Linear hydrodynamical equations coupled with radiative transfer in a non-isothermal atmosphere

In a previous paper (Schmieder, 1977), we solved simultaneously the hydrodynamical and radiative transfer equations, so we do not have to assume any relaxation time of the atmosphere. In this paper, we use that theory to interpret photospheric observations of the Mg i line at 5172 Å.For periods between 400 and 140 s, the phase-shifts observed between velocities and the phase shifts between intensity and velocity fluctuations are explained by the existence of radiative dissipation coupled with evanescent waves or upward propagating waves, according to the frequency.For smaller periods partial or total reflections must be considered.The results relative to radiative dissipation are expressed in terms of the variation of a relaxation time with frequency through the atmosphere (10^–3<₅₀₀₀<1).