The effects of the acoustic resonance induced in the atmosphere

Abstract

After strong earthquakes and volcanic eruptions, geomagnetic oscillations with frequencies of 3.5–4.0 mHz have sometimes been observed. In this paper, we theoretically study the probable cause of these phenomena, which is related to the vertical acoustic resonance arising between the Earth’s surface and the thermosphere due to the propagation of the atmospheric wave corresponding to the acoustic branch generated by surface displacements. In the plane layered model of the atmosphere and ionosphere with inclined geomagnetic field, we analyze the propagation of two-dimensional (2D) harmonic acoustic wave. The height of the reflecting atmospheric layer corresponds to the region of sharp temperature change close to the thermosphere boundary ∼80–90 km. In this case, the calculated fundamental resonant frequency is close to the frequencies of the observed oscillations. The solution of this problem is used to calculate currents and electromagnetic perturbations in the atmosphere and ionosphere. The ionospheric E-layer is considered in the thin layer approximation. In this approximation, the formulas describing the geomagnetic perturbations (GMP) in the ionosphere and on the Earth’s surface are derived. The GMP spectrum contains a sharp maximum at the frequency corresponding to the acoustic resonance. According to the calculations, close to the resonance frequency, the spectral powers of GMPs on the ground can reach 5–30 nT2/Hz, which is consistent with the results of ground-based measurements.