Residence time of spectral lines photons in the optically active media

As part of our recent series of works on the evolution of spectral lines formed in optically active media under the influence of non-stationary energy sources, the paper examines temporal changes in certain statistical averages characterising the radiation field in the medium under study. Line radiation transfer in 1D atmosphere of finite optical thickness is considered for monochromatic scattering. Of the statistical averages, the greatest attention is paid to the average residence time of various photons during diffusion in the medium. At the same time, the average number of scatterings occurring in the medium and the average number of photons undergoing thermalisation are also determined. The specified values, together with the profiles of the lines formed, are given for both photons reflected from the medium and photons passing through it. First, the problem of steady-state radiation is considered, after which the evolutionary picture of all the above-mentioned values under the action of non-stationary sources of illumination of the medium is described.