Editing Solar cycle (section)

== Observational history ==
{{Main|Solar observation}}
{{multiple image
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 |image1    = SHSchwabe.jpg
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 |caption1  = [[Heinrich Schwabe|Samuel Heinrich Schwabe]] (1789–1875), German astronomer, discovered the solar cycle through extended observations of sunspots.
 |caption2  = [[Rudolf Wolf]] (1816–1893), Swiss astronomer, carried out historical reconstruction of solar activity back to the 17th century.
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The idea of a cyclical solar cycle was first hypothesized by [[Christian Horrebow]] based on his regular observations of [[sunspot]]s made between 1761 and 1776 from the [[Rundetaarn]] observatory in [[Copenhagen]], [[Denmark]]. In 1775, Horrebow noted how "it appears that after the course of a certain number of years, the appearance of the Sun repeats itself with respect to the number and size of the spots".<ref>{{cite journal |last1=Karoff |first1=Christoffer |last2=Jørgensen |first2=Carsten Sønderskov |last3=Senthamizh Pavai |first3=V. |last4=Arlt |first4=Rainer |date=2019-06-12 |title=Christian Horrebow's Sunspot Observations – II. Construction of a Record of Sunspot Positions |journal=[[Solar Physics (journal)|Solar Physics]] |volume=294 |issue=6 |page=77 |doi=10.1007/s11207-019-1466-y |arxiv=1906.10895 |bibcode=2019SoPh..294...78K |s2cid=189841594 }}</ref> The solar cycle however would not be clearly identified until 1843 when [[Samuel Heinrich Schwabe]] noticed a periodic variation in the average number of sunspots after 17 years of solar observations.<ref name="Schwabe">{{cite journal |last1=Schwabe |title=Sonnenbeobachtungen im Jahre 1843 |journal=Astronomische Nachrichten |date=1843 |volume=21 |pages=233–236 |url=https://books.google.com/books?id=iFc_AAAAcAAJ&pg=RA1-PA50 |trans-title=Observations of the sun in the year 1843 |language=de}} From page 235: "{{lang|de|Vergleicht man nun die Zahl der Gruppen und der flecken-freien Tage mit einander, so findet man, dass die Sonnenflecken eine Periode von ungefähr 10 Jahren hatten}}" ('If one compares the number of groups [of sunspots] and the sunspot-free days with one another, then one finds that the sunspots had a period of about 10 years')</ref> Schwabe continued to observe the sunspot cycle for another 23 years, until 1867. In 1852, [[Rudolf Wolf]] designated the first numbered solar cycle to have started in February 1755 based on Schwabe's and other observations.<ref>{{cite journal |last1=Wolf |first1=R. |title=Neue untersuchungen über die periode der sonnenflecken und ihre bedeutung |journal=Mittheilungen der Naturforschenden Gesellschaft in Bern |date=1852 |volume=255 |pages=249–270 |trans-title=New investigations regarding the period of sunspots and its significance |language=de }}</ref> Wolf also created a standard sunspot number index, the [[Wolf number]], which continues to be used today.

Between 1645 and 1715, very few sunspots were observed and recorded. This was first noted by [[Gustav Spörer]] and was later named the [[Maunder minimum]] after the wife-and-husband team [[Annie S. D. Maunder]] and [[Edward Walter Maunder]] who extensively researched this peculiar interval.<ref>{{cite journal |author-link=John A. Eddy |last=Eddy |first=John A. |title=The Maunder Minimum |journal=[[Science (journal)|Science]] |volume=192 |issue=4245 |pages=1189–1202 |date=June 1976 |pmid=17771739 |doi=10.1126/science.192.4245.1189 |jstor=1742583 |bibcode=1976Sci...192.1189E |s2cid=33896851 }}</ref>

In the second half of the nineteenth century [[Richard Christopher Carrington|Richard Carrington]] and Spörer independently noted the phenomena of sunspots appearing at different [[heliographic latitude]]s at different parts of the cycle. (See [[Spörer's law]].) [[Alfred Harrison Joy]] would later describe how the magnitude at which the sunspots are "tilted"—with the leading spot(s) closer to the equator than the trailing spot(s)―grows with the latitude of these regions. (See [[Joy's law (astronomy)|Joy's law]].)

The cycle's physical basis was elucidated by [[George Ellery Hale]] and collaborators, who in 1908 showed that sunspots were strongly magnetized (the first detection of magnetic fields beyond the Earth). In 1919 they identified a number of patterns that would collectively become known as [[Hale's law]]:
* In the same heliographic hemisphere, bipolar [[active region]]s tend to have the same leading polarity.
* In the opposite hemisphere (that is, on the other side of the solar equator) these regions tend to have the opposite leading polarity.
* Leading polarities in both hemispheres flip from one sunspot cycle to the next.
Hale's observations revealed that the complete magnetic cycle—which would later be referred to as a Hale cycle—spans two solar cycles, or 22 years, before returning to its original state (including polarity). Because nearly all manifestations are insensitive to polarity, the 11-year solar cycle remains the focus of research; however, the two halves of the Hale cycle are typically not identical: the 11-year cycles usually alternate between higher and lower sums of Wolf's sunspot numbers (the [[Gnevyshev-Ohl rule]]).<ref name="hathaway_review" />

In 1961 the father-and-son team of [[Harold D. Babcock|Harold]] and [[Horace Babcock]] established that the solar cycle is a spatiotemporal magnetic process unfolding over the Sun as a whole. They observed that the solar surface is magnetized outside of sunspots, that this (weaker) magnetic field is to first order a [[dipole]], and that this dipole undergoes polarity reversals with the same period as the sunspot cycle. Horace's [[Babcock Model]] described the Sun's oscillatory magnetic field as having a quasi-steady periodicity of 22 years.<ref name="Schwabe" /><ref name="oma.be">{{cite web|url=http://sidc.oma.be/sunspot-index-graphics/wolfjmms.php |title=Sunspot Number graphics|work=Solar Influences Data Analysis Center |publisher=[[Royal Observatory of Belgium]] }}</ref> It covered the oscillatory exchange of energy between [[toroidal and poloidal]] solar magnetic field components.