Horizontal extension of acoustic resonance between the ground and the lower thermosphere

Abstract

A time-dependent, two-dimensional, nonlinear, non-hydrostatic, compressible, numerical model is used to investigate the horizontal extension of acoustic waves trapped in a thermal duct between the lower thermosphere and the ground or the stratosphere, and evanescent and gravity waves around them. The simulation shows that an impulsive point source on the ground excites acoustic waves ducted in the mesosphere. The ducted acoustic waves have horizontal wavelengths longer than 130km and frequencies of 3.7–3.8mHz. This ducted acoustic waves are identified as acoustic resonance modes expected in previous studies. They extend to 300km distance from the source after 2h. In the case of the source with a finite width, the acoustic resonance would extend to 300km out of the edge of the source. The temporal variation of two resonance modes is also declared. In the vicinity of the source, two acoustic resonance modes are formed which have frequencies of 3.7 and 4.4mHz. The mode of 4.4mHz is dominant above the mesopause for about 50min after the impulsive perturbation of the source since the mode is leaky. On the other hand, the mode of 3.7mHz is well trapped in the thermal duct and persist over 2h. In addition to the acoustic waves, evanescent gravity waves with horizontal wavelengths shorter than 100km and with frequencies between 2.4 and 3.6mHz are excited in the stratosphere. Gravity waves freely propagating are also excited with frequencies lower than 2mHz and with horizontal wavelengths shorter than 130km.