The intercalibration of aa _N data across the join between the Hartland (HAD) and Abinger (ABN) observations in 1957. The data are divided into eight equal-length fraction-of-year (F) bins, shown in the 8 rows, with the bottom row being bin 1 (0 ≤ F < 0.125) and the top row being bin 8 (0.875 ≤ F < 1). The left-hand column is for an interval of duration 11-years (approximately a solar cycle) before the join and shows scatter plots of the aa data from Abinger (after division by s(δ) to allow for secular changes in the geomagnetic field) against the similarly-corrected simultaneous NGK data, a _NGK /s(δ). The middle column is for an interval of duration 11-years after the join and shows the corresponding relationship between the already-homogenized aa data from Hartland [aa _H]_HAD and the simultaneous a _NGK /s(δ) data. All axes are in units of nT. The grey dots are daily means to which a linear regression gives the red lines which are then checked against the annual means (for the F bin in question) shown by the black dots. The right-hand column shows the fitted lines for the “before” interval, 1, against the corresponding fitted line for the “after” interval, 2: the red line would lie on the dotted line if the two stations had identical responses at the F in question. The slope and intercept of these lines, giving the intercalibration of the two stations at that F, are given in Table 1.