Top panel: plots of annual means of an index or variable x, <x>_1yr, for the years 1995–2017, inclusive, where x is: (a) the power input into the magnetosphere, P_α; (b) the am index (in black) and the number of substorm onsets derived from the SML index, N_o (in green); (c) the aσ indices for dawn (orange), noon (cyan), dusk (mauve) and midnight (blue). Middle and lower panels: year-F spectrogram plots of the means in 36 equal-sized bins of F, divided by the mean for that year, <x>_F/<x>_1yr, where x is: (d) P_α; (e) am; (f) aσ_midnight; (g) aσ_dawn, (h) aσ_noon, and (i) aσ_dusk. The data have been smoothed by applying a 3-point running mean to the time series of means in the bins of width (1/36) yr.