Response functions to variations in temperature. The left panel shows the average response function for the traced fibrils (see Sect. 4.2 and Fig. 8) for the Mg II k line, while the same is shown for the Ca II 854.2 nm line in the middle panel. The right panel shows the response function to temperature for the Ca II 854.2 nm for a representative pixel in a plage area, marked in panel (a) of Fig. 3. All panels have a common normalisation factor, which is set to the peak value of the response function for the Mg II k line in the first panel.