Analysis of the differences between semi-annual variations in the am index (left-hand panels) and in power input to the magnetosphere, P_α/P_o (right-hand panels). In all panels a thick line denotes the variation for all data, a thin line connecting open circles is for IMF [B_Y]_GSEQ > 0 and a thin line connecting filled triangles is for [B_Y]_GSEQ < 0. The lines in parts (a and b) are mean values whereas (c–f) show the average number of days per year N when the daily means of am (panels c and e) or of P_α/P_o (panels d and f) exceed their 90% quantile (blue lines), 95% quantile (orange lines) and 99% quantile (mauve lines). Parts (g) and (h) plot N/<N>, the number of days per year N normalized by their average value. Note that the variations of N in (e) and (f) for the separate IMF [B_Y]_GSEQ polarities are plotted on a scale that is different to that for all data in (c) and (d). The 90% quantile levels, q(0.9), are 2.031 for am/<am>_all and 2.081 for P_α/P_o; the 95% quantile levels, q(0.95), are 2.556 for am/<am>_all and 2.588 for P_α/P_o; the 99% quantile levels, q(0.99), are 4.193 for am/<am>_all and 4.400 for P_α/P_o. The close similarity between these pairs quantile values arises from the similarity of the normalised c.d.f.s shown in Figure 6 of the present paper and Figure 8b of Paper 1.