Contribution from velocities to emergent spectra along a cut at x = 13.95 Mm through the MURaM-ChE snapshot muram_en_499000_379s. Panel a shows the spectrum of the Mg II k line with color coded intensity. Red colors refer to the red part of the spectrum and blue colors to the blue and violet parts of the spectrum relative to the rest wavelength of Mg II k. The intensities are normalized to the maximum value in the presented range of −90 ≤ Δ v/[km s⁻¹]≤90. Panel b shows the vertical velocity component and panel c the temperature in the atmosphere. Panel d is similar to panel a but for a RT computation of the identical snapshot but neglecting velocities in the atmosphere. The green regions in panel d indicate columns where the RT computation failed. Panels b and c show only the part of the atmosphere that was used for the RT computation (see Sect. 2.2). The bottom panels e and f show comparisons of single Mg II k spectra at y = 2.49 Mm and y = 15.5 Mm between the computation with and without velocities. In panel g we show a comparison of the globally averaged spectrum between the two computations. We note that neither the y-axis nor the x-axis are the same for panels e–g.