Comparison of the disk-center portions (| x|,   |y| < 0.1   r_⊙) of two HMI magnetograms, recorded at times 2011.2086 (upper panel) and 2011.4796 (lower panel), when the average of the unsigned vertical flux density was 3.06 and 6.65 G, respectively. The upper panel thus represents the case when the basal flux density level is approximately reached. The grey-scale cuts are at  ± 50 G in both panels.