  Many studies assume that the solar irradiance in the EUV can be decomposed
into different contributions, which makes the modelling of the spectral
variability considerably easier. We consider a different approach, in which
these contributions are not imposed a priori but are effectively and robustly
inferred from spectral irradiance measurements. This is a source separation
problem with a positivity constraint, for which we use a Bayesian solution.
Using five years of daily EUV spectra recorded by the TIMED/SEE satellite, we
show that the spectral irradiance can be decomposed into three elementary
spectra. Our results suggest that they describe different layers of the solar
atmosphere rather than specific regions. The temporal variability of these
spectra is discussed.
