Variations of annual means of various forms of the aa index. (a) The published “classic” northern and southern hemisphere indices (aa _N and aa _S in red and blue, respectively). Also shown (in green) is 1.5 × a _NGK, derived from the K-indices scaled from the Niemegk data. The vertical dashed lines mark aa station changes (cyan: Melbourne to Toolangi; green: Greenwich to Abinger; red: Abinger to Hartland; and blue: Toolangi to Canberra). (b) The homogenized northern and southern hemisphere indices (aa _HN and aa _HS in red and blue, respectively) generated in the present paper. The thick green and cyan line segments are, respectively, the a _NGK and am index values used to intercalibrate segments. (c) The classic aa data series, aa = (aa _N + aa _S)/2 (in mauve) and the new homogeneous aa data series, aa _H = (aa _HN + aa _HS)/2 (in black). The orange line is the corrected aa data series aa _C generated by Lockwood et al. (2014) by re-calibration of the Abinger-to-Hartland join using the Ap index. (Note that before this join, aa and aa _C are identical and the orange line is not visible as it is underneath the mauve line). The cyan line and points show annual means of the am index. The gray-shaded area in (c) is the interval used to calibrate aa _HN and aa _HS (and hence aa _H) against am.