Solar irradiance variability
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Since November 1978 a set of total solar irradiance (TSI) measurements from space is available, yielding a time series of more than 23 years. From measurements made by different space radiometers (HF on NIMBUS 7, ACRIM I on SMM, ACRIM II on UARS and VIRGO on SOHO) a composite record of TSI can be compiled. This leads to a reliable record of TSI with an overall precision of the order of 0.05 Wm -2 . This time series is compared to an empirical model based on sunspot darkening and brightening due to faculae and network. Since early 1996 spectral measurements by filter-radiometers of VIRGO provide continuous time series of spectral solar irradiance (SSI) at 402, 500 and 862 nm. These time series are analyzed and compared to TSI yielding information about the redistribution of energy within the spectrum during changes of TSI.