Sunspot

• Top: active region 2192 in 2014 containing the largest sunspot of solar cycle 24^[1] and active region 1302 in September 2011.
• Middle: sunspot close-up in the visible spectrum (left) and another sunspot in
  TRACE
  observatory.
• Bottom: a large group of sunspots stretching about 320,000 km (200,000 mi) across.

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Sunspots are temporary spots on the

Indicating intense magnetic activity, sunspots accompany other active region phenomena such as coronal loops, prominences, and reconnection events. Most solar flares and coronal mass ejections originate in these magnetically active regions around visible sunspot groupings. Similar phenomena indirectly observed on stars other than the Sun are commonly called starspots, and both light and dark spots have been measured.^[6]

History

The earliest record of sunspots is found in the Chinese I Ching, completed before 800 BC. The text describes that a dou and mei were observed in the sun, where both words refer to a small obscuration.^[7] The earliest record of a deliberate sunspot observation also comes from China, and dates to 364 BC, based on comments by astronomer Gan De (甘德) in a star catalogue.^[8] By 28 BC, Chinese astronomers were regularly recording sunspot observations in official imperial records.^[9]

The first clear mention of a sunspot in Western literature is circa 300 BC, by ancient Greek scholar Theophrastus, student of Plato and Aristotle and successor to the latter.^[10]

The earliest known drawings of sunspots were made by English monk John of Worcester in December 1128.^[11][12]

Sunspots were first observed telescopically in December 1610 by English astronomer Thomas Harriot.^[13] His observations were recorded in his notebooks and were followed in March 1611 by observations and reports by Frisian astronomers Johannes and David Fabricius.^[14][15] After Johannes Fabricius' death at the age of 29, his reports remained obscure and were eclipsed by the independent discoveries of and publications about sunspots by Christoph Scheiner and Galileo Galilei.^[16] Galileo likely began telescopic sunspot observations around the same time as Harriot; however, Galileo's records did not start until 1612.^[17]

In the early 19th Century,

Sunspots have two main structures: a central umbra and a surrounding penumbra. The umbra is the darkest region of a sunspot and is where the magnetic field is strongest and approximately vertical, or normal, to the Sun's surface, or photosphere. The umbra may be surrounded completely or only partially by a brighter region known as the penumbra.^[22] The penumbra is composed of radially elongated structures known as penumbral filaments and has a more inclined magnetic field than the umbra.^[23] Within sunspot groups, multiple umbrae may be surrounded by a single, continuous penumbra.

The temperature of the umbra is roughly 3000–4500 K, in contrast to the surrounding material at about 5780 K, leaving sunspots clearly visible as dark spots. This is because the luminance of a heated black body (closely approximated by the photosphere) at these temperatures varies greatly with temperature. Isolated from the surrounding photosphere, a single sunspot would shine brighter than the full moon, with a crimson-orange color.^[24]

In some forming and decaying sunspots, relatively narrow regions of bright material appear penetrating into or completely dividing an umbra. These formations, referred to as light bridges, have been found to have a weaker, more tilted magnetic field compared to the umbra at the same height in the photosphere. Higher in the photosphere, the light bridge magnetic field merges and becomes comparable to that of the umbra.

The Wilson effect implies that sunspots are depressions on the Sun's surface.

Lifecycle

The appearance of an individual sunspot may last anywhere from a few days to a few months, though groups of sunspots and their associated active regions tend to last weeks or months. Sunspots expand and contract as they move across the surface of the Sun, with diameters ranging from 16 km (10 mi)^[3] to 160,000 km (100,000 mi).^[4]

Formation

Although the details of sunspot formation are still a matter of ongoing research, it is widely understood that they are the visible manifestations of

Sunspots initially appear in the photosphere as small darkened spots lacking a penumbra. These structures are known as solar pores.^[26] Over time, these pores increase in size and move towards one another. When a pore gets large enough, typically around 3,500 km (2,000 mi) in diameter, a penumbra will begin to form.^[25]

Decay

Magnetic pressure should tend to remove field concentrations, causing the sunspots to disperse, but sunspot lifetimes are measured in days to weeks. In 2001, observations from the Solar and Heliospheric Observatory (SOHO) using sound waves traveling below the photosphere (local helioseismology) were used to develop a three-dimensional image of the internal structure below sunspots; these observations show that a powerful downdraft underneath each sunspot, forms a rotating vortex that sustains the concentrated magnetic field.^[27]

Solar cycle

Solar cycles last typically about eleven years, varying from just under 10 to just over 12 years. Over the solar cycle, sunspot populations increase quickly and then decrease more slowly. The point of highest sunspot activity during a cycle is known as solar maximum, and the point of lowest activity as solar minimum. This period is also observed in most other solar activity and is linked to a variation in the solar magnetic field that changes polarity with this period.

Early in the cycle, sunspots appear at higher latitudes and then move towards the equator as the cycle approaches maximum, following Spörer's law. Spots from two sequential cycles co-exist for several years during the years near solar minimum. Spots from sequential cycles can be distinguished by direction of their magnetic field and their latitude.

The Wolf number sunspot index counts the average number of sunspots and groups of sunspots during specific intervals. The 11-year solar cycles are numbered sequentially, starting with the observations made in the 1750s.^[28]

Longer-period trends

Sunspot numbers also change over long periods. For example, during the period known as the modern maximum from 1900 to 1958 the

400-year history of sunspot numbers, showing Maunder and Dalton minima, and the Modern Maximum (left) and 11,000-year sunspot reconstruction showing a downward trend over 2000 BC – 1600 AD followed by the recent 400 year uptrend

Modern observation

Sunspots are observed with land-based and Earth-orbiting

Sunspots themselves, in terms of the magnitude of their radiant-energy deficit, have a weak effect on solar flux.[38] The total effect of sunspots and other magnetic processes in the solar photosphere is an increase of roughly 0.1% in brightness of the Sun in comparison with its brightness at the solar-minimum level. This is a difference in total solar irradiance at Earth over the sunspot cycle of close to ${\displaystyle 1.37\ \mathrm {W\cdot m^{-2}} }$. Other magnetic phenomena which correlate with sunspot activity include faculae and the chromospheric network.^[39] The combination of these magnetic factors mean that the relationship of sunspot numbers to Total Solar Irradiance (TSI) over the decadal-scale solar cycle, and their relationship for century timescales, need not be the same. The main problem with quantifying the longer-term trends in TSI lies in the stability of the absolute radiometry measurements made from space, which has improved in recent decades but remains a problem.^[40][41] Analysis shows that it is possible that TSI was actually higher in the Maunder Minimum compared to present-day levels, but uncertainties are high, with best estimates in the range ${\displaystyle \pm 0.5\ \mathrm {W\cdot m^{-2}} }$ with a ${\displaystyle 2\sigma }$ uncertainty range of ${\displaystyle \pm 1\ \mathrm {W\cdot m^{-2}} }$.^[42]

Sunspots, with their intense magnetic field concentrations, facilitate the complex transfer of energy and momentum to the upper solar atmosphere. This transfer occurs through a variety of mechanisms, including generated waves in the lower solar atmosphere^[43] and magnetic reconnection events.^[44]

Starspot

In 1947, G. E. Kron proposed that

References

Further reading

• Carl Luetzelschwab, K9LA (October 2016). "The new sunspot numbers".
  ISSN 0033-4812.{{cite journal}}: CS1 maint: numeric names: authors list (link
  )

External links

Wikimedia Commons has media related to Sunspots.

• Sunspot Database based on Terrestrial (GPR/DPD) and Satellite (SOHO/SDO) observations from 1872 to Nowadays with the newest data. ()
• Solar Cycle 24 and VHF Aurora Website (www.solarcycle24.com)
• Belgium World Data Center for the sunspot index Archived 3 August 2017 at the Wayback Machine
• High resolution sunspot image
• Sunspot images in high-res Impressive collection of sunspot images
• NOAA Solar Cycle Progression: Current solar cycle.
  • Current conditions: Space weather
• 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
• The Sharpest View of the Sun
• Daily Sunspot Update and Picture of the Sun (www.spaceweather.com)
• Animated explanation of Sunspots in the Photosphere Archived 16 November 2015 at the Wayback Machine (University of South Wales)

Sunspot data

• "11,000 Year Sunspot Number Reconstruction". Global Change Master Directory. Archived from the original on 2 November 2015. Retrieved 11 March 2005.
  • "Unusual activity of the Sun during recent decades compared with the previous 11,000 years". WDC for Paleoclimatology. Retrieved 11 March 2005.
• "Sunspot Numbers from Ancient Times to Present from NOAA/NGDC". Global Change Master Directory. Archived from the original on 14 September 2015. Retrieved 11 March 2005.
  • "Sunspot Numbers". NOAA NGDC Solar Data Services. Retrieved 21 June 2010.^{[permanent dead link]}
    • 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 1945–present
    • Ancient sunspot data 165 BC to 1684 AD
    • Group Sunspot Numbers (Doug Hoyt re-evaluation) 1610–1995
• Wilson, Robert M. (April 2014). Comparison of the Variations of Sunspot Number, Number of Sunspot Groups, and Sunspot Area, 1875–2013. Huntsville, AL: National Aeronautics and Space Administration, Marshall Space Flight Center. Retrieved 13 March 2015.