The relationship of am and normalized power input to the magnetosphere P_α/P_o for large averaging timescales τ: (a) τ = 1 year and (b) τ = 10 days. In (a) the black circles are for all data, for which the linear correlation coefficient is r = 0.97 and the best fit OLS (Ordinary Least-Squares) linear regression, shown by the green line, is (P_α/P_o) = s·am + c. Averages for the 8 UTs of the am index are shown by the squares, colored by the scale shown at the top of the figure: it can be seen that am is persistently a little smaller than the average response at 0–9 UT and persistently a little larger at 15–4 UT. In (b) the points are colored by the separation of time-of-year F from the value at the closest equinox, Δt_eq. The linear correlation for all data is r = 0.96 and the best fit linear regression is again the green line. The am response at low Δt_eq (around the equinoxes) is persistently greater than at high Δt_eq (around the solstices) showing that there is a contribution to the semi-annual variation in am is not associated with that in (P_α/P_o) and hence not directly attributable to the R–M effect on solar wind-magnetosphere coupling. The black and orange lines are, respectively, the best-fit linear regressions for around the equinoxes (Δt_eq ≤ 0.125) and around the solstices (Δt_eq > 0.125). The ordinary least squares (OLS) linear regression coefficients and errors are given in Table 1.