Fig. 3.

Upper panel: two filtered slant TEC time series. Bottom panel: Corresponding spectrograms. (a) For station 0979 observing satellite 15. It shows the primary gravito-acoustic pulse and two signals that oscillates with frequencies close to the two fundamental acoustic resonance frequencies (₀S₂₉ and ₀S₃₆) at ~3.7 and ~4.4 mHz. (b) For station 0180 observing satellite 22. It shows a signal that oscillates with a dominant frequency of ~1.8 mHz.