J. Aboudarham

Which solar EUV indices are best for reconstructing the solar EUV irradiance

Abstract The solar EUV irradiance is of key importance for space weather. Most of the time, however, surrogate quantities such as EUV indices have to be used by lack of continuous and spectrally resolved measurements of the irradiance. The ability of such proxies to reproduce the irradiance from different solar atmospheric layers is usually investigated by comparing patterns of temporal correlations. We consider instead a statistical approach. The TIMED/SEE experiment, which has been continuously operating since February 2002, allows for the first time to compare in a statistical manner the EUV spectral irradiance to five EUV proxies: the sunspot number, the f10.7, Ca K, and Mg II indices, and the He I equivalent width. Using multivariate statistical methods, we represent in a single graph the measure of relatedness between these indices and various strong spectral lines. The ability of each index to reproduce the EUV irradiance is discussed; it is shown why so few lines can be effectively reconstructed from them. All indices exhibit comparable performance, apart from the sunspot number, which is the least appropriate. No single index can satisfactorily describe both the level of variability on time scales beyond 27 days, and relative changes of irradiance on shorter time scales.

Variability of the EUV quiet Sun emission and reference spectrum using SUMER

This paper aims to build a quiet Sun EUV reference spectrum from the SOHO - SUMER database. We make use of 750 quiet Sun spectra spread over a large range of solar activity. They are carefully calibrated both in terms of frequency and intensity. The data do not show any noticeable variations versus the solar activity. This allows us to build a quiet Sun reference spectrum from data scattered over any solar activity level. We consider separately the continuum, the thick-line spectra, and the thin-line spectra. The former is computed from an average of the H Lyman and C I continua. The thick lines are directly assembled from the SUMER files. To retrieve a broad set of thin lines, we use the SUMER measurements to compute a mean Differential Emission Measure, which is then inverted. This results in a reference spectrum for the Quiet Sun, which is compared to previous studies.

Multispectral imaging the sun in the ultraviolet

Solar images in the ultraviolet (UV) are the key to the understanding of the highly dynamic and energetic solar atmosphere. Nowadays, several missions provide simultaneous observations in multiple wavelengths. Such multispectral images have traditionally been used as inputs to physical models. However, as the number of wavelengths steadily increases, empirical approaches such as hyperspectral analysis and blind source separation, become of interest. Two examples are presented, based respectively on spatial and on spectral mixtures of UV data.