The proportional change in magnetogram signal due to the time variation in instrument response. a) Cyan: the proportional change in the MDI magnetogram signal, relative to SPM, from the time variation in instrument response, ΔB_{los,MDI − SPM} derived combining the MDI(1)-SPM and MDI(2)-SPM data sets and applying the procedure of the MDI(1)-SPM comparison. Pink: the result from applying the procedure of the MDI(2)-SPM comparison to the combined data set. b) The estimates of ΔB_{los,MDI(2)−VSM} from replicating the procedure of the MDI(2)-VSM(1) (blue) and MDI(2)-VSM(2) comparisons (orange) on the combination of the data sets from these two comparisons. c) The estimates of ΔB_{los,HMI − VSM} from replicating the procedure of the HMI-VSM(2) (green) and HMI-VSM(3) comparisons (red) on the combination of the data sets from these two comparisons. The colour-coding follows Fig. 2. The dashed lines mark a) the SoHO vacation and MDI refocus and b) the VSM CCD camera upgrade and c) filling gas switch. See Sects. 2.1 and 3.1.