Average phase difference of FWHM oscillations around the dominant frequency of 4 mHz (±0.5 mHz) as a function of distance across the diameters of the swirls within the red (red curve) and cyan circle (cyan curve). The FWHM phase differences are derived relative to each swirl’s left edge that is used as a reference point, with the ends of both curves ideally associated with opposite sides of the waveguide in the case of torsional Alfvén perturbations. Only FWHM oscillations at selected time intervals when these swirls are clearly visible (see Fig. 2, panels g and h) have been considered for the phase differences derivation. Overplotted error bars correspond to the respective standard deviation of the FWHM oscillations within the considered frequency range.