Upper panel: brightness temperature T_ν as a functionof B_⊥ computed atν = 115 and 189 MHz with the CRe spectra by Orlando (2018; green line) and Padovani et al. (2018; violet line) for a slab of thickness 1 pc, with an angular resolution θ_b = 4′. Lower panel:brightness temperature spectral index β as a functionof B_⊥ (same colour coding as in the upper panel). The hatched region highlights typical values of β at intermediate and high latitudes (Mozdzen et al. 2017).