Cumulative probability distribution (c.d.f, mauve line) and histogram of number of am samples in bins Δam = 1 nT wide (N, shown by the black line as N/N_max, where N_max is the maximum value of N) for all am data in the years 1959–2017 (inclusive). The grey bars give the eight overlapping am bands employed in this paper: 0 ≤ am < 10 nT, 10 ≤ am < 20 nT, 20 ≤ am < 40 nT, 30 ≤ am < 50 nT, 40 ≤ am < 60 nT, 50 ≤ am < 90 nT, 60 ≤ am < 110 nT, and am ≥ 70 nT which contain a numbers of samples N _b of 58183, 51083, 40894, 22691, 13157, 8302, 10869, and 6060, respectively, and for which the mean am values are 5.32, 13.96, 27.50, 37.71, 47.76, 63.56, 73.90, and 109.14 nT.