The FUV (a) and MUV (b) dependence on the Mg II index and time. The effect seen in Figure 1 arises from the FUV (a), as two distinct slopes with little scatter are clearly visible, whereas (b) exhibits a much larger scatter in the data. This difference can be due to an increased ageing effect mainly in the FUV where the energetic EUV radiation alters the instrumental efficiency more than in the MUV spectral region.