The Total Solar Irradiance (TSI) incident at the top of the Earth atmosphere is affected by the Sun's activity and thus varies slightly on a daily and seasonal basis. A reconstruction of the daily TSI since 1976, based on measurements from spaceborne radiometers and appropriate corrections , is shown in Fig. 1. The long-term mean TSI value is referred to as the "Solar Constant". Its latest determination, based on the reconstruction just mentioned, amounts to 1361.1 W/m2  at normal incidence, which is 5.0 W/m2 lower than a previous determination . This TSI value corresponds to the whole solar spectrum, whose distribution resembles that of a blackbody at ≈5772 K, albeit with various disturbances (mainly in the UV). A complete reference extraterrestrial spectrum (here) corresponding to the mean sun-earth distance and a solar constant of 1366.1 W/m2 was proposed in 2004 . A revision, for wavelengths beyond 200 nm, was published in 2018 , based on newer spaceborne measurements. A comparison between the latter spectrum and various Planck distributions for an ideal blackbody are shown in Fig. 2.
A determination of the extraterrestrial spectrum is necessary for various space applications, and also as a primary input to a spectral radiation model, such as SMARTS, for the prediction of the solar spectrum at the surface. This, in turn, is required in various terrestrial applications that are affected by spectral variations, such as photovoltaics (PV).
Fig. 1: Time series of a daily TSI reconstruction sine 1976 according to Ref. .
Fig. 2: Extraterrestrial spectrum according to Ref. , compared to ideal Planck distributions for various blackbody temperatures.
 C.A. Gueymard, The sun’s total and spectral irradiance for solar energy applications and solar radiation models. Solar Energy, vol. 76, 423–452 (2004).
 C.A. Gueymard, Revised composite extraterrestrial spectrum based on recent solar irradiance observations. Solar Energy, 169, 434–440 (2018).