Reference solar spectra: Their evolution, standardization issues, and comparison to recent measurements

Conversely, large differences are noted when comparing the same reference spectrum (as well as other older reference spectra) to spectral data from the SORCE instruments, from 120 to 1600 nm. Whereas these instruments have been designed for extremely high accuracy, the current datasets (versions 3 and 4) must be considered preliminary due to various unresolved problems. Nevertheless, the precision of these instruments seems to be sufficiently high over most of the spectrum to detect short-term changes of 0.1% or less in solar irradiance due to daily changes in activity. Considerably larger daily variations (up to ≈20%) in the UV-C spectrum are observed during the intense activity period of October-November 2003. Furthermore, using the latest version of SORCE data, a comparison between periods of moderate activity and periods of elevated sunspot activity (with low total solar irradiance) suggests that the most part of this loss in total irradiance can be explained by small and smooth spectral changes in the 400-1600 nm waveband, thus confirming that sunspots are dark over all or most of the spectrum.