Analysis of the sensitivity of the Hartland (HAD) station. Time-of-day (UT)/time-of-year (F) plots of: (left column) the modelled sensitivity for the am index, S_am, for the current stations and sector weighting functions; (middle column) modelled values of the ratio s_HAD/S_am where s_HAD is the sensitivity of the Hartland magnetometer station for measuring its a_K values, a_HAD; and (right column) means of the observed values of the ratio 〈a_HAD〉/〈am〉 = s_HAD/S_am. All data are for eight UT bins 3 h wide and 20 F bins 18.25 days wide over the years 1959–2017 (inclusive). The panels are for am ranges (from top to bottom) of: am ≥ 70 nT; 60 ≤ am < 110 nT; 50 ≤ am < 90 nT; 40 ≤ am < 60 nT; 30 ≤ am < 50 nT; 20 ≤ am < 40 nT; 10 ≤ am < 20 nT; and 0 ≤ am < 10 nT shown in Figure 6. The modelled values are based on the mean am in each band which equals, respectively, 109.14, 75.94, 63.56, 47.76, 37.71 27.50, 13.96, and 5.32 nT. Modelled sensitivities are computed at points 1 h apart in UT and 1/365 apart in F and then averaged into the same sized UT-F bins (3 h by 0.05) as used for the observations. Note that the left-hand plots are colour-contoured using the 0.8–1.2 scale given by the lower colour bar while the modelled and observed s_HAD/S_am sensitivity ratios both use the 0.5–1.6 scale given by the upper colour bar. In all plots unity values are coloured yellow.