Radiative transfer in a moving medium

We illustrate first, for a one-dimensional medium, how to generalize the concept of the probability of quantum exit to the case of a moving atmosphere. The transfer of radiation takes place in a two-level atom with total redistribution in frequency at each scattering. The values of the reflection coefficient and of the source function at the surface of a semi-infinite medium with constant properties (especially concerning the velocity gradient) are given. The results are then extended to the three-dimensional case in the Appendix.