Editing Sunspot (section)

== Physics ==
=== Morphology ===
[[File:Sunspot from Hinode in G band (4305) 2007-04-30 T001456.gif|thumb|A decaying sunspot shown over the course of two hours. The umbra is separated into two pieces within the penumbra by a light bridge.<ref name="felipe16">{{cite journal |last1=Felipe |first1=T. |last2=Collados |first2=M. |last3=Khomenko |first3=E. |last4=Kuckein |first4=C. |last5=Asensio Ramos |first5=A. |last6=Balthasar |first6=H. |last7=Berkefeld |first7=T. |last8=Denker |first8=C. |last9=Feller |first9=A. |last10=Franz |first10=M. |last11=Hofmann |first11=A. |last12=Joshi |first12=J. |last13=Kiess |first13=C. |last14=Lagg |first14=A. |last15=Nicklas |first15=H. |last16=Orozco Suárez |first16=D. |last17=Pastor Yabar |first17=A. |last18=Rezaei |first18=R. |last19=Schlichenmaier |first19=R. |last20=Schmidt |first20=D. |last21=Schmidt |first21=W. |last22=Sigwarth |first22=M. |last23=Sobotka |first23=M. |last24=Solanki |first24=S. K. |last25=Soltau |first25=D. |last26=Staude |first26=J. |last27=Strassmeier |first27=K. G. |last28=Volkmer |first28=R. |last29=von der Lühe |first29=O. |last30=Waldmann |first30=T. |title=Three-dimensional structure of a sunspot light bridge |journal=Astronomy & Astrophysics |date=December 2016 |volume=596 |pages=A59 |doi=10.1051/0004-6361/201629586 |arxiv=1611.04803 |bibcode=2016A&A...596A..59F |s2cid=119419693 |url=https://www.aanda.org/articles/aa/pdf/2016/12/aa29586-16.pdf |access-date=5 January 2022}}</ref> Solar pores are also visible to the left of the penumbra.]]
Sunspots have two main structures: a central [[wikt:umbra|umbra]] and a surrounding [[wikt:penumbra|penumbra]]. The umbra is the darkest region of a sunspot and is where the magnetic field is strongest and approximately vertical, or [[Normal (geometry)|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.<ref name="schlichenmaier10">{{cite journal |last1=Schlichenmaier |first1=R. |last2=Rezaei |first2=R. |last3=Bello González |first3=N. |last4=Waldmann |first4=T. A. |title=The formation of a sunspot penumbra |journal=Astronomy and Astrophysics |date=March 2010 |volume=512 |pages=L1 |doi=10.1051/0004-6361/201014112|bibcode=2010A&A...512L...1S |doi-access=free }}</ref> The penumbra is composed of radially elongated structures known as penumbral filaments and has a more inclined magnetic field than the umbra.<ref>{{cite journal |last1=Mathew |first1=S. K. |last2=Lagg |first2=A. |last3=Solanki |first3=S. K. |last4=Collados |first4=M. |last5=Borrero |first5=J. M. |last6=Berdyugina |first6=S. |last7=Krupp |first7=N. |last8=Woch |first8=J. |last9=Frutiger |first9=C. |title=Three dimensional structure of a regular sunspot from the inversion of IR Stokes profiles |journal=Astronomy & Astrophysics |date=November 2003 |volume=410 |issue=2 |pages=695–710 |doi=10.1051/0004-6361:20031282|bibcode=2003A&A...410..695M |doi-access=free }}</ref> Within sunspot groups, multiple umbrae may be surrounded by a single, continuous penumbra.

The temperature of the umbra is roughly 3000–4500&nbsp;K, in contrast to the surrounding material at about 5780&nbsp;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.<ref>{{cite web|url=http://image.gsfc.nasa.gov/poetry/workbook/sunspot.html|title=Sunspots|publisher=NASA|date=1 April 1998|access-date=22 February 2013|archive-date=3 April 2013|archive-url=https://archive.today/20130403033824/http://image.gsfc.nasa.gov/poetry/workbook/sunspot.html|url-status=dead}}</ref>

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. [[Gas pressure]] in light bridges has also been found to dominate over [[magnetic pressure]], and convective motions have been detected.<ref name="felipe16" />

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

=== Lifecycle ===
[[File:Growing Sunspots Tracking Closeup - February 2011.ogv|thumb|left|The emergence and evolution of a sunspot group over a period of two weeks]]
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 region]]s tend to last weeks or months. Sunspots expand and contract as they move across the surface of the Sun, with diameters ranging from {{convert|16|km|mi|sigfig=1|sp=us|abbr=on}}<ref name="nascom"/> to {{convert|160000|km|mi|sigfig=1|sp=us|abbr=on}}.<ref name="howstuffworks"/>

==== 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 [[magnetic flux tube]]s in the Sun's [[convective zone]] projecting through the photosphere within active regions.<ref name="solanki03">{{cite journal |last1=Solanki |first1=Sami K. |title=Sunspots: An overview |journal=Astronomy and Astrophysics Review |date=1 April 2003 |volume=11 |issue=2–3 |pages=153–286 |doi=10.1007/s00159-003-0018-4|bibcode=2003A&ARv..11..153S |s2cid=120721248 }}</ref> Their characteristic darkening occurs due to this strong magnetic field inhibiting [[convection]] in the photosphere. As a result, the energy flux from the Sun's interior decreases, and with it, surface temperature, causing the surface area through which the magnetic field passes to look dark against the bright background of [[Granule (solar physics)|photospheric granules]].

Sunspots initially appear in the photosphere as small darkened spots lacking a penumbra. These structures are known as solar pores.<ref>{{cite journal |last1=Sobotka |first1=Michal |last2=Vazquez |first2=Manuel |last3=Bonet |first3=Jose Antonio |last4=Hanslmeier |first4=Arnold |last5=Hirzberger |first5=Johann |title=Temporal Evolution of Fine Structures in and around Solar Pores |journal=The Astrophysical Journal |date=20 January 1999 |volume=511 |issue=1 |pages=436–450 |doi=10.1086/306671 |bibcode=1999ApJ...511..436S |s2cid=121691780 |url=https://iopscience.iop.org/0004-637X/511/1/436/pdf/0004-637X_511_1_436.pdf |access-date=5 January 2022}}</ref> Over time, these pores increase in size and move towards one another. When a pore gets large enough, typically around {{convert|3500|km|mi|sigfig=1|sp=us|abbr=on}} in diameter, a penumbra will begin to form.<ref name="solanki03" />

==== 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 lies beneath each sunspot, forms a rotating [[vortex]] that sustains the concentrated magnetic field.<ref>{{cite news|title=SOHO reveals how sunspots take stranglehold on the Sun|url=http://www.spaceflightnow.com/news/n0111/06sunspots/|newspaper=SpaceFlight Now|date=6 November 2001|author=NASA News Release|access-date=9 March 2013|archive-date=17 January 2015|archive-url=https://web.archive.org/web/20150117214759/http://www.spaceflightnow.com/news/n0111/06sunspots/|url-status=dead}}</ref>

=== Solar cycle ===
{{main|Solar cycle}}
[[Image:Sunspot butterfly graph.gif|thumb|upright=1.35|[[Solar cycle#Sunspots|Butterfly diagram]] showing paired [[Spörer's law]] behavior|alt=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.]]
[[File:Sunspot Growth in June 2012.ogv|left|thumb|The full solar disk over the course of 13 days during the rise of [[solar cycle 24]]]]

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 phenomena|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.

[[Wolf number|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.<ref name="testtest1">{{cite book
| author=Tribble, A.  |date=2003
| title=The Space Environment, Implications for Spacecraft Design
| publisher=Princeton University Press | pages=15–18}}</ref>

[[George Ellery Hale]] first linked magnetic fields and sunspots in 1908.<ref name="doi10.1086/141602">{{Cite journal | last1 = Hale | first1 = G. E. | title = On the Probable Existence of a Magnetic Field in Sun-Spots | journal = The Astrophysical Journal | volume = 28 | pages = 315 | year = 1908 | doi = 10.1086/141602|bibcode = 1908ApJ....28..315H | doi-access = free }}</ref> Hale suggested that the sunspot cycle period is 22&nbsp;years, covering two periods of increased and decreased sunspot numbers, accompanied by 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.

=== Longer-period trends ===
{{main|Solar cycle#Cycle history}}

Sunspot numbers also change over long periods. For example, during the period known as the modern maximum from 1900 to 1958 the [[solar maxima]] trend of sunspot count was upwards; for the following 60 years the trend was mostly downwards.<ref>{{cite web |url=http://sidc.oma.be/html/wolfaml.html |title=Sunspot index graphics |publisher=Solar Influences Data Analysis Center |access-date=27 September 2007}}</ref> Overall, the Sun was last as active as the modern maximum over 8,000 years ago.<ref name="pmid15510145">{{cite journal
| display-authors=4
| author=Solanki SK
| author2=Usoskin IG
| author3=Kromer B
| author4=Schüssler M
| author5=Beer J |date=October 2004
| title=Unusual activity of the Sun during recent decades compared to the previous 11,000 years
| journal=Nature |volume=431 |issue=7012 |pages=1084–1087
| url=http://www.ncdc.noaa.gov/paleo/pubs/solanki2004/solanki2004.html
| doi=10.1038/nature02995 |pmid=15510145 |bibcode=2004Natur.431.1084S
| s2cid=4373732
}}</ref>

Sunspot number is correlated with the intensity of [[solar radiation]] over the period since 1979, when satellite measurements became available. The variation caused by the sunspot cycle to solar output is on the order of 0.1% of the solar constant (a peak-to-trough range of 1.3&nbsp;W·m<sup>−2</sup> compared with 1366&nbsp;W·m<sup>−2</sup> for the average solar constant).<ref name="IPCCtarWG1244">{{cite web
 |title       = Solar Forcing of Climate
 |work        = Climate Change 2001: Working Group I: The Scientific Basis
 |url         = http://www.grida.no/climate/ipcc_tar/wg1/244.htm
 |access-date  = 2005-03-10
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 |archive-url  = https://web.archive.org/web/20050315081547/http://www.grida.no/climate/ipcc_tar/wg1/244.htm
 |archive-date = 15 March 2005
}}</ref><ref name="AIPsolar">{{Cite web
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| title=The Discovery of Global Warming – Changing Sun, Changing Climate?
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| url=http://www.aip.org/history/climate/solar.htm
| publisher=[[American Institute of Physics]]
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{{multiple image
 |direction = horizontal
 |align= center
 |width1= 320
 |width2= 260
 |image1=Sunspot Numbers.png
 |image2=Sunspots 11000 years.svg
 |footer=400-year history of [[Wolf number|sunspot numbers]], showing [[Maunder Minimum|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
}}
[[File:Power spectrum of sunspot number, from 1945 to 2017.png|thumb|The daily sunspot number from 1945 to 2017, and its [[Spectral density|power spectrum]]. There are two prominent peaks corresponding to its 11-year cycle and its 27-day cycle due to solar rotation.<ref>{{Cite journal |last1=Le Mouël |first1=Jean-Louis |last2=Shnirman |first2=Mikhail G. |last3=Blanter |first3=Elena M. |date=2007-12-01 |title=The 27-Day Signal in Sunspot Number Series and the Solar Dynamo |url=https://doi.org/10.1007/s11207-007-9065-8 |journal=Solar Physics |language=en |volume=246 |issue=2 |pages=295–307 |doi=10.1007/s11207-007-9065-8 |bibcode=2007SoPh..246..295L |issn=1573-093X}}</ref>]]