Abstract
Abstract. We develop theoretical basics of active experiments with two beams of acoustic waves, radiated by a ground-based sound generator. These beams are transformed into atmospheric acoustic gravity waves (AGWs), which have parameters that enable them to penetrate to the altitudes of the ionospheric E and F regions where they influence the electron concentration of the ionosphere. Acoustic waves are generated by the ground-based parametric sound generator (PSG) at the two close frequencies. The main idea of the experiment is to design the output parameters of the PSG to build a cascade scheme of nonlinear wave frequency downshift transformations to provide the necessary conditions for their vertical propagation and to enable penetration to ionospheric altitudes. The PSG generates sound waves (SWs) with frequencies f1 = 600 and f2 = 625 Hz and large amplitudes (100–420 m s−1). Each of these waves is modulated with the frequency of 0.016 Hz. The novelty of the proposed analytical–numerical model is due to simultaneous accounting for nonlinearity, diffraction, losses, and dispersion and inclusion of the two-stage transformation (1) of the initial acoustic waves to the acoustic wave with the difference frequency Δf = f2 − f1 in the altitude ranges 0–0.1 km, in the strongly nonlinear regime, and (2) of the acoustic wave with the difference frequency to atmospheric acoustic gravity waves with the modulational frequency in the altitude ranges 0.1–20 km, which then reach the altitudes of the ionospheric E and F regions, in a practically linear regime. AGWs, nonlinearly transformed from the sound waves, launched by the two-frequency ground-based sound generator can increase the transparency of the ionosphere for the electromagnetic waves in HF (MHz) and VLF (kHz) ranges. The developed theoretical model can be used for interpreting an active experiment that includes the PSG impact on the atmosphere–ionosphere system, measurements of electromagnetic and acoustic fields, study of the variations in ionospheric transparency for the radio emissions from galactic radio sources, optical measurements, and the impact on atmospheric aerosols. The proposed approach can be useful for better understanding the mechanism of the acoustic channel of seismo-ionospheric coupling.
Highlights
The generation of low-frequency sound waves in advance of earthquakes forms a part of the modern description of the processes in the upper crust
The influence on ionospheric transparency for the electromagnetic VLF and HF wave caused by the parametric soundgenerator-excited acoustic waves has been observed in a number of experiments
This effect was observed for electromagnetic waves artificially excited from the ground and detected at ground-based observatories and satellites in different seasons, days, and times of day
Summary
The generation of low-frequency sound waves in advance of earthquakes forms a part of the modern description of the processes in the upper crust. A paper by Xia et al (2011) described 240 abnormal infrasound signals that have been observed in Beijing and, in total, 92 cases worldwide for earthquakes with M ≥ 7.0 during 2002–2008. It is found that about 85 % of the earthquakes are preceded by infrasound signals 1–30 days in advance, whilst earthquakes with M ≥ 7.6 are most likely led by infrasound 1–10 days. Rapoport et al.: Ground-based acoustic parametric generator impact in advance. According to Xia et al (2011), the frequency of co-seismic infrasound waves is roughly equal to 1 Hz, while the frequency of waves observed before earthquakes is lower and in the 0–0.005 Hz frequency band
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