Spectrum of Mg II h&k lines. Shown are spatial averages of the spectra observed by IRIS (black) and synthesized from snapshot muram_en_499000_379s (red). For comparison, we added the publicly available spectrum from Bifrost (blue). A comparison between the two synthetic spectra is presented in Sect. 3.3. The inset panel shows a zoom onto the Mg II k line as a function of Doppler velocity centered on the rest wavelength.