Time variation of the spatially averaged Mg II h&k spectrum. Panel a shows a time series of 10 min in length at a cadence of ≈1.2 s. We computed the spatial average from eight slits that are evenly spaced in the simulation at positions x, y ∈ [0, 6, 12, 18] Mm. We then subtracted the average spectrum over the whole 10 min and show the relative difference. Panel b shows the time-averaged spectrum together with the standard deviation. The red curve shows for comparison the spatially averaged spectrum from snapshot muram_en_499000_379s, which was discussed in detail in Sect. 3.2. The spectra have been degraded to the IRIS spatial and spectral resolution.