Phase speeds υ_ph100 (in km s⁻¹) for a height difference of 100 km derived from the phase difference corresponding to the maximum cross-power of the considered I–I pairs (white lines in Figs. 2 and 3) within the dominant frequency range and for different areas. Here, υ_ph100 has been calculated only for phase differences higher than 10° within the vortex area (black curves), and the swirls within the red, cyan, and green circles (respective colour curves) shown in Fig. 1. Coloured legends provide the weighted mean υ_ph100 values with the respective normalised cross-power as weight. The derivation of actual speeds, υ_ph, further requires the actual formation height separation, dh, as they are equal to (dh/100 km) × υ_ph100, according to Eq. (2).