Left to right: time evolution of amplitude solutions for LOFAR core stations for the observation shown in Fig. 3. Each plot is separated from the next by 10 s. Each point represents a core station and each is positioned in the plot according to its geographical location. Colour (blue to red) and size (small to large) are related to the value of the amplitude correction (from − 10% to + 10%). A wave travelling south–north is visible. Similar structures were also reported with single-station observations. The full movie is available in the online material.