The UT variations of means of the (black lines) am index and its two hemispheric sub-indices an (for the northern hemisphere, red lines) and as (for the southern hemisphere, blue line). The solid lines are for 1959–2017, the dot-dash lines for 1995–2019. The top panel shows the mean absolute values of the index (generically termed ax) in 1-hour bins. The lower panel shows the mean values as a fraction of the overall mean for the interval, ax/〈ax〉_all. The hourly values were obtained by linearly interpolating the 3-hour index values.