Top: Scaled variations of modern a _K values from various stations using the station location correction procedure used in this paper. For all stations, the observed a _K values have been corrected for any secular magnetic field change by dividing by the s(δ) factor and then scaled to the am index using the linear regression coefficients m and i obtained from the calibration interval (2002–2009, inclusive). The plot shows 27-day Bartels rotation means for data from: (mauve) Sodankylä, SOD; (brown) Eskdalemuir, ESK; (orange) Niemegk, NGK; (red) Hartland, HAD; (blue) Canberra, CNB; and (green) a spline of Gangara, GNA and nearby Gingin, GNG (see text for details). The black line is the am index. Bottom: the rms. fit residual of the re-scaled station a _K indices compared with the am index, ε _rms, for the 27-day means. The average of ε _rms for the whole interval shown (1995–2017), is 〈ε _rms〉 = 9.7%