Snapshots of spatial distribution of travelling wave fronts during the UF for the N5 extended source. Duration of propagatingthe 3-min waves along a magnetic waveguide of about one period. The observational wavelength is 1600 Å. A continuous line represents a positive half-period of propagating waves, and the dashed line separates the negative half-period. The background image is the distribution of the brightness of the source at the time of the maximum flash. The minimum negative half-period is indicated in green. The time resolution is 24 s.