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Analysis of the effect of secular change in the geomagnetic field on the aa magnetometer stations using a spline of the IGRF-12 and the gufm1 geomagnetic field models (for after and before 1900, respectively). (a) The modulus of the corrected geomagnetic latitude, |ΛCG| of the stations; (b) the angular separation of the closest approach to the station of a nominal nightside auroral oval (at |ΛCG| = 69°), δ; and (c) the scale factor s(δ) = L/L o where L is given as a function of δ by Figure 3 and L o = 500 nT, the reference value for the Niemegk station (for which δ is taken to be 19°) except for Canberra which, because of its more equatorward location, is scaled using L o = 450 nT. The northern hemisphere stations are Greenwich (code GRW, in mauve), Abinger (ABN, in green) and Hartland (HAD, in red). The southern hemisphere stations are Melbourne (MEL, in black), Toolangi (TOO, in cyan) and Canberra (CNB, in blue). Also shown is Niemegk (NGK, in orange: data available since 1890). Vertical dashed lines mark aa station changes.

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