Time-of-year (F), time-of-day (UT) plots of observed ratios of geomagnetic indices. All indices are 3-hourly so there are 8 UT bins and the 3-hourly ratios have been averaged into 20 equal-sized bins in F. (a) ; (b) ; (c) aa′/am; and (d) the aa′ hemispheric anisotropy ratio . This plot is for all data in 1959–2017 with 70 ≤ aa′ < 110 nT, for which the mean aa′ is 88.15 nT.