  Variations of the solar UV irradiance are an important driver of chemical and
physical processes in the Earth's upper atmosphere and may also influence
global climate. Here we reconstruct solar UV irradiance in the range 115-400 nm
over the period 1974-2007 by making use of the recently developed empirical
extension of the SATIRE models employing SUSIM data. The evolution of the solar
photospheric magnetic flux, which is a central input to the model, is described
by the magnetograms and continuum images recorded at the Kitt Peak National
Solar Observatory between 1974 and 2003 and by the MDI instrument on SoHO since
1996. The reconstruction extends the available observational record by 1.5
solar cycles. The reconstructed Ly-alpha irradiance agrees well with the
composite time series by Woods et al (2000). The amplitude of the irradiance
variations grows with decreasing wavelength and in the wavelength regions of
special interest for studies of the Earth's climate (Ly-alpha and oxygen
absorption continuum and bands between 130 and 350 nm) is one to two orders of
magnitude stronger than in the visible or if integrated over all wavelengths
(total solar irradiance).
