Acoustic and Acousto-Gravity Wave Pulses Caused by Sources of Seismic Origin

The nonlinear excitation of an ultra-low frequency (ULF, ≤ 1 Hz) acoustic wave (AW) by extremely low frequency (ELF ∼ 10–1000 Hz) acoustic bursts is discussed. The ELF wave is excited as a burst-like envelope of finite transverse scale by the seismic motion of the Earths surface. Then, it propagates upwards and is subject to both non-linearity and dissipation. The non-linearity leads to the generation of higher harmonics and, thus, to a sawtooth-like wave structure, and to an increase of the ULF part of the wave spectrum at altitudes 0–100 km. Only the ULF AW can reach the ionosphere dynamo layer (120–200 km), where it excites ionospheric plasma waves. Simulations of the nonlinear transformation of the acoustic waves are based on the slowly varying profile equation. In the present paper, a simulation of the propagation of acoustic gravity wave (AGW) monopulses is described and the role of sources of various origins and corresponding boundary conditions are discussed. The results indicate that dispersion and diffraction of the wave change the initial shape of the pulse quickly, so that the envelope-like pulse can reach the F-layer of the ionosphere with some delay, in comparison with the AW. The period of the oscillations is about 1–5 min, and the velocity of the particles within the AGW may be > 1 m/s. The vertical and horizontal components of that velocity are of the same order, but they have different distributions.