Scatter plot comparing pairs of SO/PHI-HRT 60-second ME-B_LOS and HMI B_LOS. The log density of the pixels is shown and saturated at 100 (a) and 1000 (b) pixels per plotted point for clarity. The averaged linear fit (of HMI vs. SO/PHI-HRT and SO/PHI-HRT vs. HMI) is shown with the solid grey line, and a one-to-one correspondence is indicated by the dashed black line. Panel a: seven pairs with HMI 720-second magnetograms. Panel b: 56 pairs with HMI 45-second magnetograms. See the main text for a more detailed description.