Panel (a) shows a snapshot of the main sunspot of NOAA 12127 at the passband of −0.4 Å off the Hα spectral line center, on which three umbrae of interest are highlighted by U1–U3. Four virtual slots (denoted by S1–S4) and five virtual slits are also marked to be investigated. Evolutions of the averaged intensities within the slots are shown in panel (b), where the curves for S3 and S4 are plotted with a shift of −0.2. Panels (c) and (d) show the cross-wavelet transform for two pairs of curves, S1 and S2, and S3 and S4, respectively. The 95% confidence level against noise is shown with thick contours. The phase difference is shown as arrows (with in-phase pointing right and anti-phase pointing left). Panels (e) and (f) show cross-correlation for the above two pairs of curves.