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Solar Irradiance
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The "solar constant" is, in fact, not constant. Recent satellite observations have found that the Total Solar Irradiance (TSI), the amount of solar radiation received at the top of the Earth’s atmosphere, does vary - see the graph for the results from six satellites(ftp://ftp.ngdc.noaa.gov/STP/SOLAR_DATA/SOLAR_IRRADIANCE/IRRAD97.PDF). "The variations on solar rotational and active region time scales are clearly seen. The large, short-term decreases are caused by the TSI blocking effect of sunspots in magnetically active regions as they rotate through our view from Earth. The peaks of TSI preceding and following these sunpot "dips" are caused by the faculae of solar active regions whose larger areal extent causes them to be seen first as the region rotates onto our side of the sun and last as they rotate over the opposite solar limb." [Excerpted from the UARS descriptive text] The TSI provides the energy that determines the Earth’s climate.Variations of the total solar irradiance (solar constant) have become an important new tool for studying the sun since the deployment of a new generation of precise solar flight instrumentation, such as the ACRIM I satellite experiment on the Solar Maximum Mission (SMM) in 1980. The study of variations of the spectral irradiance observed in the EUV also has developed rapidly. The largest variations of the total irradiance occur on time scales of a day to several weeks and are caused by solar active regions. Efforts to model the radiative effects of active regions are proceeding and the first round of results from these have appeared in literature. Disagreements have quickly surfaced in this new field and a topical workshop was convened at the California Institute of Technology in June 1983, to provide both formal and informal opportunities for dialog between those actively working in this area. The papers resulting from this workshop are collected in the report by LaBonte et al. (1984). NGDC Boulder holds the SMM satellite irradiance data for February 1980 to May 1989 and the Nimbus data for November 1978 to the present. Also available are the Hoyt and Eddy model data for the period 1974 to 1981. Early data by Abbott, Smithsonian Institution, from many locations worldwide covering the period 1902 to 1962 are available also.

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http://www.ngdc.noaa.gov/stp/SOLAR/ftpsolarirradiance.html
ftp://ftp.ngdc.noaa.gov/STP/SOLAR_DATA/SOLAR_IRRADIANCE
http://www.ngdc.noaa.gov/stp/SOLAR/ftpsolarirradiance.html
ftp://ftp.ngdc.noaa.gov/STP/SOLAR_DATA/SOLAR_IRRADIANCE
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