Vertical acoustic resonance in the atmosphere as a source of Pc5 geomagnetic pulsations

Abstract

Some strong earthquakes and volcano eruptions are accompanied by atmospheric pressure variations and geomagnetic perturbations (GMPs), which contain spectral spikes in the Pc5 frequency range. GMPs are usually observed in epicentral areas of several hundred kilometers in size and in a geomagnetically conjugated region. This effect can be explained by the impact on the ionosphere of an atmospheric acoustic wave caused by seismic vibration of the earth's surface. A partial reflection of the acoustic wave from the region of a sharp change in atmospheric temperature near the bottom of the thermosphere can lead to a vertical acoustic resonance (VAR) in the area between the earth's surface and the thermosphere boundary. The acoustic wave partially penetrates into the ionosphere, thereby exciting ionospheric currents and GMPs in the Pc5 frequency range. In this study we develop a plane-stratified model of the atmosphere and ionosphere in order to examine spectrum of the first VAR harmonics. A set of 2D gas dynamics equations is treated to derive dispersion relations which determine the VAR resonant frequencies. Ionospheric currents and GMP are found in the approximation of a thin ionospheric E-layer with an inclined geomagnetic field. Analysis of the VAR dispersion relation shows that the resonant frequencies of the first three harmonics lies within 3.7−5.9 mHz, that is, in the Pc5 frequency range, while their logarithmic decrements range from zero to 0.6. The weak attenuation of VAR harmonics can explain the observed long duration of VAR and GMPs after a seismic event. Unlike previous studies, the effect of wind in the upper atmosphere is taken into account. It has been found that wind velocity weakly influences the resonant frequencies but it may greatly affect the attenuation coefficients of VAR harmonics. The model predicts a similar shape of GMPs detected near the epicenter of strong seismic events and in the magnetically conjugated arears, which is consistent with geomagnetic response to VAR detected by ground-based magnetometers.