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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.
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