Editing Sunlight (section)

==Solar irradiance==
{{Main|Solar irradiance}}
===Solar constant===

[[File:Solar irradiance spectrum 1992.gif|thumb|upright=1.3|Solar irradiance spectrum at top of atmosphere, on a linear scale and plotted against [[wavenumber]]]]
{{Main|Solar constant}}

The '''solar constant''' is a measure of [[flux density]], is the amount of incoming solar [[electromagnetic radiation]] per unit area that would be incident on a plane perpendicular to the rays, at a distance of one [[astronomical unit]] (AU) (roughly the mean distance from the Sun to Earth). The "solar constant" includes all types of solar radiation, not just the [[visible light]]. Its average value was thought to be approximately 1,366&nbsp;W/m<sup>2</sup>,<ref name=TSI20>{{cite web |url=http://acrim.com/TSI%20Monitoring.htm |title=Satellite observations of total solar irradiance |publisher=Acrim.com |access-date=2012-02-12 |url-status=live |archive-url=https://web.archive.org/web/20030204191816/http://acrim.com/TSI%20Monitoring.htm |archive-date=2003-02-04 }}</ref> varying slightly with [[solar cycle|solar activity]], but recent recalibrations of the relevant satellite observations indicate a value closer to 1,361&nbsp;W/m<sup>2</sup> is more realistic.<ref name=KoppLean11>{{cite journal|last1=G. Kopp|author2=J. Lean |title=A new, lower value of total solar irradiance: Evidence and climate significance|journal=Geophys. Res. Lett.|date=2011|pages=L01706|doi=10.1029/2010GL045777|bibcode = 2011GeoRL..38.1706K|first1=Greg|volume=38 |issue=1 |doi-access=free}}</ref>

===Total solar irradiance (TSI) and spectral solar irradiance (SSI) upon Earth===
{{Anchor|Total Solar Irradiance|TSI}}
Since 1978, a series of overlapping NASA and ESA satellite experiments have measured '''[[total solar irradiance]]''' (TSI) – the amount of solar radiation received at the top of Earth's atmosphere – as 1.365 kilo⁠watts per square meter (kW/m<sup>2</sup>).<ref name=TSI20 /><ref>{{cite journal | last1 = Willson | first1 = R. C. | last2 = Mordvinov | first2 = A. V. | year = 2003 | title = Secular total solar irradiance trend during solar cycles 21–23 | journal = Geophys. Res. Lett. | volume = 30 | issue = 5 | page = 1199 | doi = 10.1029/2002GL016038 | bibcode = 2003GeoRL..30.1199W | doi-access = free }}</ref><ref name=TSI>{{cite web |title= Construction of a Composite Total Solar Irradiance (TSI) Time Series from 1978 to present |url= http://www.pmodwrc.ch/pmod.php?topic=tsi/composite/SolarConstant |access-date= 2005-10-05 |url-status= dead |archive-url= https://web.archive.org/web/20110830221302/http://www.pmodwrc.ch/pmod.php?topic=tsi%2Fcomposite%2FSolarConstant |archive-date= 2011-08-30 }}</ref><ref>{{cite web|url= http://www.acrim.com/index.htm|title= Current Projects|website= www.acrim.com|access-date= 25 January 2018|url-status= live|archive-url= https://web.archive.org/web/20171016112445/http://www.acrim.com/Index.htm|archive-date= 16 October 2017}}</ref> TSI observations continue with the [[ACRIMSAT]]/ACRIM3, [[Solar and Heliospheric Observatory|SOHO]]/VIRGO and [[Solar Radiation and Climate Experiment|SORCE]]/TIM satellite experiments.<ref>{{cite web|url= http://www.acrim.com/Comparison%20of%20TSI%20Results.htm|website= ACRIM.com|title= Comparison: ACRIMSAT/ACRIM3, SOHO/VIRGO and SORCE/TIM results|access-date= 25 January 2018|url-status= live|archive-url= https://web.archive.org/web/20171016112535/http://www.acrim.com/Comparison%20of%20TSI%20Results.htm|archive-date= 16 October 2017}}</ref> Observations have revealed variation of TSI on many timescales, including the solar magnetic cycle<ref name="acrim">{{cite web |url= http://www.acrim.com/TSI%20Monitoring.htm |title= Graphics Gallery |publisher= Acrim.com |access-date= 2014-04-21 |url-status= live |archive-url= https://web.archive.org/web/20140502013019/http://acrim.com/TSI%20Monitoring.htm |archive-date= 2014-05-02 }}</ref> and many shorter periodic cycles.<ref>{{cite web |url= http://www.acrim.com/Comparison%20of%20TSI%20Results.htm |title= Comparison: ACRIMSAT/ACRIM3, SOHO/VIRGO and SORCE/TIM results |website= ACRIM.com |access-date= 2013-03-14 |url-status= live |archive-url= https://web.archive.org/web/20130530223744/http://www.acrim.com/Comparison%20of%20TSI%20Results.htm |archive-date= 2013-05-30 }}</ref> TSI provides the energy that drives Earth's climate, so continuation of the TSI time-series database is critical to understanding the role of solar variability in climate change.

Since 2003, the SORCE Spectral Irradiance Monitor (SIM) has monitored '''Spectral solar irradiance''' (SSI) – the spectral distribution of the TSI. Data indicate that SSI at UV (ultraviolet) wavelength corresponds in a less clear, and probably more complicated fashion, with Earth's climate responses than earlier assumed, fueling broad avenues of new research in "the connection of the Sun and stratosphere, troposphere, biosphere, ocean, and Earth's climate".<ref name=goddard>
{{cite web
 |url= http://atmospheres.gsfc.nasa.gov/climate/index.php?section=136
 |title= NASA Goddard Space Flight Center: Solar Radiation
 |publisher= Atmospheres.gsfc.nasa.gov |date= 2012-02-08
 |access-date= 2012-02-12 |url-status= dead
 |archive-url= http://archive.wikiwix.com/cache/20110920123747/http://atmospheres.gsfc.nasa.gov/climate/index.php?section=136
 |archive-date= 2011-09-20 
}}
</ref>