Solar ultraviolet spectral-irradiance observations from the SUSTM-UARS experiment

The Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) instrument on board the Upper Atmosphere Research Satellite (UARS) has measured solar ultraviolet spectral-irradiance values since October 1991. The calibration and tracking methods are described. The intensity values have a precision of 1 % at wavelengths longer than 200 nm. Short-term ultraviolet (UV) variability is correlated with other chromospheric indices at wavelengths shorter than 280 nm. At longer wavelengths, this correlation changes gradually into an anticorrelation. There exists a long-term solar-cycle component at all wavelengths, which seems to be independent of the rotational modulation. The variability in the integrated UV from 110 nm to 300 nm amounts to 33% of the variability in the total solar irradiance.