Numerical Study of Global ELF Electromagnetic Wave Propagation with Respect to Lithosphere–Atmosphere–Ionosphere Coupling

Zhuangkai Wang,Yi Liu,Xuhui Shen,Xiang Xu,Chen Zhou,Li Liao,Moran Liu,Shufan Zhao

doi:10.3390/rs13204107

Abstract

Before and after earthquakes, abnormal physical and chemical phenomena can be observed by gathering ground-based and satellite data and interpreted by the lithosphere–atmosphere–ionosphere coupling (LAIC) mechanism. In this study, we focused on the mechanism of LAIC electromagnetic radiation and investigated the seismic electromagnetic (EM) wave generated in the lithosphere by earthquakes and its global propagation process from the lithosphere through the atmosphere and into the bottom of ionosphere, in order to analyze the abnormal disturbance of ground-based and space-based observation results. First, analytic formulas of the electrokinetic effect were used to simulate the generation and propagation process of the seismic EM wave in the lithosphere, interpreted as the conversion process of the seismic wave and EM wave in porous media. Second, we constructed a three-dimensional Earth–ionosphere waveguide by applying the finite-difference time-domain (FDTD) algorithm to model the global propagation process of the seismic EM wave into the atmosphere and cavity between the bottom of the ionosphere and the surface of the Earth. By combining the model of the electrokinetic effect in the lithosphere with the numerical model of the Earth–ionosphere waveguide in the atmosphere and ionosphere, we numerically simulated the global transmission process of extremely low-frequency (ELF: 3 Hz–3000 Hz) EM waves which are related to earthquakes. The propagation parameters of coseismic ELF EM waves with different duration times and center frequencies were analyzed and summarized. The simulation results demonstrate that the distribution characteristics of an electric field along longitude, latitude and altitude with time are periodic and the time interval during which an EM wave travels around the whole Earth is approximately 0.155 s when adopting the conductivity of the knee profile. We also compared the observation data with the simulation results and found that the attenuating trends of the ELF electric field are consistent. This proposed ELF EM wave propagation model of lithosphere–atmosphere–ionosphere coupling is very promising for the explanation of abnormal disturbances of ground-based and space-based observation results of ELF EM fields which are associated with earthquakes.

Highlights

Seismo-ionospheric effects have become a topic of intensive research in recent years since researchers observed a large number of abnormal ionospheric disturbance events before and after earthquakes which may be related to earthquakes
The scientific interpretation of the lithosphere–atmosphere– ionosphere coupling (LAIC) mechanism mainly focuses on three aspects: first, that anomalies in the lithosphere, atmosphere and ionosphere are caused by the penetration of the electric field or current induced by earthquakes, namely the LAIC direct current (DC) electric field mechanism; second, that anomalies are caused by the upward propagation of acoustic gravity waves generated by earthquakes, namely the LAIC acoustic gravity wave (AGW) mechanism; third, that anomalies are caused by abnormal EM radiation, namely the LAIC electromagnetic radiation mechanism
Scholars have put forward lots of physical hypotheses attempting to interpret these abnormal phenomena, such as the propagation of positive holes [6,7] and radon emanation to explain the LAIC DC electric field mechanism preceding earthquakes; internal gravity wave theory [8] to explain the LAIC AGW mechanism after earthquakes; and the electrokinetic effect [9,10] to explain the LAIC electromagnetic radiation mechanism after earthquakes

Summary

Introduction

Seismo-ionospheric effects have become a topic of intensive research in recent years since researchers observed a large number of abnormal ionospheric disturbance events before and after earthquakes which may be related to earthquakes. The scientific interpretation of the LAIC mechanism mainly focuses on three aspects: first, that anomalies in the lithosphere, atmosphere and ionosphere are caused by the penetration of the electric field or current induced by earthquakes, namely the LAIC direct current (DC) electric field mechanism; second, that anomalies are caused by the upward propagation of acoustic gravity waves generated by earthquakes, namely the LAIC acoustic gravity wave (AGW) mechanism; third, that anomalies are caused by abnormal EM radiation, namely the LAIC electromagnetic radiation mechanism These three physical mechanisms have been verified by corresponding observational results [3,4,5]. Various models have been proposed to simulate the corresponding physical mechanisms, including the electric field penetration model in ref. [11] and the acoustic and gravity wave propagation model in ref. [12]

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Conclusion