Over the last century, abnormal electromagnetic (EM) emissions associated with earthquake (EQ) activities have been widely reported and recorded by ground-based and satellite observations. The mechanism of extremely low-frequency (ELF) EM waves radiated from earthquakes has been gradually established. However, whether EM waves radiated from earthquakes can be detected by low Earth orbit (LEO) satellites remains controversial. In this paper, to address these concerns, a lithosphereatmosphere-ionosphere model of ELF wave propagation is constructed. The features of the simulated EM field at LEO satellite altitudes radiated from earthquakes have been studied. The simulated EM field at the altitude of the China Seismo-Electromagnetic Satellite (CSES) is compared with the sensitivity of electromagnetic (EM) sensors onboard the CSES. The results illustrate that an earthquake with a magnitude over 6.0 can be detected by the EM sensors of the CSES. However, this depends on the focal depth, seismogenic environment and ionospheric parameters. earthquakes, CSES satellite, sensitivity, trans-lithosphere-atmosphere-ionosphere ELF wave propagation model Citation:Zhao S F, Shen X H, Liao L, et al. A lithosphere-atmosphere-ionosphere coupling model for ELF electromagnetic waves radiated from seismic sources and its possibility observed by the CSES.

The factors affecting the reflection and transmission coefficient of the ionosphere have been analyzed. These factors include wave frequency, incident angle, geomagnetic inclination, electron density and collision frequency in the ionosphere. The ionosphere refractive index is also analyzed. The ionosphere above 70 km is considered to be homogeneous and anisotropic, and the reflection and transmission coefficient matrix is calculated using matrix method. Simultaneously the Booker quartic equation is solved to get the refractive index in the ionosphere. The results show that when the wave frequency is higher, it is easier to penetrate into the ionosphere from its bottom boundary and the propagation attenuation in the ionosphere is smaller. TE (traverse electric) wave and TM (traverse magnetic) wave can both penetrate into the ionosphere with a small incident angle, while TE wave can hardly transmit into the ionosphere when the incident angle is large. The transmission coefficient decreases as the geomagnetic inclination increases. TE and TM wave cannot penetrate into the ionosphere at magnetic equator. When the electron collision frequency is higher, it is easier for VLF wave to penetrate into the ionosphere and the attenuation of ordinary wave is weaker, which may be caused by the energy transportation between the waves and the particles. The ordinary (O) wave experiences severer attenuation than extraordinary (X) wave, and X wave is a penetration mode whereas O wave is a non-penetration mode in the ionosphere. All the results indicate that VLF wave with higher frequency is easier to penetrate into the ionosphere and to be recorded by the satellites at high latitude. It is hard for ULF and the lower frequency VLF wave to transmit into the ionosphere directly for the severe reflection and attenuation. It may transmit into the ionosphere with a small incident angle due to the nonlinear effect, for example, the interaction between the waves and the particles or cross modulation, and then propagate along the whistle duct with small attenuation. This work may be a preliminary theoretical exploration for the future calculation on the response of ground based VLF artificial transmitter in the ionosphere and further study on the seismic ionosphere coupling model.

Earthquakes may disturb the lower ionosphere through various coupling mechanisms during their seismogenic and coseismic periods. The VLF signal radiated from ground-based transmitters is affected when it passes near the disturbed region above the seismogenic area, and this anomaly can be recorded by ground-based VLF receivers. In this paper, the seismic anomalies before two strong earthquakes (M>7) that occurred in western China were detected using the ground-based observation of VLF signal; the possible reasons for the anomalies were discussed using full wave simulation. The amplitude of the VLF signals observed by the link between NOV, KHA transmitter, and VLF receivers at Ya’an and Tonghai show obvious anomaly by nighttime fluctuation analysis. The simulated results demonstrate that the anomalies could have been induced by ascending/descending of the bottom height of the ionosphere, caused by depletion/increase in D region electron density. The simulated result also illustrates that terminator time shift could have been induced by descending of the bottom boundary of the ionosphere, which is due to modal interference between different wave modes.

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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.