Recent progress in the global description of atomic transitions

The model of Unresolved Transition Arrays (UTA) has been proposed some years ago for modelling a transition array, without resorting to any large energy-matrix diagonalization. For each UTA, the low-order moments of the distribution of the line energies weighted by their strengths are computed exactly, in the form of compact formulae. Then, this distribution is represented in the spectrum by one Gaussian, or one skewed Gaussian whose axis, variance and asymmetry are obtained from the calculated moments. This model is well suited to the case of emission spectra of highly-charged atomic ions, where the low configurations generally do not overlap, and where the recorded lines often coalesce into broad bands. Recently, some progress in the global approach has been made for the cases (i) where configuration-interaction effects are important and (ii) where the lines are resolved. It is now possible to propose simulations of a transition array where the total strength is corrected for the interaction with a large number of configurations, and/or where the lines are obtained by a Monte Carlo method. In such simulations, the low-order moments of the weighted and unweighted distributions of line wavelengths and amplitudes are correct, and the number of lines and the total strength in small wavelength ranges are in good agreement with a line-by-line calculation. Thus, it is possible to introduce thousands of transition arrays in the interpretation of laser-produced plasma spectra, or in the calculation of astrophysical opacities.