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Geomagnetic index at minimum, aamin, vs. observed maximum amplitude of the dipolar field strength, DMmax. The ordinates of the stars and dots are the observations of DMmax as deduced from Mount Wilson and Wilcox solar observatories data, respectively. The abscissae are the yearly values of the standard aa index (left-hand panel) and the homogenized data from Lockwood (2009, priv. comm.). The filled triangles in the left-hand panel are a measure for the prediction of aamin #24 as derived from the regression line and the value of DMmax for cycle 24 (De Jager & Duhau 2009) and the open triangle is the observed value. The open triangle in the right-hand panel is the value of aamin obtained from the corresponding regression line. This diagram proves that aamin is a suitable proxy for the polar field flux.