Retrieving the solar EUV spectrum from a reduced set of spectral lines
- 1LPCE, CNRS and University of Orléans, 3A avenue de la Recherche Scientifique, 45 071 Orléans cedex 2, France
- 2LPG, CNRS and Joseph Fourier University, Bâtiment D de Physique, BP 53, 38 041 Saint-Martin d’Hères cedex 9, France
- 3LESIA, Paris Observatory, 5 Place Jules Janssen, 92 195 Meudon, France
- 4LIS, CNRS, 961 Rue de la Houille Blanche, BP 46, 38 402 St. Martin d’Hères cedex, France
- 5IFSI, INAF, Via del Fosso del Cavaliere, 00 133 Roma, Italy
Abstract. The solar EUV irradiance is a key input for thermospheric and ionospheric models. Difficulties in continuously measuring the calibrated spectrum has prompted the use of various surrogate quantities. Although most proxies correlate quite well with the spectral variability, their use for modelling purposes becomes increasingly unsatisfactory. A different and data-driven approach is considered here, in which the EUV spectrum is reconstructed from a linear combination of a few, calibrated and carefully selected spectral lines. This approach is based on a statistical analysis of the temporal variability of EUV spectra, as recorded by the TIMED satellite. A basic set of lines is extracted, from which the salient features of the spectral variability can be reconstructed. The best results are achieved with a selection of 5 to 8 of these lines. This study focuses on the methodology for selecting these lines, which can also be used for instrument specification and provides new insight into the comparison of solar proxies against the EUV irradiance.