Left: temporal variations of the Hα vertical velocity (a), Ti II vertical velocity (b), and TiO intensity (c) at the position marked with a cross symbol in Fig. 5. Right: wavelet power spectra of Hα vertical velocity (d) and Ti II vertical velocity (e). The purple contours represent the 95% local confidence level. The coherency between the two time series of velocity is shown by contours in (f). The phase differences at the points of coherency above 0.7 are denoted by the directions of the arrows.