Scatter plots of mean modelled sensitivity as a function of mean index ratio in each of the 160 UT-F bins used in Figures 2–5. This plot is for all 8 bins in aa′ used to generate values of A and B: 0 ≤ aa′ < 10 nT; 10 ≤ aa′ < 20 nT; 20 ≤ aa′ < 30 nT; 30 ≤ aa′ < 40 nT; 40 ≤ aa′ < 60 nT; 50 ≤ aa′ < 90 nT; 70 ≤ aa′ < 110 nT; and aa′ ≥ 100 nT. Plots are for the modelled sensitivity against observed ratios for (a) ; (b) ; (c) aa′/am; and (d) the aa′ hemispheric anisotropy ratio . The red lines are where the model sensitivity map perfectly reproduces the index ratios and all points would lie on the horizontal blue line if the sensitivities were assumed constant.