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Scatter of fraction of auroral electrojet disturbance indices exceeding the 95-percentile in each year, as a function of the annual mean value for that year for (a) AU and (b) AL. The analysis is of hourly means so f[AU > AUo] is the fraction of strong substorm growth- and expansion-phase hours defined by AU > AUo and so are in the top 5% of all observed values. Similarly f[AL<ALo] is the fraction of strong substorm expansion phases defined by AL < ALo. The 95 percentiles are defined from the distributions of all available hourly means over the interval 1967–2016 (inclusive) and are AUo = 228 nT and ALo = −444 nT. The AE index, AE = AU − AL shows the same behaviour and is discussed further below.
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