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The relationship of am and normalized power input to the magnetosphere Pα/Po 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 (Pα/Po) = 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, Δteq. 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 Δteq (around the equinoxes) is persistently greater than at high Δteq (around the solstices) showing that there is a contribution to the semi-annual variation in am is not associated with that in (Pα/Po) 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 (Δteq ≤ 0.125) and around the solstices (Δteq > 0.125). The ordinary least squares (OLS) linear regression coefficients and errors are given in Table 1.
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