Letter to the Editor

The EUV Sun as the superposition of elementary Suns

¹ GIPSA-lab, DIS (UMR CNRS 5216, INP Grenoble), BP 46, 38402 Saint-Martin d'Hères, France e-mail: bidou.amblard@gipsa-lab.inpg.fr
² IRCCYN (UMR CNRS 6597, ECNantes), 1 rue de la Noé, BP 92101, 44321 Nantes Cedex 3, France
³ LPCE (UMR 6115 CNRS-Université d'Orléans), 3A avenue de la Recherche Scientifique, 45071 Orléans, France
⁴ LESIA (UMR CNRS 8109, Observatoire de Paris), 5 place Jules Janssen, 92195 Meudon, France
⁵ LPG (UMR 5109 CNRS-Université Joseph Fourier), Bâtiment de Physique D, BP 53, 38041 Saint-Martin d'Hères Cedex, France
⁶ IAS (UMR 8617 CNRS-Université Paris-Sud), 91045 Orsay, France

Received: 15 February 2008
Accepted: 27 May 2008

Abstract

Aims. Many studies assume that the solar irradiance in the EUV can be decomposed into different contributions, which makes modelling the spectral variability considerably easier. We consider a different approach in which these contributions are not imposed a priori but effectively and robustly inferred from spectral irradiance measurements.

Methods. This is a source separation problem with a positivity constraint, for which we use a Bayesian solution.

Results. 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.