Geomagnetic index at minimum, aa_min, vs. observed maximum amplitude of the dipolar field strength, DM_max. The ordinates of the stars and dots are the observations of DM_max 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 aa_min #24 as derived from the regression line and the value of DM_max 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 aa_min obtained from the corresponding regression line. This diagram proves that aa_min is a suitable proxy for the polar field flux.