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Figure 1

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The semi-annual variation in the aaH index, available since 1868. (a) Annual means of the international sunspot number, R; (b) mean values of aaH in 30 equal-sized bins of time-of-year F as a function of F and year; (c) the mean aaH values normalized by dividing by the annual mean, 〈aaHyr; (d) a comparison of the means of aaH in quarter-year intervals around the March and September equinoxes, 〈aaHme and 〈aaHse respectively, where req = {〈aaHme − 〈aaHse}/{〈aaHme + 〈aaHse} and (e) the occurrence frequency of 3-hourly aaH values exceeding its 95 percentile, q(0.95) = 56 nT, f[aaH > q(0.95)].

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