Vertical slice from a Bifrost radiation-MHD simulation, indicating the approximate formation height of different diagnostics. The solid lines indicate the τ = 1 layer at the core of the Mg II k line (purple), Ca II K line (navy) and Hα line (orange). The black solid line indicates the location where T = 20 kK and the dashed black line the plasma β = 1 layer. The gray shades illustrate the He I 10830 line opacity. We indicated the entire formation range of the Fe I 6301 line in green and the Ca II 8542 line in red.