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(a) The 13-month smoothed mean total sunspot number. Grey shading indicates solar maximum years. Occurrence of (b) solar X-ray flux for 01 January 1980 until 28 June 2017, (c) Kp for 1980–2020, and (d) D-RAP modelled absorption >1 dB for all PCA event observed in 1986–2020. Red horizontal lines indicate MOD and SEV event thresholds for (b) SWF (X1 and X10 solar X-ray flare), (c) AA (Kp = 8, 9), and (d) PCA (2 dB and 5 dB absorption at 30 MHz). (e) Maximum intensity of PSD events, represented by hourly relative foF2.
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