurrence is high) than during solar minimum (when flare occurrence is very low). A radio operator can experience great difficulty in transmitting or receiving signals during solar flares due to more noise and different propagation patterns.

Significant events

An extremely powerful flare was emitted toward Earth on 1 September 1859. It interrupted electrical 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.


Main article: Solar cycle

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 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.

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. 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.

Sunspot observation

Sunspots are observed with land-based solar telescopes as well as ones on Earth-orbiting satellites. These telescopes use filtration and projection techniques for direct observation, in additional to filtered cameras of various types. Specialized tools such as spectroscopes and spectrohelioscopes are used to examine sunspots and areas of sunspots. 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, through binoculars or a telescope is extremely dangerous, amateur observation of sunspots with the unaided eye is generally done by projection or via using proper filtration. Small sections of very dark filter glass, such as a #14 welder's glass is sometimes employed. The eyepiece of a telescope is also used in the role of a "projector" to project the image, without filtration, on to a white screen where it can be viewed indirectly, and even traced, so sunspot evolution can be followed. 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) are also used on the front of a telescope to provide safe direct observation through the eyepiece.


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.

Starspots on other stars

Periodic changes in brightness had been first seen on red dwarfs and in 1947 G. E. Kron proposed that spots were the cause.[2] Since the mid 1990s observations of starspots have been made using increasingly powerful techniques yielding more and more detail: photometry determined starspot regions grew and decayed and showed cyclic behaviour similar to the Sun's; spectroscopy examined the structure of starspot regions; 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 3500 kelvin, together with a warm spot of 4800 kelvin.[2][9]


  1. ^ First sunspot of new solar cycle glimpsed, NewScientist (retrieved 8 January 2008)
  2. ^ a b c 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. ^ Solar Forcing of Climate. Climate Change 2001: Working Group I: The Scientific Basis. Retrieved on March 10, 2005.
  5. ^ Weart, Spencer (2006), "Changing Sun, Changing Climate?", in Weart, Spencer, The Discovery of Global Warming, American Institute of Physics, <http://www.aip.org/history/climate/solar.htm>. Retrieved on 14 April 2007 
  6. ^ Recent Greenhouse Gas Concentrations, CDIAC (retrieved 27 September 2007).
  7. ^ Wainwright, G. (2004). Jupiter's influence. New Scientist 2439, 30 (retrieved 27 September 2007).
  8. ^ Hellemans, Alexander; Bryan Bunch (1988). The Timetables of Science. New York, New York: Simon and Schuster, 317. ISBN 0671621300. 
  9. ^ derived images showing rotation of cool and warm starspots


  • 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 conditions: Space weatherr
  • Lockheed Martin Solar and Astrophysics Lab
  • Sun|trek website An educational resource for teachers and students about the Sun and its effect on the Earth
  • Tools to display the current sunspot number in a browser
    • Propfire - displays current sunspot number in browser status bar
    • HamLinks Toolbar - displays solar flux, A Index and K Index data in a toolbar

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

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