Short-term ionospheric precursors of earthquakes using vertical and oblique ionosondes

Abstract

The possibility to use short-term ionospheric parameters as a new precursor of earthquakes (EQs) is examined both theoretically and experimentally by using ground-based vertical and oblique ionosondes and broadcasting stations operating in the frequency ranges of 5–40 MHz and 150–180 kHz, respectively. A rigorous self-matching three-dimensional (3D) electrodynamic model of interaction of the acoustic branch of acoustic-gravity waves (AGWs) generated by the lithosphere–atmosphere interactions during the seismic event preparation, with the cold dense ionospheric plasma, accompanied by the ambient magnetic and perturbed electric fields (both in vertical and horizontal directions), was constructed in conjunction with the actual height variations of the plasma particles mobility and conductivity tensors in the ionosphere. We present a generation mechanism of ionospheric inhomogeneities with different scales and with various degree of perturbations, Δ N/ N 0 = 10 −3−10 −4 ( N 0 is the concentration of electrons (or ions) of the background ionospheric plasma and Δ N, its perturbation). A 3D model of probing UHF and HF wave scattering from the disturbed ionospheric area above the seismic epicenter is proposed both for oblique and vertical propagation through the disturbed area, based on the well-known 2D models. The corresponding observations are arranged by using the oblique and vertical ionosondes, and the broadcasting stations with traces passing through the seismic ionospheric area above the epicenter. The anomalous effects of ionospheric plasma, quasi-regular structure stratification and generation of additional sporadic layers in whole thickness of the ionosphere are observed 20–24 hours before the seismic event using sounding the disturbed ionosphere by radio signals operated in LF-frequency band from 150 to 180 kHz and in HF-frequency band from 5 to 30–40 MHz. We observe an additional effect of irregular “flashes” and sharp fading of broadcasting signals at 150–180 kHz, passing through the disturbed ionosphere above the epicenter several hours before the seismic event. The plasma density disturbances with respect to the background ionospheric plasma, the “worked” volume and area of disturbed ionosphere above the epicenter, as well as the magnitude of seismic event and the area of seismic zone, can be estimated using both models and the results of experimental measurements.