(a) Same as Figure 1(b), but with an illustrative few-degree-order drift on the magnetic dipole inclination θ₁, marked by the orange dashed curve. The inset shows a zoom-in view, highlighting a specific window marked in a dashed box. (b) Same as panel (a), but with an illustrative few-degree-order drift on the magnetic dipole initial azimuth ϕ₁. Both drifts allow two minima to satisfy the viewing condition, leading to a transition from a single-burst to a double-burst pattern.