Editing Solar cycle (section)

=== Atmospheric ===

==== 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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==== Short-wavelength radiation ====
[[File:The Solar Cycle XRay hi.jpg|thumb|left|upright=1.35|A solar cycle: a montage of ten years' worth of [[Yohkoh]] SXT images, demonstrating the variation in solar activity during a solar cycle, from after August 30, 1991, to September 6, 2001. Credit: the Yohkoh mission of [[Institute of Space and Astronautical Science|ISAS]] (Japan) and [[NASA]] (US).]]
With a temperature of 5870 K, the [[photosphere]] emits a proportion of radiation in the [[extreme ultraviolet]] (EUV) and above. However, hotter upper layers of the Sun's atmosphere ([[chromosphere]] and [[solar corona|corona]]) emit more short-wavelength radiation. Since the upper atmosphere is not homogeneous and contains significant magnetic structure, the solar ultraviolet (UV), [[Extreme ultraviolet|EUV]] and X-ray flux varies markedly over the cycle.

The photo montage to the left illustrates this variation for soft [[X-ray]], as observed by the Japanese satellite [[Yohkoh]] from after August 30, 1991, at the peak of cycle 22, to September 6, 2001, at the peak of cycle 23. Similar cycle-related variations are observed in the flux of solar UV or EUV radiation, as observed, for example, by the [[Solar and Heliospheric Observatory|SOHO]] or [[TRACE]] satellites.

Even though it only accounts for a minuscule fraction of total solar radiation, the impact of solar UV, EUV and X-ray radiation on the Earth's upper atmosphere is profound. Solar UV flux is a major driver of [[Stratosphere|stratospheric chemistry]], and increases in ionizing radiation significantly affect [[ionosphere]]-influenced temperature and [[electrical conductivity]].
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==== Solar radio flux ====
Emission from the Sun at centimetric (radio) wavelength is due primarily to coronal plasma trapped in the magnetic fields overlying active regions.<ref>{{cite journal |author=Tapping K.F. |title=Recent solar radio astronomy at centimeter wavelength: the temporal variability of the 10.7-cm flux |journal=J. Geophys. Res. |volume=92 |issue=D1 |pages=829–838 |date=1987 |doi=10.1029/JD092iD01p00829 |bibcode=1987JGR....92..829T}}</ref> The F10.7 index is a measure of the solar radio flux per unit frequency at a wavelength of 10.7&nbsp;cm, near the peak of the observed solar radio emission. F10.7 is often expressed in SFU or [[solar flux unit]]s (1 SFU = 10<sup>−22</sup> W m<sup>−2</sup> Hz<sup>−1</sup>). It represents a measure of diffuse, nonradiative coronal plasma heating. It is an excellent indicator of overall solar activity levels and correlates well with solar UV emissions.

Sunspot activity has a major effect on long distance [[radio communications]], particularly on the [[shortwave]] bands although medium wave and low [[VHF]] frequencies are also affected. High levels of sunspot activity lead to improved signal propagation on higher frequency bands, although they also increase the levels of solar noise and ionospheric disturbances. These effects are caused by impact of the increased level of solar radiation on the [[ionosphere]].

10.7&nbsp;cm solar flux could interfere with point-to-point terrestrial communications.<ref>{{cite journal |title=The Effect of 10.7 cm Solar Radiation on 2.4 GHz Digital Spread Spectrum Communications |journal=NARTE News |volume=17 |issue=3 |date=July–October 1999 }}</ref>

==== Clouds ====
Speculations about the effects of cosmic-ray changes over the cycle potentially include:
* Changes in ionization affect the aerosol abundance that serves as the condensation nucleus for cloud formation.<ref name="Tinsley2004">{{Cite book|contribution = Atmospheric Ionization and Clouds as Links Between Solar Activity and Climate|first1 = Brian A.|last1 = Tinsley|first2 = Fangqun|last2 = Yu|year = 2004|volume = 141|pages = 321–339|editor1-first = Judit M.|editor1-last = Pap|editor2-first = Peter|editor2-last = Fox|title = Solar Variability and its Effects on Climate|isbn = 978-0-87590-406-1|contribution-url = http://www.utdallas.edu/physics/pdf/Atmos_060302.pdf|publisher = [[American Geophysical Union]]|series = Geophysical monograph series|bibcode = 2004GMS...141..321T|doi = 10.1029/141GM22|citeseerx = 10.1.1.175.5237|access-date = 2015-08-10|archive-date = 2007-06-04|archive-url = https://web.archive.org/web/20070604183050/http://www.utdallas.edu/physics/pdf/Atmos_060302.pdf}}{{cite web
 |url=http://www.utdallas.edu/physics/
 |title=Department of Physics – the University of Texas at Dallas
 |access-date=2015-08-10
 |archive-url=https://web.archive.org/web/20150815202558/http://www.utdallas.edu/physics/
 |archive-date=2015-08-15
 }}</ref> During solar minima more cosmic rays reach Earth, potentially creating ultra-small aerosol particles as precursors to [[cloud condensation nuclei]].<ref name="CERN Clouds">{{cite press release|title=CERN's CLOUD experiment provides unprecedented insight into cloud formation |publisher=[[CERN]] |url=http://press.cern/press-releases/2011/08/cerns-cloud-experiment-provides-unprecedented-insight-cloud-formation |date=25 August 2011 |access-date=12 November 2016}}</ref> Clouds formed from greater amounts of condensation nuclei are brighter, longer lived and likely to produce less precipitation.
* A change in cosmic rays could affect certain types of clouds.<ref>{{Cite journal |last1=Kumar |first1=Vinay |last2=Dhaka |first2=Surendra K. |last3=Hitchman |first3=Matthew H. |last4=Yoden |first4=Shigeo |date=2023-03-06 |title=The influence of solar-modulated regional circulations and galactic cosmic rays on global cloud distribution |journal=Scientific Reports |language=en |volume=13 |issue=1 |page=3707 |doi=10.1038/s41598-023-30447-9 |issn=2045-2322 |pmc=9988889 |pmid=36878955|bibcode=2023NatSR..13.3707K }}</ref>
* It was proposed that, particularly at high [[latitude]]s, cosmic ray variation may impact terrestrial low altitude cloud cover (unlike a lack of correlation with high altitude clouds), partially influenced by the solar-driven interplanetary magnetic field (as well as passage through the galactic arms over longer timeframes),<ref name="shaviv2005">{{Cite journal |title = On climate response to changes in the cosmic ray flux and radiative budget|journal = Journal of Geophysical Research|volume = 110|year = 2005|url = http://www.phys.huji.ac.il/~shaviv/articles/sensitivity.pdf|doi = 10.1029/2004JA010866|access-date = 17 June 2011|author = Shaviv, Nir J|issue = A08105|pages = A08105|bibcode = 2005JGRA..110.8105S|arxiv = physics/0409123|s2cid = 16364672}}</ref><ref name="Svensmark2007">{{Cite journal |title = Cosmoclimatology: a new theory emerges|journal = Astronomy & Geophysics|volume = 48|year = 2007|pages = 1.18–1.24|doi = 10.1111/j.1468-4004.2007.48118.x|author = Svensmark, Henrik|issue = 1|bibcode = 2007A&G....48a..18S|doi-access = free}}</ref><ref name="Svensmark1998">{{Cite journal |first = Henrik|last = Svensmark|author-link = Henrik Svensmark|title = Influence of Cosmic Rays on Earth's Climate|journal = [[Physical Review Letters]]|year = 1998|volume = 81|issue = 22|pages = 5027–5030|url = http://www.cosis.net/abstracts/COSPAR02/00975/COSPAR02-A-00975.pdf|doi = 10.1103/PhysRevLett.81.5027|access-date = 17 June 2011|bibcode = 1998PhRvL..81.5027S|citeseerx = 10.1.1.522.585}}</ref><ref>{{Cite journal |title = Celestial driver of Phanerozoic climate?|journal = Geological Society of America|volume = 13|year = 2003|page = 4|doi = 10.1130/1052-5173(2003)013<0004:CDOPC>2.0.CO;2|author1=Shaviv, Nir J  |author2=Veizer, Ján |name-list-style=amp |issue = 7|doi-access = free| bibcode=2003GSAT...13g...4S }}</ref> but this hypothesis was not confirmed.<ref>{{Cite journal |author1 = Sun, B.|author2 = Bradley, R.|title = Solar influences on cosmic rays and cloud formation: A reassessment|journal = Journal of Geophysical Research|volume = 107|issue = D14|page = 4211|year = 2002|doi=10.1029/2001jd000560|bibcode = 2002JGRD..107.4211S |doi-access = free}}</ref>
Later papers showed that production of clouds via cosmic rays could not be explained by nucleation particles. Accelerator results failed to produce sufficient, and sufficiently large, particles to result in cloud formation;<ref>{{Cite journal |author1 = Pierce, J.|author2 = Adams, P.|title = Can cosmic rays affect cloud condensation nuclei by altering new particle formation rates?|journal = Geophysical Research Letters|volume = 36|issue = 9|page = 36|year = 2009|doi=10.1029/2009gl037946|bibcode = 2009GeoRL..36.9820P |s2cid = 15704833|doi-access = free}}</ref><ref>{{Cite journal |author = Snow-Kropla, E.|display-authors = etal|title = Cosmic rays, aerosol formation and cloud-condensation nuclei: sensitivities to model uncertainties|journal = Atmospheric Chemistry and Physics|volume = 11|issue = 8|date = Apr 2011|page = 4001|doi=10.5194/acp-11-4001-2011|bibcode = 2011ACP....11.4001S |doi-access = free}}</ref> this includes observations after a major solar storm.<ref name="Erlykin, A., et al. 137">{{Cite journal |author = Erlykin, A.|display-authors = etal|title = A review of the relevance of the 'CLOUD' results and other recent observations to the possible effect of cosmic rays on the terrestrial climate|journal = Meteorology and Atmospheric Physics|volume = 121|issue = 3|page = 137|date = Aug 2013|doi=10.1007/s00703-013-0260-x|arxiv = 1308.5067 |bibcode = 2013MAP...121..137E |s2cid = 118515392}}</ref> Observations after [[Chernobyl disaster|Chernobyl]] do not show any induced clouds.<ref>{{Cite conference |author1 = Sloan, T.|author2 = Wolfendale, A.|author-link2=Arnold Wolfendale|title = Cosmic Rays and Global Warming|book-title = 30TH INTERNATIONAL COSMIC RAY CONFERENCE, Merida, Mexico|date = Jun 2007}}</ref>