Fig. 3

Power spectral density calculated by the MEM from continuous data for each 1 h with elevation angle more than 50 degrees. The enhanced PSD peaks encircled by red broken line have the acoustic wave periods. The enhanced PSD peaks encircled by a blue broken line may be caused by the internal gravity waves. Note that the PSD intensities are plotted with tapering factor \(\mathrm{F}={T}^{1.5}\), where T is the period (s), to plot with a linear scale rather than a log-scale in one frame. After the squall around 08UT on March 18, 2017, the PSDs show clear enhancement