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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.
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