Phase tracks for the beginning (black) and the end (red) of the same B mode (observation L99010), with an artificial vertical offset added. The average profiles of the corresponding pulse sequences are plotted in the background with matching colours. The serifs mark the midpoint between profile components (see text for details). Lines show the fit with standard geometry/aliasing parameters. Solid lines correspond to the fit with fiducial longitude ϕ₀ matching the midpoint between profile components. The light blue dashed line shows the fit to the late-mode phase track with ϕ₀ fixed at the position of the midpoint between components at the onset of the B mode. Both constant and moving ϕ₀ yield the same quality fits and an almost identical number of sparks N, and the change in ϕ₀ is absorbed by fitted θ₀.