Influence of the stratification structure on the internal radiation field. The anisotropy of the radiation field in the line core (Δλ = 0) of Hα is shown in the left-hand panel. Here, the arrows represent the μ angles considered for the internal energy balance, and their length/colour their corresponding amplitude. The green box around the lower region of the two-part model indicates that this contribution is contained within the boundary condition. On the right the associated temperature stratification (solid lines) of the atmospheres and ratio of populations responsible for forming Hα (dashed lines) are shown.