Time series from the VIRGO/SPM instrument obtained with the green and red channels as described in Section 3.1 (top-left). Standard deviation for VIRGO data using subseries of size k × P_rot with k = 1, 2, 3, 4, 5, 6, 10, 20, and 30 (from bottom right to top right). The black dot dash line represents the global magnetic index S_ph and the red triple dot-dash line corresponds to the mean magnetic index, , computed as described in Section 3.