Scatter plots of the data used for the determination of the proportionality constant c₀ (panel a), of the scaling factor for the magnetograms (panel b), of the conversion constant for (panel c), and the conversion constant for (panel d). The red lines indicate the best fits. The red shaded areas represent the errors on the slopes, obtained from the fits. The numbers in the top left corners are the values of the Pearson correlation coefficient, of the slope, and of the error on the slope. In all cases, the p values of the correlation coefficients are too small to be given here (≪0.01).