The Effect of Atmospheric Acoustic Resonance Induced by Earthquakes and Volcanic Eruptions on the Ionosphere and Geomagnetic Field

Abstract—It has been observed that strong earthquakes and volcanic eruptions are sometimes followed by geomagnetic oscillations with frequencies of 3.5–4.0 mHz. This paper describes the theoretical study of the probable cause of these phenomena, which is related to the vertical acoustic resonance between the Earth’s surface and the thermosphere, produced by the propagation of the atmospheric wave corresponding to the acoustic branch generated by surface displacements. The propagation of two-dimensional (2D) harmonic acoustic wave is analyzed in a plane layered model of the atmosphere and ionosphere with oblique geomagnetic field. The altitude of the reflecting atmospheric layer corresponds to the region of a sharp temperature change in the vicinity of the thermosphere boundary ~80–90 km. The calculated fundamental resonant frequency is close to the observed oscillations. The obtained solution is used to calculate currents and electromagnetic perturbations in the atmosphere and ionosphere. Assuming thin-layer approximation for the ionospheric E-layer, we derive formulas describing the geomagnetic disturbances (GMD) in the ionosphere and on the Earth’s surface. The GMD spectrum has a sharp peak at the frequency corresponding to the acoustic resonance. According to the calculations, close to the resonance frequency, the power spectral density of GMD on the ground can reach 5–30 nT²/Hz, which is consistent with the results of ground-based measurements.