The time evolution of a hydrogen spectral line. In this case, the detailed calculations of Ly-α line were made using the MC-RT code SIROCCO (Matthews et al. 2025) under conditions described in Sects. 2 and 5. The individual rows show a particular model, labelled on the left; each column corresponds to a different viewing polar angle θ denoted within each panel. The times of shown simulations range from 50 to 730 days with the particular time-snapshots emphasised by different colours and labelled directly in the plot. Spectral line profiles are plotted on a linear scale and normalised for clarity of presentation; for this reason, we do not display units on the vertical axis (cf. fig. 12 in PK20). The originally slightly blue-shifted peak of emission transfers after about 300-500 days to a red-shifted peak, as corresponds to observations of the spectral lines of SNe (see, e.g. Smith et al. 2015).