2D cross-correlation function calculated for one train of 3 min oscillations, observed at different wavelengths. The base signal is the image taken at 02:49 UT at 304 Å when the spiral wave front was most pronounced. The value of the cross-correlation function is shown by the brightness. The time lag in the formation of the spiral wave front is highlighted by the dashed line. The wavelengths of different observational channels are shown in Å.