(a) The observed variations of the conductivity-corrected aσ geomagnetic indices with fraction of year, F, shown as means in 36 equal-sized bins in F as a fraction of their overall mean. In all panels, red lines are for the northern hemisphere index, blue for the south and black for the average of the two. (a) aσ_cc(dawn)(F))/(〈aσ_cc(dawn)〉_all; (b) aσ_cc(noon)(F))/(〈aσ_cc(noon)〉_all; (c) aσ_cc(dusk)(F))/(〈aσ_cc(dusk)〉_all; and (d) aσ_cc(midn)(F))/(〈aσ_cc(midn〉_all.