Mechanism of the bidirectional (zigzag-shaped) PA distortion. a) Effective microbeam pattern (dot-dashed line), and the macroscopic X-mode emissivity η_⊥ (thick solid) are convolved into the net intensity profile I_⊥ (thin solid). b) PA as a function of phase. The slanted solid line marks ψ_⊥, i.e. the net value of the X-mode PA calculated as a fixed-phase average of the bifurcated grey band. The dotted line marks the RVM-based reference (ψ_B). The double-peaked microbeams, with arrows emerging from points A and B, show how the PA is distributed within the neighbouring pulse longitudes. The void assumed in η_⊥ at D (φ = 0) creates the horizontal break in the bifurcated PA band (grey). The resulting imbalance of the PA averaging produces the thick solid PA wiggle.