Blue lines as chromospheric diagnostics: The Si I lines at 3906 and 4103 Å

Abstract

We present a complete atomic model for Si I line synthesis. We study how the computed profiles of two blue lines of this atom are influenced by the choice of the atomic parameters and find that, although several cross sections are not known accurately, the line profiles do not depend strongly on them and are therefore useful as diagnostics of the atmospheric structure. We study which transitions need not be included in the model, in order to reduce as much as possible the computing time. We compare the profiles computed for a standard model of the quiet solar atmosphere with the observations and find very good agreement. We confirm that irradiation by UV lines originating in the transition region above sunspot umbrae or plages strongly enhances the continuum between 1300 and 1700 Å, which is due to Si I bound-free transitions. If line fluxes typical of the impulsive phase of flares are assumed, the line profiles are also affected.