Formation of the Hα intensity profile for the EB at t = 0 from column A marked in Fig. 1. The contribution function to emergent intensity C_I in the lower right panel can be calculated as the product of the three factors shown in the other panels. The height where τ_λ = 1 (solid red) and the velocity profile in the z-direction (solid white) are plotted in all panels (negative velocities correspond to blueshift, i.e. upflows, while positive values are downflows). The top left panel shows the factor χ_λ/τ_λ, i.e. the ratio between the opacity and optical depth; the dotted cyan line shows the ionised to neutral hydrogen ratio as a function of height. The top right panel shows the monochromatic source function S_λ; the dotted green line shows the atmosphere’s temperature profile. The lower left panel shows the factor τ_λ/e^−τ_λ, i.e. a function that peaks in the region where τ_λ ≈ 1. The lower right panel shows the contribution to emergent intensity C_I. The solid cyan line shows the Hα spectral profile of the EB, while the dotted white line shows an Hα background profile, i.e. the average profile over many columns and scans. The spectral profile is normalised to I_wing, which is the average intensity of the outermost wavelength points in the blue and red wings of the background profile.