Top panel: The B_y-index over the time period 1968–2020. Red mark periods of 2-sector structures, while blue mark periods of 4-or irregular sector structures. Middle panel: Scalogram obtained by a wavelet analysis of the B_y-index over the same time period. The dominating frequencies can be seen by the lighter color bands around 13.5 and 27 days per period. Bottom panel: Yearly occurrence rate of 2-sector structures for the B_y-index over the same period.