Editing Solar cycle (section)

== Effects ==

=== Sun ===
[[File:Solar-cycle-data.png|thumb|upright=1.35|Activity cycles 21, 22 and 23 seen in sunspot number index, TSI, 10.7cm radio flux, and flare index. The vertical scales for each quantity have been adjusted to permit overplotting on the same vertical axis as TSI. Temporal variations of all quantities are tightly locked in phase, but the degree of correlation in amplitudes is variable to some degree.]]

==== Surface magnetism ====
[[Sunspot]]s eventually decay, releasing magnetic flux in the photosphere. This flux is dispersed and churned by turbulent convection and solar large-scale flows. These transport mechanisms lead to the accumulation of magnetized decay products at high solar latitudes, eventually reversing the polarity of the polar fields (notice how the blue and yellow fields reverse in the Hathaway/NASA/MSFC graph above).

The dipolar component of the solar magnetic field reverses polarity around the time of solar maximum and reaches peak strength at the solar minimum.

=== Space ===

==== Spacecraft ====
CMEs ([[coronal mass ejection]]s) produce a radiation flux of high-energy [[protons]], sometimes known as solar cosmic rays. These can cause radiation damage to electronics and [[solar cell]]s in [[satellites]]. Solar proton events also can cause [[single-event upset]] (SEU) events on electronics; at the same, the reduced flux of galactic cosmic radiation during solar maximum decreases the high-energy component of particle flux.

CME radiation is dangerous to [[astronaut]]s on a space mission who are outside the shielding produced by the [[Earth's magnetic field]]. Future mission designs (''e.g.'', for a [[human mission to Mars|Mars Mission]]) therefore incorporate a radiation-shielded "storm shelter" for astronauts to retreat to during such an event.

Gleißberg developed a CME forecasting method that relies on consecutive cycles.<ref>{{cite book |author=Wolfgang Gleißberg |title=Die Häufigkeit der Sonnenflecken |publisher=Ahademie Verlag |location=Berlin |date=1953 |language=de}}</ref>

The increased irradiance during solar maximum expands the envelope of the Earth's atmosphere, causing low-orbiting [[space debris]] to re-enter more quickly.

==== Galactic cosmic ray flux ====
The outward expansion of solar ejecta into interplanetary space provides overdensities of plasma that are efficient at scattering high-energy [[cosmic rays]] entering the [[Solar System]] from elsewhere in the galaxy. The frequency of solar eruptive events is modulated by the cycle, changing the degree of cosmic ray scattering in the outer Solar System accordingly. As a consequence, the cosmic ray flux in the inner Solar System is anticorrelated with the overall level of solar activity.<ref>{{cite journal|last1=Potgeiter|first1=M.|title=Solar Modulation of Cosmic Rays|journal=Living Reviews in Solar Physics|volume=10|issue=1|page=3|doi=10.12942/lrsp-2013-3|arxiv = 1306.4421 |bibcode = 2013LRSP...10....3P |year=2013|doi-access=free |s2cid=56546254}}</ref> This anticorrelation is clearly detected in cosmic ray flux measurements at the Earth's surface.

Some high-energy cosmic rays entering Earth's atmosphere collide hard enough with molecular atmospheric constituents that they occasionally cause nuclear [[Cosmic ray spallation|spallation reactions]]. Fission products include radionuclides such as [[Carbon-14|<sup>14</sup>C]] and [[Beryllium-10|<sup>10</sup>Be]] that settle on the Earth's surface. Their concentration can be measured in tree trunks or ice cores, allowing a reconstruction of solar activity levels into the distant past.<ref>{{Cite journal
 | first1=Sami K.| last1=Solanki
 | author-link=Sami Solanki
 | first2=Ilya G.| last2=Usoskin
 | first3=Bernd  | last3=Kromer
 | first4=Manfred| last4=Schüssler
 | first5=Jürg   | last5=Beer
 | title=Unusual activity of the Sun during recent decades compared to the previous 11,000 years
 | journal=Nature
 | volume=431 | date=2004 | pages=1084–7
 | url=http://cc.oulu.fi/%7Eusoskin/personal/nature02995.pdf
 | doi=10.1038/nature02995
 | pmid=15510145
 | issue=7012
 |bibcode = 2004Natur.431.1084S | s2cid=4373732
 }}</ref> Such reconstructions indicate that the overall level of solar activity since the middle of the twentieth century stands amongst the highest of the past 10,000 years, and that epochs of suppressed activity, of varying durations have occurred repeatedly over that time span.{{Citation needed|date=February 2024}}

=== 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>

=== Terrestrial ===

==== Organisms ====
The impact of the solar cycle on living organisms has been investigated (see [[chronobiology]]). Some researchers claim to have found connections with human health.<ref>{{cite journal | journal = Neuroendocrinology Letters | title = Cross-spectrally coherent ~10.5- and 21-year biological and physical cycles, magnetic storms and myocardial infarctions | date = 2000 | pages = 233–258 | url = http://www.nel.edu/21_3/3StoryBeh_Halb.htm | volume = 21 | issue = 3 | pmid = 11455355 | last2 = Cornélissen | first2 = G | last3 = Otsuka | first3 = K | last4 = Watanabe | first4 = Y | last5 = Katinas | first5 = GS | last6 = Burioka | first6 = N | last7 = Delyukov | first7 = A | last8 = Gorgo | first8 = Y | last9 = Zhao | first9 = Z | last1 = Halberg | first1 = F | archive-url = https://web.archive.org/web/20080729003640/http://www.nel.edu/21_3/3StoryBeh_Halb.htm | archive-date = 2008-07-29 }}</ref>

The amount of ultraviolet UVB light at 300&nbsp;nm reaching the Earth's surface varies by a few percent over the solar cycle due to variations in the protective [[ozone layer]]. In the stratosphere, [[ozone]] is [[Ozone-oxygen cycle|continuously regenerated]] by the [[Photodissociation|splitting]] of [[Oxygen|O<sub>2</sub>]] molecules by ultraviolet light. During a solar minimum, the decrease in ultraviolet light received from the Sun leads to a decrease in the concentration of ozone, allowing increased UVB to reach the Earth's surface.<ref>{{cite book |chapter-url=https://www.nap.edu/read/4778/chapter/5#66 |title=Solar Influences on Global Change |chapter=Solar Variations, Ozone, and the Middle Atmosphere |author=National Research Council |year=1994 |pages=66–68 |location=Washington DC |publisher=National Academies Press |doi=10.17226/4778|hdl=2060/19950005971 |isbn=978-0-309-05148-4 }}</ref><ref>{{cite journal |first1=E |last1=Echer |first2=VWJH |last2=Kirchhoff |first3=Y |last3=Sahai |first4=N |last4=Paes Leme |title=A study of the solar cycle signal on total ozone over low-latitude Brazilian observation stations |journal=Advances in Space Research |volume=27 |issue=12 |year=2001 |pages=1983–1986 |doi=10.1016/S0273-1177(01)00270-8|bibcode=2001AdSpR..27.1983E }}</ref>

==== Radio communication ====
{{Main|Skywave}}
Skywave modes of radio communication operate by bending ([[refracting]]) radio waves ([[electromagnetic radiation]]) through the [[Ionosphere]]. During the "peaks" of the solar cycle, the ionosphere becomes increasingly ionized by solar photons and [[cosmic rays]]. This affects the [[Radio propagation|propagation]] of the radio wave in complex ways that can either facilitate or hinder communications. Forecasting of skywave modes is of considerable interest to commercial [[Marine (ocean)|marine]] and [[aircraft]] [[communications]], [[amateur radio operators]] and [[shortwave]] [[Broadcasting|broadcasters]]. These users occupy frequencies within the [[High Frequency]] or 'HF' radio spectrum that are most affected by these solar and ionospheric variances. Changes in solar output affect the [[maximum usable frequency]], a limit on the highest [[frequency]] usable for communications.

==== Climate ====
Both long-term and short-term variations in solar activity are proposed to potentially affect global climate, but it has proven challenging to show any link between solar variation and climate.<ref name="haigh">Joanna D. Haigh "[http://solarphysics.livingreviews.org/Articles/lrsp-2007-2/ The Sun and the Earth's Climate]", ''Living Reviews in Solar Physics'' (access date 31 January 2012)</ref>

Early research attempted to correlate weather with limited success,<ref name="spencer">{{Cite book | first=Spencer | last=Weart | author-link=Spencer Weart | title=The Discovery of Global Warming | chapter=Changing Sun, Changing Climate? | publisher=Harvard University Press | date=2003 | isbn=978-0-674-01157-1 | url=http://www.aip.org/history/climate/ | chapter-url=http://www.aip.org/history/climate/solar.htm | access-date=17 April 2008 | archive-date=4 August 2011 | archive-url=https://web.archive.org/web/20110804232058/http://www.aip.org/history/climate/ }}</ref> followed by attempts to correlate solar activity with global temperature. The cycle also impacts regional climate. Measurements from the SORCE's Spectral Irradiance Monitor show that solar UV variability produces, for example, colder winters in the U.S. and northern Europe and warmer winters in Canada and southern Europe during solar minima.<ref name="SolarForcing">{{cite journal | title=Solar forcing of winter climate variability in the Northern Hemisphere | journal=[[Nature Geoscience]] |date=October 9, 2011 |author=Ineson S. |author2=Scaife A.A. |author3=Knight J.R.|author4=Manners J.C. |author5=Dunstone N.J.|author6=Gray L.J. |author7=Haigh J.D. |volume=4 |pages=753–7 |doi=10.1038/ngeo1282 | issue=11|bibcode = 2011NatGe...4..753I | hdl=10044/1/18859 |url=http://spiral.imperial.ac.uk/bitstream/10044/1/18859/2/Nature%20Geoscience_4_11_2011.pdf |hdl-access=free }}</ref>

Three proposed mechanisms mediate solar variations' climate impacts:
* Total solar irradiance ("[[Radiative forcing]]").
* Ultraviolet irradiance. The UV component varies by more than the total, so if UV were for some (as yet unknown) reason having a disproportionate effect, this might affect climate.
* Solar wind-mediated galactic [[cosmic ray]] changes, which may affect cloud cover.

The solar cycle variation of 0.1% has small but detectable effects on the Earth's climate.<ref>{{cite journal |author=Labitzke K.|author2=Matthes K. |title=Eleven-year solar cycle variations in the atmosphere: observations, mechanisms and models |journal=The Holocene |volume=13 |issue=3 |pages=311–7 |date=2003 |doi=10.1191/0959683603hl623rp  |bibcode=2003Holoc..13..311L|s2cid=129100529 }}</ref><ref>Pablo J.D. Mauas & Andrea P. Buccino. "[https://arxiv.org/abs/1003.0414 Long-term solar activity influences on South American rivers]" page 5. Journal of Atmospheric and Solar-Ter
restrial Physics on Space Climate, March 2010. Accessed: 20 September 2014.</ref><ref>{{cite journal | last1 = Zanchettin | first1 = D. | last2 = Rubino | first2 = A. | last3 = Traverso | first3 = P. | last4 = Tomasino | first4 = M. | date = 2008 | title = [Impact of variations in solar activity on hydrological decadal patterns in northern Italy] | journal = Journal of Geophysical Research | volume = 113 | issue = D12 | page = D12102 | doi = 10.1029/2007JD009157 | bibcode=2008JGRD..11312102Z| s2cid = 54975234 | doi-access = free }}</ref> Camp and Tung suggest that solar irradiance correlates with a variation of 0.18&nbsp;K ±0.08&nbsp;K (0.32&nbsp;°F ±0.14&nbsp;°F) in measured average global temperature between solar maximum and minimum.<ref name="solar-climate">{{cite journal |author=C. D. Camp|author2=K. K. Tung|name-list-style=amp |journal=Geophysical Research Letters |volume=34 |issue= 14|pages= L14703 | title=Surface warming by the solar cycle as revealed by the composite mean difference projection |date=2007 |doi= 10.1029/2007GL030207  |bibcode=2007GeoRL..3414703C|s2cid=16596423|doi-access=free}}</ref>

Other effects include one study which found a relationship with wheat prices,<ref>[https://www.newscientist.com/article.ns?id=dn6680 Sunspot activity impacts on crop success] [[New Scientist]], 18 November 2004</ref> and another one that found a weak correlation with the flow of water in the [[Paraná River]].<ref>[https://www.newscientist.com/channel/earth/mg20026814.100-sunspot-activity-may-be-linked-to-rainfall.html "Sunspot activity may be linked to rainfall"], [[New Scientist]], 8 Nov., 2008, p. 10.</ref> Eleven-year cycles have been found in tree-ring thicknesses<ref name=Luthardt2017 /> and layers at the bottom of a lake<ref name=NeoP /> hundreds of millions of years ago.

The current [[scientific consensus on climate change]] is that solar variations only play a marginal role in driving [[global climate change]],<ref name="haigh" /> since the measured magnitude of recent solar variation is much smaller than the forcing due to greenhouse gases.<ref name="grida fig6-6">{{Cite book | editor1-first=J.T. | editor1-last=Houghton | editor1-link=John T. Houghton | editor2-first=Y. | editor2-last=Ding | editor3-first=D.J. | editor3-last=Griggs | editor4-first=M. | editor4-last=Noguer | title=Climate Change 2001: Working Group I: The Scientific Basis | url=http://www.grida.no/climate/ipcc_tar/wg1/index.htm | date=2001 | publisher=[[Intergovernmental Panel on Climate Change]] | chapter=6.11 Total Solar Irradiance—Figure 6.6: Global, annual mean radiative forcings (1750 to present) | chapter-url=http://www.grida.no/climate/ipcc_tar/wg1/fig6-6.htm | access-date=15 April 2007}}; see also the IPCC Fourth Assessment Report, in which the magnitude of variation in solar irradiance was revised downward, although the evidence of connections between solar variation and certain aspects of climate increased over the same time period: [http://www.ipcc.ch/publications_and_data/ar4/wg1/en/ch2s2-7.html#2-7-1 Assessment Report-4, Working group 1, chapter 2] {{Webarchive|url=https://web.archive.org/web/20131207151831/http://www.ipcc.ch/publications_and_data/ar4/wg1/en/ch2s2-7.html#2-7-1 |date=2013-12-07 }}</ref> Also, average solar activity in the 2010s was no higher than in the 1950s (see above), whereas average global temperatures had risen markedly over that period. Otherwise, the level of understanding of solar impacts on weather is low.<ref>{{Citation
 | year=2007
 | isbn=978-0-521-88009-1
 | chapter=Changes in Atmospheric Constituents and Radiative Forcing: §&nbsp;2.9.1 Uncertainties in Radiative Forcing
 | chapter-url=https://archive.ipcc.ch/publications_and_data/ar4/wg1/en/ch2s2-9-1.html#table-2-11
  | title=Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, 2007
  | url=https://archive.ipcc.ch/publications_and_data/ar4/wg1/en/ch2.html
  | author1=Forster, P. |author2=V. Ramaswamy |author3=P. Artaxo |author4=T. Berntsen |author5=R. Betts |author6=D.W. Fahey |author7=J. Haywood |author8=J. Lean |author9=D.C. Lowe |author10=G. Myhre |author11=J. Nganga |author12=R. Prinn |author13=G. Raga |author14=M. Schulz |author15=R. Van Dorland
| publisher=Cambridge University Press
 |editor=Solomon, S. |editor2=D. Qin |editor3=M. Manning |editor4=Z. Chen |editor5=M. Marquis |editor6=K.B. Averyt |editor7=M. Tignor |editor8=H.L. Miller
  }}</ref>

Solar variations also affect the [[orbital decay]] of objects in [[low Earth orbit]] (LEO) by altering the density of the upper [[thermosphere]].<ref name=sair>{{cite journal |last=Molaverdikhani|first=Karan|author2=Ajabshirizadeh, A.|title=Complexity of the Earth's space–atmosphere interaction region (SAIR) response to the solar flux at 10.7 cm as seen through the evaluation of five solar cycle two-line element (TLE) records|journal=Advances in Space Research|date=2016|volume=58|issue=6|pages=924–937
|doi=10.1016/j.asr.2016.05.035 |bibcode= 2016AdSpR..58..924M|doi-access=free}}</ref>