Universal Time-year spectrograms of normalised geomagnetic activity indices. In each panel the mean value in 3-hour bins of UT for a given calendar year are shown as a ratio of the overall mean for that year (generically x/〈x〉_yr) where x and 〈x〉_yr are, respectively, 3-hour and 1-year means of the index in question), colour-coded as a function of UT and year. (a) The SuperMAG SML index; (b) the auroral electrojet AL index; (c) the northern hemisphere component of the am index, an; (d) the southern hemisphere component of the am index, as; (e) the am index, am = ((an + as))/2; (f) the homogenous aa index, aa_H = (aa_HN + aa_HS)/2.