Cumulative probability distributions (c.d.f.s) of the am geomagnetic index, as a ratio of its mean value 〈am〉, for data from (a) the full interval of am data (1959–2017) and (b) the interval of near-continuous interplanetary data (1995–2017). The am data are divided into 288 bins, 36 equal-sized bins in fraction of year F and the 8 3-hour Universal Time (UT) bins over which the range am index is evaluated. For each F-UT bin the c.d.f. of am as a ratio of the mean value for that bin (〈am〉_F,UT) is plotted and colored according to the value of the ratio 〈am〉_F,UT/〈am〉_all, where 〈am〉_all is the mean value for all 175,320 three-hourly am samples in the interval studied. The black line is for all am data, the orange line for the F-UT bin giving the largest 〈am〉_F,UT and the green line for the F-UT bin giving the smallest 〈am〉_F,UT.