The data shown in Fig. 6 have been binned by year and the mean of each bin is plotted here. Top panel: all data from Fig. 6 are included. Bottom panel: only measurements with a field above 1500 Gauss are included. The error bars correspond to the standard error on the mean. The solid line shows the evolution of the international sunspot number over the same period for reference. Assuming a linear trend gives a gradient of −23.6  ±  3.9 Gauss per year and −22.3  ±  3.9 Gauss per year respectively.