Fig. 4.
(a) Left panel: dispersion curve (frequency versus angular order) of the modes of the Earth-atmosphere system, computed using a standard atmosphere model (USSA with free surface at 150 km) and reprinted from Lognonné and Clévédé (2002). Right panel: zoom on the dispersion curves (frequency versus wavelength) of the two acoustic branches of trapped modes (up), with color-coded quality factor Q, and branches of atmospheric gravity modes (bottom), with color-coded group velocity vg, computed using the MSISE-00 (thermosphere of the day and radiating a surface at a 600 km altitude. The plain magenta line shows the atmospheric gravity modes that propagate at ~225 m/s. The tsunami modes propagating at 171 m/s are also shown in dashed magenta line. (b) Multitaper power spectrum of 4 hours starting after the shock wave for station 0979 observing satellite 15 on the top and for station 0180 observing satellite 22 on the bottom. The 3 first multitapers are used (Slepian, 1978). The first resonance is a peak at 3.73 mHz which corresponds to the theoretical acoustic fundamental trapped modes of angular order of about 150 (about 265 km of wavelength) for the MSISE atmosphere of the day. The second peaks at about 4.5 mHz and correspond to the first overtone. Note the doublet structure in this second resonance, a likely consequence of the vicinity of the theoretical first and second harmonics of the atmospheric wave guide e.g. Lognonné et al. (1998).