Time-of-year (F)/time-of-day (UT) plots of hemispheric geomagnetic indices for 1959–2017. The means are for 8 bins UT and 20 equal-length bins in F. (a) Shows the variation for the classic northern hemisphere aa index, aa _N and (b) is for the classic southern hemisphere aa index, aa _S. (c) and (d) are for the homogenised northern and southern (respectively) hemisphere aa indices, aa _HN and aa _HS, which include the correction of the secular variation described in Paper 1 (Lockwood et al., 2018) and allowance for the variation of the station sensitivity developed in the present paper. (e) and (f) are for the northern and southern (respectively) hemisphere am indices, an and as.