Correlation between daily values of the solar S-index from TIGRE (in the Mount Wilson scale) and the USET area fraction of plages and enhanced network, A_PEN. The parameter n represents the number of data, and the chronological order is color coded. The first and last dates of the data are given next to the color bar. Additionally, a linear fit to the data was performed (solid red line). A mean error bar is displayed in the bottom right corner to give an idea of the uncertainties on the data. Image taken from Vanden Broeck et al. (2024).