Dependence of the magnetically-induced CMB power spectrum due to passive tensor modes on the spectral index for a GUT-scale PMF (left) and comparison between the two extremes for the time ratio τ_ν/τ_B (right). The black lines show the primary CMB anisotropies; for the other colours we refer to the legend. Solid lines represent PMFs generated at the GUT scale, τ_ν/τ_B = 10¹⁷, whereas dashed lines represent PMFs generated at late times, τ_ν/τ_B = 10⁶.