Time-of-year (F)/time-of-day (UT) plots of mean geomagnetic indices. The means are for 8 bins UT and 20 equal-length bins in F. (a) is for the am index, which covers the interval 1959–2017 (inclusive); (b) is for the classic aa index, aa, for the same interval (1959–2017); (c) is for the corrected aa index, aa _H, for the same interval (1959–2017); and (d) is for aa _H for the prior interval 1868–1958. Parts (a)–(c) use the same (upper) colour scale, but because average values are lower before 1859, a slightly different scale is used in (d), given by the lower colourbar. The am index and the corrected aa index, aa _H, both show the “equinoctial” pattern, but the classic aa index does not.