As a function of time, how the various magnetographs compare to one another. a) The top plot shows, for the MDI(1)-SPM comparison, the proportion of the solar disc above the magnetogram noise threshold, n_mag in the two sets of magnetograms (blue and red). The bottom plot shows the proportional difference in n_mag between the MDI(1) and SPM magnetograms, Δn_{mag,MDI(1)−SPM} (black, Eq. (1)) and the proportional change in B_los and ϕ_tot in the MDI(1) magnetograms from the time variation in instrument response, ΔB_{los,MDI(1)−SPM} (green, Eq. (2)) and Δϕ_{tot,MDI(1)−SPM} (purple, Eq. (3)). b) to g) The corresponding plots for the other instrument comparisons. Due to similarity, the Δn_mag and ΔB_los profiles are largely hidden behind the Δϕ_tot profiles. See Sect. 3.1.