Editing Solar cycle (section)

==== Solar irradiance ====
{{Main|Solar irradiance}}The total solar irradiance (TSI) is the amount of solar radiative energy incident on the Earth's upper atmosphere. TSI variations were undetectable until satellite observations began in late 1978. A series of [[radiometers]] were launched on [[satellites]] since the 1970s.<ref>{{cite journal | title=Magnitudes and timescales of total solar irradiance variability |author=Kopp G | journal=Journal of Space Weather and Space Climate | date=2016-07-01 |doi=10.1051/swsc/2016025 | volume=6 | pages=A30|arxiv=1606.05258 |bibcode = 2016JSWSC...6A..30K| doi-access=free }}</ref> TSI measurements varied from 1355 to 1375 W/m<sup>2</sup> across more than ten satellites. One of the satellites, the [[ACRIMSAT]] was launched by the ACRIM group. The controversial 1989–1991 "ACRIM gap" between non-overlapping ACRIM satellites was interpolated by the ACRIM group into a composite showing +0.037%/decade rise. Another series based on the ACRIM data is produced by the PMOD group and shows a −0.008%/decade downward trend.<ref>{{cite journal | title=ACRIM3 and the Total Solar Irradiance database |author=Richard C. Willson | journal=Astrophysics and Space Science | date=2014-05-16 |doi=10.1007/s10509-014-1961-4 | volume=352 |issue=2 | pages=341–352|bibcode = 2014Ap&SS.352..341W | doi-access=free }}</ref> This 0.045%/decade difference can impact climate models. However, reconstructed total solar irradiance with models favor the PMOD series, thus reconciling the ACRIM-gap issue.<ref>{{cite journal | title=ACRIM-gap and total solar irradiance revisited: Is there a secular trend between 1986 and 1996? |vauthors=Krivova NA, Solanki SK, Wenzler T | journal=Geophysical Research Letters | date=2009-10-01 |doi=10.1029/2009GL040707 | volume=36 |issue=20 | pages=L20101|arxiv=0911.3817 |bibcode = 2009GeoRL..3620101K | doi-access=free }}</ref><ref>{{Cite journal |last1=Amdur |first1=T. |last2=Huybers |first2=P. |date=2023-08-16 |title=A Bayesian Model for Inferring Total Solar Irradiance From Proxies and Direct Observations: Application to the ACRIM Gap |url=https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2023JD038941 |journal=Journal of Geophysical Research: Atmospheres |language=en |volume=128 |issue=15 |doi=10.1029/2023JD038941 |bibcode=2023JGRD..12838941A |s2cid=260264050 |issn=2169-897X}}</ref><ref>{{Cite journal |last=Chatzistergos |first=Theodosios |last2=Krivova |first2=Natalie A. |last3=Solanki |first3=Sami K. |last4=Leng Yeo |first4=Kok |date=2025 |title=Revisiting the SATIRE-S irradiance reconstruction: Heritage of Mt Wilson magnetograms and Ca II K observations |url=https://www.aanda.org/10.1051/0004-6361/202554044 |journal=Astronomy & Astrophysics |volume=696 |pages=A204 |doi=10.1051/0004-6361/202554044 |issn=0004-6361|doi-access=free }}</ref>

Solar irradiance varies systematically over the cycle,<ref>{{cite journal | last1 = Willson | first1 = R.C. | display-authors = etal   | date = 1981 | title = Observations of Solar Irradiance Variability | journal = Science | volume = 211 | issue = 4483| pages = 700–2 |doi= 10.1126/science.211.4483.700 | pmid=17776650|bibcode = 1981Sci...211..700W }}</ref> both in total irradiance and in its relative components (UV vs visible and other frequencies). The [[solar luminosity]] is an estimated 0.07 percent brighter during the mid-cycle solar maximum than the terminal solar minimum. [[Photosphere|Photospheric]] magnetism appears to be the primary cause (96%) of 1996–2013 TSI variation.<ref>{{cite journal | title=Reconstruction of total and spectral solar irradiance from 1974 to 2013 based on KPVT, SoHO/MDI and SDO/HMI observations | author= K.L. Yeo | display-authors= etal | journal= Astronomy & Astrophysics | date=2014-09-23 | doi=10.1051/0004-6361/201423628 | bibcode=2014A&A...570A..85Y | volume=570 | pages=A85|arxiv = 1408.1229 | s2cid= 56424234 }}</ref> The ratio of ultraviolet to visible light varies.<ref name="InvertedForcingpaper">{{cite journal |journal=Nature |volume=467 |issue=7316 |title=An influence of solar spectral variations on radiative forcing of climate |date=October 6, 2010|doi=10.1038/nature09426 |pmid=20930841 |pages=696–9|bibcode = 2010Natur.467..696H |last1=Haigh |first1=J. D |last2=Winning |first2=A. R |last3=Toumi |first3=R |last4=Harder |first4=J. W |hdl=10044/1/18858 |s2cid=4320984 |url=http://spiral.imperial.ac.uk/bitstream/10044/1/18858/2/Nature_467_7316_2010.pdf |hdl-access=free }}</ref>

TSI varies in phase with the solar magnetic activity cycle<ref>{{cite journal |author=Willson RC|author2=Hudson HS |title=The Sun's luminosity over a complete solar cycle |journal=Nature |volume=351 |issue=6321 |pages=42–4 |date=1991 |doi= 10.1038/351042a0|bibcode=1991Natur.351...42W |s2cid=4273483 }}</ref> with an amplitude of about 0.1% around an average value of about 1361.5 W/m<sup>2</sup><ref>{{cite journal | doi = 10.1007/s10509-014-1961-4 | bibcode=2014Ap&SS.352..341W | volume=352 | title=ACRIM3 and the Total Solar Irradiance database | year=2014 | journal=Astrophysics and Space Science | pages=341–352 | last1 = Willson | first1 = Richard C.| issue=2 | doi-access=free }}</ref> (the "[[solar constant]]"). Variations about the average of up to −0.3% are caused by large sunspot groups and of +0.05% by large faculae and the bright network on a 7-10-day timescale<ref>{{cite journal |author=Willson R.C.|author2=Gulkis S.|author3=Janssen M. |author4=Hudson H.S.|author5=Chapman G.A. |title=Observations of solar irradiance variability |journal=Science |volume=211 |issue=4483 |pages=700–2 |date=1981 |doi=10.1126/science.211.4483.700 |pmid=17776650|bibcode = 1981Sci...211..700W }}</ref><ref name="ACRIM-graphic">{{Cite web | publisher = ACRIM project web page | url = http://acrim.com/Acrim1%20Results.htm | title = Total Solar Irradiance Graph from ACRIM page |archive-url=https://web.archive.org/web/20151017073029/http://acrim.com/Acrim1%20Results.htm |archive-date=2015-10-17 | access-date = 2015-11-17}}</ref> Satellite-era TSI variations show small but detectable trends.<ref>{{cite journal |author=Willson R.C.|author2=Mordvinov A.V. |title=Secular total solar irradiance trend during solar cycles 21–23 |journal=Geophys. Res. Lett. |volume=30 |issue=5 |page=1199 |date=2003 |doi=10.1029/2002GL016038 |bibcode=2003GeoRL..30.1199W|s2cid=55755495 |doi-access=free }}</ref><ref>{{cite journal |author=Scafetta N. |author2=Willson R.C. |title=ACRIM-gap and TSI trend issue resolved using a surface magnetic flux TSI proxy model |journal=Geophys. Res. Lett. |volume=36 |issue= 5|pages=L05701 |date=2009 |doi=10.1029/2008GL036307 |bibcode=2009GeoRL..36.5701S |s2cid=7160875 |doi-access=free }}</ref>

TSI is higher at solar maximum, even though sunspots are darker (cooler) than the average photosphere. This is caused by magnetized structures other than sunspots during solar maxima, such as faculae and active elements of the "bright" network, that are brighter (hotter) than the average photosphere. They collectively overcompensate for the irradiance deficit associated with the cooler, but less numerous sunspots.<ref>{{cite journal |vauthors=Chatzistergos T, Krivova NA, Ermolli I, Kok Leng Y, Mandal S, Solanki SK, Kopp G, Malherbe JM |title=Reconstructing solar irradiance from historical Ca II K observations. I. Method and its validation |journal=Astronomy and Astrophysics |volume=656 |pages=A104 |date=2021-12-01 |doi=10.1051/0004-6361/202141516 |arxiv=2109.05844 |bibcode=2021A&A...656A.104C|doi-access=free }}</ref> The primary driver of TSI changes on solar rotational and solar cycle timescales is the varying photospheric coverage of these radiatively active solar magnetic structures.<ref>{{cite journal |vauthors=Solanki SK, Schuessler M, Fligge M |title=Secular variation of the Sun's magnetic flux |journal=Astronomy and Astrophysics |volume=383 |pages=706–712|date=2002-02-01 |issue=2 |doi=10.1051/0004-6361:20011790  |bibcode=2002A&A...383..706S |doi-access=free }}</ref>

Energy changes in UV irradiance involved in production and loss of [[ozone]] have atmospheric effects. The 30 [[hPa]] [[atmospheric pressure]] level changed height in phase with solar activity during solar cycles 20–23. UV irradiance increase caused higher ozone production, leading to stratospheric heating and to poleward displacements in the [[Stratosphere|stratospheric]] and [[Troposphere|tropospheric]] wind systems.<ref>{{cite journal|title = The Impact of Solar Variability on Climate|last = Haigh|first = J D|journal = Science|date = May 17, 1996|volume = 272|pages = 981–984|doi = 10.1126/science.272.5264.981|pmid = 8662582|issue = 5264|bibcode = 1996Sci...272..981H |s2cid = 140647147}}</ref>
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