A sunspot is a region on the Sun's surface (photosphere) that is marked by a lower temperature than its surroundings, and intense magnetic activity. Although they are blindingly bright, at temperatures of roughly 5000 K, the contrast with the surrounding material at some 6000 K leaves them clearly visible as dark spots. Interestingly, if they were isolated from the surrounding photosphere they would be brighter than an electric arc.
Sunspot numbers have been measured since 1700 and estimated back to 1500. The trend is upward from 1900 to 1960s then somewhat downward  (http://sidc.oma.be/html/wolfaml.html).
The number of sunspots correlates with the intensity of solar radiation. Since sunspots are dark it is natural to assume that more sunspots means less solar radiation. However the surrounding areas are brighter and the overall effect is that more sunspots means a brighter sun. The variation is small (of the order of 0.1%) and was only established once satellite measurements of solar variation became available in the 1980s. During the Maunder Minimum there were hardly any sunspots at all and the earth may have cooled by up to 1°C (see Little Ice Age).
Active region 9393 as seen by the MDI instrument on SOHO hosted the largest sunspot group observed so far during the current solar cycle. On 30 March 2001, the sunspot area within the group spanned an area more than 13 times the entire surface of the Earth. 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
. Caused by intense magnetic fields
emerging from the interior, a sunspot appears to be dark only when contrasted against the rest of the solar surface, because it is slightly cooler than the unmarked regions.
Apparent references to sunspots were made by first millennium AD Chinese astronomers, 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.
They were first observed telescopically in late 1610 by 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.
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.
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.
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 :
- umbra (temperatures around 2200°C)
- penumbra (temperatures around 3000°C)
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 (Solar Max for short), and the point of lowest activity is Solar Minimum (Solar Min). 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 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 photo of a sun spot (seen slightly left of the centre) taken without specialist equipment.
Sunspots are relatively easily observed -- a small telescope with a projection facility suffices. In some circumstances (low sunsets) sunspots can be observed with the naked eye. Note: Never look directly into the Sun; it can cause permanent, incurable damage to the retina before you know that anything is happening.
Due to their link to other kinds of solar activity, they can be used to predict the space weather and with it the state of the ionosphere. Thus they can help predict conditions of short-wave propagation or satellite communications.
- - Belgium World Data Center for the sunspot index (http://sidc.oma.be/index.php3)
- Reconstructed sunspot activity for last 11,400 years (http://www.mpg.de/english/illustrationsDocumentation/documentation/pressReleases/2004/pressRelease20041028/genPDF.pdf)