Variation in CC and its wavelet power spectrum. Panel a is the distribution of CC vs. time for the SST Hα line core chromospheric swirls. See Sect. 2 for the definition of the CC. The inset in panel a is a zoomed-in view of the region before the dashed green line. The dashed blue line corresponds to a CC of 0.1. Panel b shows the variation in CC after applying a highpass filter and subtracting its average value. Panel c is the corresponding wavelet power spectrum with darker colours for higher powers. The solid black curves are the local 95% confidence levels. The solid black curve in panel d is the global wavelet power. Purple text indicates peaks above the 95% confidence level (dashed black line). The dotted black lines are used to determine the extension of each peak.