Upper panel: zonally averaged magnetic diagram produced after re-gridding individual synoptic magnetograms from sine latitude to latitude with a spatial resolution of one degree and then taking longitudinal average of the magnetic field in each latitude by using only the data points with weak magnetic field strength (|B| < 10 G) to suppress the effects of sunspots in the mid-latitudes and to reproduce the magnetic flux transport to the poles with higher contrast. Lower panel: temporal profiles of the mean magnetic field in every 10° latitude bin obtained from the butterfly diagram in the upper panel, with the red and blue points denoting positive and negative polarity, respectively.