The EUV Sun as a superposition of 3 elementary Suns
Abstract
Many studies assume that the solar irradiance in the EUV can be decomposed into different contributions, which considerably eases the modelling of the spectral variability for thermosphere/ionosphere models. We consider a different approach, in which these contributions are not imposed a priori but inferred from the spectral irradiance measurements. This problem can be considered as a source separation problem with positivity constraint, for which we use a recent Bayesian method. Using five years of daily EUV spectra recorded by the SEE instrument onboard TIMED, we show that the spectral irradiance can be decomposed into three elementary spectra. The first one is associated with the quiet Sun, the second one with active regions, whereas the third one captures the cold contribution from the chromosphere.