Scatter plots of the maximum value of the AE auroral electrojet index in 3 h intervals, AE_max, as a function of the homogeneous aa index value, aa_H, from the same 3-hour interval. Plots are for three averaging timescales τ of both data series: (a) τ = 1 day (note that 〈aa_H〉_τ = 1 day is called Aa_H); (b) τ = 27 days and (c) τ = 1 year. Data are for 1995 to 2017 inclusive. In the middle panel, the data points have been colour-coded according to the difference in F between the centre of the 27-day interval and the closest June solstice, ΔF.