RADIO PHYSICS AND RADIO ASTRONOMY

PACS numbers: 93, 96.30.s 

Purpose: The study is concerned with the features of variations in the amplitude of infrasound wave propagating through the terrestrial atmosphere at global-scale distances. As a powerful source of infrasound, the passage and explosion of the Tunguska celestial body that occurred in the atmosphere at an altitude of 6–10 km over the Central Siberia on June 30, 1908 have been chosen. The aim of this study is investigating the dependence of the amplitude on distance, developing simple approximating relations between the pressure in the infrasound wave and the distance between the infrasound source and the infrasound detector location.

Design/methodology/approach: The data acquired from the Europe-
Asia array of 23 microbarographs located at distances of 0.49–35 Mm from the Tunguska catastrophe site are used to construct the correlation diagrams for the distance and amplitude, which are used to study the dependence of the infrasound amplitude on distance. In analyzing the dependence of the infrasound amplitude on the distance between the source generating infrasound and the location of the infrasound detector, the model for weakening the amplitude with distance, which best fits the observations, has being found. The following models for propagation of infrasound waves have been tested: propagation in free space with a spherical wavefront, propagation in the air-Earth boundary waveguide with a cylindrical wavefront, propagation with a gradual transition from a spherical divergence to a cylindrical one (both neglecting and accounting for attenuation).

Findings: The relations approximating the dependence of the infrasound amplitude on distance have been developed for the different models of propagation of infrasound waves along the Earth’s surface in the distance range of 0.49–35 Mm. For the base models of the global-scale propagation of infrasound waves, the following models have been chosen: (1) propagation of a spherical wavefront without attenuation, (2) propagation of a cylindrical wavefront without attenuation, (3) the combination of a spherical wavefront and a cylindrical wavefront without attenuation, (4) beam spreading loss described by an arbitrary power law without attenuation, (5) propagation of a cylindrical wavefront with attenuation, (6) propagation of a spherical wavefront with attenuation. The approximating relations obtained have been analyzed and compared. The rate of attenuation is estimated for a cylindrical and spherical wavefronts in the ground–stratosphere and the ground–thermosphere waveguides. It is equal to about 0.16 Mm^–1 and 0.17 Mm^–1, respectively.

Conclusions: The following conclusions have been drawn from this study: (1) the dependence of the amplitude of the infrasound wave generated by the Tunguska celestial body on distance is determined to be complex and difficult to fit with simple mathematical relations based on reasonable physical models of the propagation of infrasound waves along the Earth’s surface at global-scale distances, (2) the intercomparison of the approximating relations determined permitted the selection of preferable relations from their entire set. To these latter belong the relations based on the following models for propagation of infrasound waves in the waveguides formed by the ground and atmospheric layers (primarily, by the stratosphere, and, to a lesser degree, by the thermosphere): (1) a spherical wavefront gradually becoming a cylindrical wavefront, and (2) a cylindrical wavefront with attenuation.

Key words: Tunguska celestial body, infrasonic wave, wave amplitude, amplitude dependence on distance, amplitude wave attenuation model, spherical wavefront, waveguide propagation, cylindrical wavefront, power law divergence of the wavefront, wave attenuation, attenuation coefficient, approximating relations

Manuscript submitted  26.01.2018

Radio phys. radio astron. 2018, 23(2): 94-103

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