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Universal Time (UT) variations of the am index and of its northern and southern hemisphere components, an and as. The left-hand plots are for averages and the right-hand plots are for the frequency of high activity events, defined by am exceeding its 90-percentile, q(0.9) = 44 nT. Black lines are for am, red for an and blue for as. Parts (a) and (b) show the sensitivities of the three indices, Sx, computed using the model of Lockwood et al. (2019d) and averaged over all F for each of the 8 UTs: (a) is for am = 〈am〉 = 21 nT and (b) is for am = q(0.9) = 44 nT. (c) The indices averaged over all years and all F for the 8 UT bins (am, an and as are given the generic axis label x); (d) corresponding occurrence frequency f of the index x exceeding its 90-percentile, f[x > q(0.90)]; (e) The indices allowing for modelled sensitivity x/Sx. (f) The large event occurrence frequencies allowing for index sensitivity, f[x/Sx > q(0.90)]. (g) and (h) The mean am/Sam and f[am/Sam > q(0.90)], respectively, for each year separately, as a function of UT and year. Data are for 1959–2017, inclusive.
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