Time-distance stack plots obtained from AIA 193 Å, 171 Å, and 211 Å running-difference images along sectors S1 (top) and S2 (bottom), as shown in Fig. 1b. The horizontal dashed lines indicate the positions for extracting signals for wavelet analysis, whereas the oblique dotted red lines depict the linear fitting results for estimating the speeds of wavefronts. The dashed green lines in the top panels represent the boundary of the southern polar CH.