Geomagnetic Effect of the Atmospheric Acoustic Resonance Excited by Earthquakes and Volcano Eruptions

An analytical model of the vertical acoustic resonance between the thermosphere and the earth's surface and its geomagnetic effect has been constructed. Resonance with a frequency of several mHz occurs when an atmospheric acoustic wave (AW) is excited by vibrations of the earth or ocean surface. The derived analytical relationships give a possibility to examine the dependence of the fundamental resonant frequency on the height of the reflecting atmospheric layer and horizontal AW number. Geomagnetic disturbances caused by the impact of AW on the ionosphere are calculated within the framework of a multi-layered model of the ground-atmosphere-ionosphere system. The E-layer of the ionosphere is treated in the approximation of a thin layer with an inclined geomagnetic field, while the topside ionosphere is assumed to consist of a cold collisionless plasma. The dependence of the spectral power of magnetic perturbations on the direction of the horizontal AW propagation is examined. The magnetic perturbation spectra are shown to have maxima at frequencies close to the acoustic resonance frequency. The spectral powers of magnetic perturbations and barometric variations measured during the Iwate-Miyagi Nairik earthquake are in accordance with the model predictions for realistic media and AW parameters. Field-aligned currents that arise when AWs enter the ionospheric E-layer is shown to carry measurable electromagnetic disturbances into the geomagnetically conjugated region. Geophysical phenomena with large energy yield, such as earthquakes and volcano eruptions, can excite specific oscillations of the geomagnetic field and ionospheric plasma with frequencies around a few mHz. These oscillations are spatially localized in the vicinity of earthquake/volcano epicenter and last up to 1-2 hr. This phenomenon was interpreted as the vertical resonance of the acoustic wave (AW) trapped between the ground/ocean surface and thermosphere (& SIM;80 km). It is important to know geomagnetic response to the acoustic resonance because it will give possibility to use data from magnetometer array for in-depth study of this phenomenon. Presented theoretical model of the acoustic resonance provides analytical relationships to examine the dependence of geomagnetic signature of the acoustic resonance on atmosphere/ionosphere parameters. These simple relationships anybody can use to estimate without sophisticated computer modeling an expected geomagnetic effect of the acoustic resonance for any atmosphere/ionosphere parameters. Recorded spectral powers of magnetic perturbations and variations of atmospheric pressure are found to be in accordance with the model predictions. Currents along the geomagnetic field line that arise when AW interacts with ionospheric plasma can carry electromagnetic disturbances into the opposite hemisphere. This theoretical prediction has been confirmed by observations by Pacific magnetometers. Theory describes analytically magnetic pulsations observed after seismic/volcano events driven by vertical acoustic resonance in atmosphereA dominant magnetic component of pulsations excited on the ground is directed along the horizontal wave vector of the acoustic waveTheory predicts that ionospheric field-aligned currents can cause detectable magnetic response to acoustic resonance in conjugate ionosphere