Origin of the zigzag-shaped PA wiggle in the case of the filled-in microbeam of the O-mode. The layout is similar to that of Fig. 5, except for the fixed-phase PA distributions which are plotted at 1° phase intervals in panel b). The minimum in η_∥ produces the single-minimum dip in the net O-mode intensity (I_∥, thin solid line in a)) as well as the blank gap in the non-bifurcated grey PA band (panel b)). The PA wiggle is again produced by the imbalance of the PA averaging.