Annual (left column) and diurnal (right column) variations in mean geomagnetic indices for 1959–2017. In all panels, the red line shows the northern hemisphere index, the blue line is the corresponding southern hemisphere index and the black line is the global index, being the average of the two hemispheric indices. (a) and (b) are for the classic aa indices with aa _N in red, aa _S in blue and aa in black. (c) and (d) are for the am indices with an in red, as in blue and am in black. (e) and (f) are for the new homogenized aa indices derived in this paper with aa _HN in red, aa _HS in blue and aa _H in black.