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Time-of-day (UT)/time-of-year (F) plots of the observed sensitivity of the aa index, Saa = (〈aa〉/〈am〉) × Sam, for the years 1959–2017. The left-hand and right-hand columns are for high and low geomagnetic activity respectively. The low activity range is 10 ≤ aa < 20 nT as used in Figures 7 and 8, but to get sufficient samples, high activity is here defined as am ≥ 40 nT both when averaging the observed 〈aa〉 and 〈am〉 values and when calculating the model sensitivity of am, Sam.
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