Reconstructed annual values for the AE index over the past 400 years. (a) The mean AE. (b) The fraction of substorm-like hours with AE in the top 5% of the overall distribution, so that <AE>_{τ = 1 hr} ≥ AEo = 650 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).