Sunspot#17th and 18th centuries

{{Broader|Solar observation}}

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.{{cite journal |doi=10.1016/0146-6364(80)90034-1 |title=The hexagram "Feng" in "the book of changes" as the earliest written record of sunspot |journal=Chinese Astronomy |volume=4 |issue=4 |pages=406 |year=1980 |last1=Xu Zhen-Tao |bibcode=1980ChA.....4..406X }} 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.{{Cite web |title=Early Astronomy and the Beginnings of a Mathematical Science |date=2007 |work=NRICH (University of Cambridge) |url=http://nrich.maths.org/6843 |access-date=2010-07-14}} By 28 BC, Chinese astronomers were regularly recording sunspot observations in official imperial records.{{cite journal |title=The Observation of Sunspots |date=1988 |journal=UNESCO Courier |url=http://findarticles.com/p/articles/mi_m1310/is_1988_Oct/ai_6955852/ |access-date=2010-07-14 |url-status=dead |archive-url=https://web.archive.org/web/20110702095337/http://findarticles.com/p/articles/mi_m1310/is_1988_Oct/ai_6955852/ |archive-date=2011-07-02}}

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."[http://adsabs.harvard.edu/full/2007JBAA..117..346V Letter to the Editor: Sunspot observations by Theophrastus revisited]", and see Theophrastus' Fragment VI, De Signis Tempestatum, 11.4–5.

The earliest known drawings of sunspots were made by English monk John of Worcester in December 1128.{{cite journal |last1=Stephenson |first1=F. R. |last2=Willis |first2=D. M. |title=The earliest drawing of sunspots |journal=Astronomy & Geophysics |volume=40 |issue=6 |year=1999 |pages=6.21–6.22 |issn=1366-8781 |doi=10.1093/astrog/40.6.6.21|bibcode=1999A&G....40f..21S |doi-access=free }}Stefan Hughes, Catchers of the Light: The Forgotten Lives of the Men and Women Who First Photographed the Heavens, ArtDeCiel Publishing, 2012 p. 317

Sunspots were first observed telescopically in December 1610 by English astronomer Thomas Harriot.{{cite journal |last1=Vokhmyanin |first1=M. |last2=VArlt |first2=R. |last3=Zolotova |first3=N. |title=Sunspot Positions and Areas from Observations by Thomas Harriot |journal=Solar Physics |date=10 March 2020 |volume=295 |issue=3 |pages=39.1–39.11 |doi=10.1007/s11207-020-01604-4 |bibcode=2020SoPh..295...39V |s2cid=216259048 |url=https://doi.org/10.1007/s11207-020-01604-4|url-access=subscription }} His observations were recorded in his notebooks and were followed in March 1611 by observations and reports by Frisian astronomers Johannes and David Fabricius.{{cite web | title=Great Moments in the History of Solar Physics 1| work =Great Moments in the History of Solar Physics | url=http://web.hao.ucar.edu/public/education/sp/great_moments.html| access-date = 2006-03-19 |archive-url = https://web.archive.org/web/20060301083022/http://web.hao.ucar.edu/public/education/sp/great_moments.html |archive-date = 1 March 2006}}{{cite book |last1=Vaquero |first1=J.M. |last2=Vázquez |first2=M |title=The Sun Recorded Through History: Scientific Data Extracted from Historical Documents vol. 361 of the series Astrophysics and Space Science Library |doi=10.1007/978-0-387-92790-9 |date=2009 |volume=361 |publisher=Springer, New York |isbn=978-0-387-92789-3 }} After Johannes Fabricius' death at the age of 29, his reports remained obscure and were overshadowed by the independent discoveries of and publications about sunspots by Christoph Scheiner and Galileo Galilei.{{cite book |last1=Carlowicz |first1=Michael J. |last2=López |first2=Ramón |title=Storms from the Sun: The Emerging Science of Space Weather |date=2002 |publisher=Joseph Henry Press |pages=1–382 |isbn=978-0309076425 |url=https://books.google.com/books?id=RJO_IsMDiccC&pg=PP66 |access-date=19 June 2020}} Galileo likely began telescopic sunspot observations around the same time as Harriot; however, Galileo's records did not start until 1612.{{cite journal |last1=Vokhmyanin |first1=M. |last2=Zolotova |first2=N. |title=Sunspot Positions and Areas from Observations by Galileo Galilei |journal=Solar Physics |date=5 February 2018 |volume=293 |issue=2 |pages=31.1–31.21 |doi=10.1007/s11207-018-1245-1 |bibcode=2018SoPh..293...31V |s2cid=126329839 |url=https://doi.org/10.1007/s11207-018-1245-1|url-access=subscription }} During the next decades numerous astronomers of that era participated in the pursuit of sunspots. One of these was the famous astronomer Johannes Hevelius who recorded 19 sunspot groups during the period of the early Maunder Minimum (1653-1679) in the book Machina Coelestis.{{Cite journal |last1=Hoyt |first1=Douglas V. |last2=Schatten |first2=Kenneth H. |date=1995-09-01 |title=Overlooked sunspot observations by Hevelius in the early Maunder Minimum, 1653–1684 |url=https://link.springer.com/article/10.1007/BF00732815 |journal=Solar Physics |language=en |volume=160 |issue=2 |pages=371–378 |doi=10.1007/BF00732815 |bibcode=1995SoPh..160..371H |issn=1573-093X|url-access=subscription }}

In the early 19th Century, William Herschel was one of the first to hypothesize a connection of sunspots with temperatures on Earth and believed that certain features of sunspots would indicate increased heating on Earth.{{cite journal |last1=Herschel |first1=William |title=XIII. Observations tending to investigate the nature of the sun, in order to find the causes or symptoms of its variable emission of light and heat; with remarks on the use that may possibly be drawn from solar observations |journal=Philosophical Transactions of the Royal Society of London |date=31 December 1801 |volume=91 |pages=265–318 |doi=10.1098/rstl.1801.0015 |s2cid=122986696 |doi-access=free }} During his recognition of solar behavior and hypothesized solar structure, he inadvertently picked up the relative absence of sunspots from July 1795 to January 1800 and was perhaps the first to construct a past record of observed or missing sunspots. From this he found that the absence of sunspots coincided with high wheat prices in England. The president of the Royal Society commented that the upward trend in wheat prices was due to monetary inflation.Soon, W., and Yaskell, S.H., The Maunder Minimum and the Variable Sun-earth Connection (World Scientific Press: 2003) pp. 87–88 Years later scientists such as Richard Carrington in 1865 and John Henry Poynting in 1884 tried and failed to find a connection between wheat prices and sunspots, and modern analysis finds that there is no statistically significant correlation between wheat prices and sunspot numbers.{{cite journal |last1=Love |first1=Jeffrey J. |title=On the insignificance of Herschel's sunspot correlation |journal=Geophysical Research Letters |date=27 August 2013 |volume=40 |issue=16 |pages=4171–4176 |doi=10.1002/grl.50846 |bibcode=2013GeoRL..40.4171L |s2cid=1654166 }}

File:Sunspot from Hinode in G band (4305) 2007-04-30 T001456.gif

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.{{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 }} The penumbra is composed of radially elongated structures known as penumbral filaments and has a more inclined magnetic field than the umbra.{{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 }} 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.{{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}}

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.

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

File:Growing Sunspots Tracking Closeup - February 2011.ogv

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 {{convert|16|km|mi|sigfig=1|sp=us|abbr=on}} to {{convert|160000|km|mi|sigfig=1|sp=us|abbr=on}}.

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 tubes in the Sun's convective zone projecting through the photosphere within active regions.{{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 }} 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 photospheric granules.

Sunspots initially appear in the photosphere as small darkened spots lacking a penumbra. These structures are known as solar pores.{{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}} 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.

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.{{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}}

{{main|Solar cycle}}

Image:Sunspot butterfly graph.gif 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]]

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.{{cite book

| author=Tribble, A. |date=2003

| title=The Space Environment, Implications for Spacecraft Design

| publisher=Princeton University Press | pages=15–18}}

George Ellery Hale first linked magnetic fields and sunspots in 1908.{{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 }} Hale suggested that the sunspot cycle period is 22 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.

{{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.{{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}} Overall, the Sun was last as active as the modern maximum over 8,000 years ago.{{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

| url-access=subscription

}}

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 W·m⁻² compared with 1366 W·m⁻² for the average solar constant).{{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

|url-status = dead

|archive-url = https://web.archive.org/web/20050315081547/http://www.grida.no/climate/ipcc_tar/wg1/244.htm

|archive-date = 15 March 2005

}}{{Cite web

| first=Spencer

| last=Weart

| title=The Discovery of Global Warming – Changing Sun, Changing Climate?

| editor-first=Spencer

| editor-last=Weart

| url=http://www.aip.org/history/climate/solar.htm

| publisher=American Institute of Physics

| date=2006

| access-date=2007-04-14

| archive-date=17 June 2006

| archive-url=https://web.archive.org/web/20060617030502/http://www.aip.org/history/climate/solar.htm

| url-status=dead

}}

{{multiple image

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|image1=Sunspot Numbers.png

|image2=Sunspots 11000 years.svg

|footer=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

}}

File:Power spectrum of sunspot number, from 1945 to 2017.png. There are two prominent peaks corresponding to its 11-year cycle and its 27-day cycle due to solar rotation.{{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|url-access=subscription }}]]

{{Main|Solar observatory}}

File:Swedish Solar Telescope.jpg at Roque de los Muchachos Observatory on La Palma in the Canary Islands|alt=Photo of six-story building with fenced balcony containing large telescope]]

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 human vision, amateur observation of sunspots is generally conducted 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 and 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.

Due to their correlation with other kinds of solar activity, sunspots can be used to help predict space weather, the state of the ionosphere, and conditions relevant to short-wave radio propagation or satellite communications. High sunspot activity is celebrated by members of the amateur radio community as a harbinger of excellent ionospheric propagation conditions that greatly increase radio range in the HF bands. During peaks in sunspot activity, worldwide radio communication can be achieved on frequencies as high as the 6-meter VHF band.{{cite web|url=https://hamradioschool.com/sunspots-and-propagation/|title=Sunspots and Propagation|author=Stu Turner|publisher=Ham Radio School.com|access-date=5 January 2020|archive-url=https://web.archive.org/web/20170626024320/http://www.hamradioschool.com/sunspots-and-propagation/|archive-date=26 June 2017|url-status=dead}}

Solar activity (and the solar cycle) have been implicated as a factor in global warming. The first possible example of this is the Maunder Minimum period of low sunspot activity which occurred during the Little Ice Age in Europe.{{cite journal |author=Eddy J.A. |title=The Maunder Minimum |journal=Science |volume=192 |issue=4245 |pages=1189–1202 |date=June 1976 |pmid=17771739 |doi=10.1126/science.192.4245.1189 |bibcode=1976Sci...192.1189E |s2cid=33896851 }} [http://bill.srnr.arizona.edu/classes/182h/Climate/Solar/Maunder%20Minimum.pdf PDF Copy] {{webarchive|url=https://web.archive.org/web/20100216030712/http://bill.srnr.arizona.edu/classes/182h/Climate/Solar/Maunder%20Minimum.pdf |date=16 February 2010 }} However, detailed studies from multiple paleoclimate indicators show that the lower northern hemisphere temperatures in the Little Ice Age began while sunspot numbers were still high before the start of the Maunder Minimum, and persisted until after the Maunder Minimum had ceased. Numerical climate modelling indicates that volcanic activity was the main driver of the Little Ice Age.{{Cite journal |last1=Owens |first1=M.J. |display-authors=et al. |date=October 2017 |title= The Maunder Minimum and the Little Ice Age: An update from recent reconstructions and climate simulations |url=https://centaur.reading.ac.uk/74244/ |journal=J. Space Weather and Space Climate |language=en |volume=7 |pages=A25 |issn=2115-7251 |doi=10.1051/swsc/2017019 |s2cid=37433045 |doi-access=free |arxiv=1708.04904 }}

Sunspots themselves, in terms of the magnitude of their radiant-energy deficit, have a weak effect on solar flux.{{Cite journal

| author=Hudson H |date=2008

| title=Solar activity |journal=Scholarpedia

|volume=3

|issue=3

|page=3967

|doi=10.4249/scholarpedia.3967

|bibcode=2008SchpJ...3.3967H

|doi-access=free

}} 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 $1.37\backslash \; \backslash mathrm\{W\backslash cdot\; m^\{-2\}\}$. Other magnetic phenomena which correlate with sunspot activity include faculae and the chromospheric network.{{Cite journal

| title=Observations of solar irradiance variability |date=1981

| bibcode=1981Sci...211..700W|doi = 10.1126/science.211.4483.700| pmid=17776650| volume=211|issue=4483

| journal=Science| pages=700–702|last1=Willson

|first1=R. C.

|last2=Gulkis

|first2=S.

|last3=Janssen

|first3=M.

|last4=Hudson

|first4=H. S.

|last5=Chapman

|first5=G. A.

}}

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.{{Cite journal |last1=Kopp |first1=G. |date=April 2014|title=An assessment of the solar irradiance record for climate studies|url=https://www.swsc-journal.org/articles/swsc/abs/2014/01/swsc130036/swsc130036.html |journal=J. Space Weather and Space Climate |language=en |volume=4|pages=A14 |doi=10.1051/swsc/2014012|bibcode=2014JSWSC...4A..14K |doi-access=free }}{{Cite journal |last1=Kopp |first1=G. |date=July 2016|title=Magnitudes and timescales of total solar irradiance variability |journal=J. Space Weather and Space Climate |language=en |volume=6 |pages=A30 |doi=10.1051/swsc/2016025|arxiv=1606.05258 |bibcode=2016JSWSC...6A..30K |s2cid=55902879 }} 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 $\backslash pm0.5\backslash \; \backslash mathrm\{W\backslash cdot\; m^\{-2\}\}$ with a $2\backslash sigma$ uncertainty range of $\backslash pm1\backslash \; \backslash mathrm\{W\backslash cdot\; m^\{-2\}\}$.{{Cite journal |last1=Lockwood |first1=M. |last2= Ball |first2=W. |date=May 2020 |title=Placing limits on long-term variations in quiet-Sun irradiance and their contribution to total solar irradiance and solar radiative forcing of climate |url=https://centaur.reading.ac.uk/90804/ |journal=Proceedings of the Royal Society A|language=en |volume=476 |issue=2238 |pages=20200077 |issn=1364-5021 |doi=10.1098/rspa.2020.0077|pmid=32831591 |pmc=7428030 |bibcode=2020RSPSA.47600077L }}

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{{cite journal | vauthors=((Jess, D. B., Jafarzadeh, S., Keys, P. H., Stangalini, M., Verth, G., Grant, S. D. T.)) | title=Waves in the lower solar atmosphere: the dawn of next-generation solar telescopes | volume=20 | issue=1 | publisher=Springer Nature | date=19 January 2023 |journal=Living Reviews in Solar Physics | url=https://link.springer.com/article/10.1007/s41116-022-00035-6 | doi=10.1007/s41116-022-00035-6| arxiv=2212.09788 | bibcode=2023LRSP...20....1J }} and magnetic reconnection events.{{cite journal | vauthors=((Shibata, K., Magara, T.)) | title=Solar Flares: Magnetohydrodynamic Processes | volume=8 | issue=1 | pages=6 |journal=Living Reviews in Solar Physics | publisher=Springer International Publishing | date=15 December 2011 | doi=10.12942/lrsp-2011-6| doi-access=free | bibcode=2011LRSP....8....6S | hdl=2433/153022 | hdl-access=free }}

{{main|Starspot}}

In 1947, G. E. Kron proposed that starspots were the reason for periodic changes in brightness on red dwarfs. 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 Trianguli (HD 12545) with a temperature of {{convert|3500|K|C}}, together with a warm spot of {{convert|4800|K|C}}.{{cite web |url=http://www.aip.de/groups/activity/pics/DI/12545s4e.gif |title=Derived images showing rotation of cool and warm starspots |publisher=Leibniz Institute for Astrophysics |access-date=2013-01-14 |archive-url=https://web.archive.org/web/20100529200045/http://www.aip.de/groups/activity/pics/DI/12545s4e.gif |archive-date=29 May 2010 |url-status=dead }}

{{Clear}}

{{Div col|colwidth=30em}}

• Hale's law – empirical law for the orientation of magnetic fields in solar active regions
• Joy's law – empirical law for the distribution of sunspots in solar active regions
• Letters on Sunspots
• List of solar cycles
• Radio propagation
• Solar cycle
• Solar rotation
• Space weather
• Spörer's law {{small|(predictive)}}
• Starspot
• Wolf number {{small|sunspot number}}

{{Div col end}}

{{Reflist}}

{{Refbegin}}

• {{cite journal |author=Carl Luetzelschwab, K9LA |title=The new sunspot numbers |journal=QST |date=October 2016 |volume=100 |issue=10 |issn=0033-4812 |pages=38–41}}

{{Refend}}

{{Commons category|Sunspots}}

• [https://web.archive.org/web/20150521142828/http://fenyi.solarobs.unideb.hu/ Sunspot Database based on Terrestrial (GPR/DPD) and Satellite (SOHO/SDO) observations from 1872 to Nowadays with the newest data.] (File:Flag of Hungary.svg)
• [https://web.archive.org/web/20100820163335/http://solarcycle24.com/ Solar Cycle 24 and VHF Aurora Website (www.solarcycle24.com)]
• [http://sidc.oma.be/index.php3 Belgium World Data Center for the sunspot index] {{Webarchive|url=https://web.archive.org/web/20170803104838/http://sidc.oma.be/index.php3 |date=3 August 2017 }}
• [https://web.archive.org/web/20051212113926/http://www.universetoday.com/am/publish/sharpest_image_sunspot.html?6102005 High resolution sunspot image]
• [https://web.archive.org/web/20100116124450/http://www.perseus.gr/Astro-Solar-Sunspots.htm Sunspot images in high-res] Impressive collection of sunspot images
• [https://archive.today/20120604110852/http://www.swpc.noaa.gov/SolarCycle/ NOAA Solar Cycle Progression]: Current solar cycle.
• Current conditions: Space weather
• [http://www.lmsal.com Lockheed Martin Solar and Astrophysics Lab]
• [https://web.archive.org/web/20181001081332/http://www.suntrek.org/ 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
• [http://www.n0hr.com/Propfire.htm Propfire – displays current sunspot number in browser status bar]
• [http://www.n0hr.com/Ham_Radio_Toolbar.htm HamLinks Toolbar – displays solar flux, A Index and K Index data in a toolbar]
• [http://antwrp.gsfc.nasa.gov/apod/image/0211/sunspot_swedish_c1.jpg The Sharpest View of the Sun ]
• [http://www.spaceweather.com/ Daily Sunspot Update and Picture of the Sun (www.spaceweather.com)]
• [http://alienworlds.southwales.ac.uk/sunStructure.html#/photosphere Animated explanation of Sunspots in the Photosphere] {{Webarchive|url=https://web.archive.org/web/20151116133527/http://alienworlds.southwales.ac.uk/sunStructure.html#/photosphere |date=16 November 2015 }} (University of South Wales)

• {{cite web | title=11,000 Year Sunspot Number Reconstruction | work=Global Change Master Directory | url=http://gcmd.nasa.gov/KeywordSearch/Metadata.do?Portal=GCMD&KeywordPath=%5BParameters%3ACategory%3D%27EARTH+SCIENCE%27%2CTopic%3D%27SUN-EARTH+INTERACTIONS%27%2CTerm%3D%27SOLAR+ACTIVITY%27%2CVariable%3D%27SUNSPOTS%27%5D&OrigMetadataNode=GCMD&EntryId=NOAA_NCDC_PALEO_2005-015&MetadataView=Brief&MetadataType=0&lbnode=gcmd3b | access-date=11 March 2005 | archive-date=2 November 2015 | archive-url=https://web.archive.org/web/20151102112535/http://gcmd.nasa.gov/KeywordSearch/Metadata.do?Portal=GCMD&KeywordPath=%5BParameters%3ACategory%3D%27EARTH+SCIENCE%27%2CTopic%3D%27SUN-EARTH+INTERACTIONS%27%2CTerm%3D%27SOLAR+ACTIVITY%27%2CVariable%3D%27SUNSPOTS%27%5D&OrigMetadataNode=GCMD&EntryId=NOAA_NCDC_PALEO_2005-015&MetadataView=Brief&MetadataType=0&lbnode=gcmd3b | url-status=dead }}
• {{cite web | title= Unusual activity of the Sun during recent decades compared with the previous 11,000 years | work=WDC for Paleoclimatology | url=http://www.ncdc.noaa.gov/paleo/pubs/solanki2004/ | access-date=11 March 2005}}
• {{cite web | title=Sunspot Numbers from Ancient Times to Present from NOAA/NGDC | work=Global Change Master Directory | url=http://gcmd.nasa.gov/KeywordSearch/Metadata.do?Portal=GCMD&KeywordPath=%5BParameters%3ACategory%3D%27EARTH+SCIENCE%27%2CTopic%3D%27SUN-EARTH+INTERACTIONS%27%2CTerm%3D%27SOLAR+ACTIVITY%27%2CVariable%3D%27SUNSPOTS%27%5D&OrigMetadataNode=GCMD&EntryId=FE00140&MetadataView=Brief&MetadataType=0&lbnode=gcmd3b | access-date=11 March 2005 | archive-date=14 September 2015 | archive-url=https://web.archive.org/web/20150914054517/http://gcmd.nasa.gov/KeywordSearch/Metadata.do?Portal=GCMD&KeywordPath=%5BParameters%3ACategory%3D%27EARTH+SCIENCE%27%2CTopic%3D%27SUN-EARTH+INTERACTIONS%27%2CTerm%3D%27SOLAR+ACTIVITY%27%2CVariable%3D%27SUNSPOTS%27%5D&OrigMetadataNode=GCMD&EntryId=FE00140&MetadataView=Brief&MetadataType=0&lbnode=gcmd3b | url-status=dead }}
• {{cite web | title=Sunspot Numbers | work=NOAA NGDC Solar Data Services | url=ftp://ftp.ngdc.noaa.gov/STP/SOLAR_DATA/SUNSPOT_NUMBERS/ | access-date=21 June 2010 }}{{dead link|date=January 2018 |bot=InternetArchiveBot |fix-attempted=yes }}
• International Sunspot Number{{snd}}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
• {{cite book | last1=Wilson|first1=Robert M.|title=Comparison of the Variations of Sunspot Number, Number of Sunspot Groups, and Sunspot Area, 1875–2013|publisher=National Aeronautics and Space Administration, Marshall Space Flight Center|location=Huntsville, AL| date=April 2014| url=https://purl.fdlp.gov/GPO/gpo54398|access-date=13 March 2015}}

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