ACOUSTIC-RESONANCE OF THE ATMOSPHERE AT 3.7 MHZ

Vertical propagation of plane acoustic-gravity waves with horizontal wave fronts is discussed using a layer model with realistic temperature structure based on the Cospar International Reference Atmosphere, 1986. Acoustic resonance of a normal mode occurs between the ground and upper mesosphere, at a frequency of 3.64-3.69 mHz. Since acoustic waves with these frequencies are evanescent in the upper mesosphere where the acoustic cutoff is normally around 4.1 mHz, part of the wave energy propagated from below is reflected downward to interfere with the upward-propagating waves, thus causing modal resonance. The computed resonant frequency coincides with the lower (3.7 mHz) of the two dominant frequencies found in the Rayleigh wave from the eruptions of El Chichon in 1982 and Mount Pinatubo in 1991. It also agrees with the lower spectral component of the ionospheric infrasonic waves observed by the high-frequency Doppler radar technique in association with severe convective storms below in the troposphere.