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Reconstructed annual values for the Ap index over the past 400 years. (a) The mean Ap. (b) The fraction of storm-like days with Ap in the top 5% of the overall distribution, so that <Ap>τ = 1 day ≥ Apo = 38 nT. In both cases the green area defines the uncertainty at the 1-sigma level. The blue bars define the Maunder minimum (MM) and the Dalton Minimum (DM) and the pink bar the Modern Grand Maximum (MGM).
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