Correlation between northern and southern maxima A) and period B) in the time series of toroidal proxies B_N and B_S. In the two panels the thick blue cross indicates the mean values ±one standard deviation. In A) the horizontal and vertical dashed lines, drawn at B_N = 0.6 and B_S = 0.6, delineate the cycles belonging to the two modes of the distributions in Fig. 10. The diagonal light gray line indicates a perfect correlation (B_N = B_S).