Plot of the average maximum magnetic field in the core of sunspot umbrae measured in SoHO/MDI magnetograms over cycle 23. The smoothed monthly sunspot index is overplotted to show the evolution of the solar cycle (black curve). The overlaid green line corresponds to the linear trend measured by Penn & Livingston (2011). The red curve corresponds to a 11-year periodic modulation, while the blue line connecting the two last minima suggests a slight decrease in the recent minimum, though much lower than the Penn-Livingston trend (Figure adapted from Watson & Fletcher 2011).