In this study, we investigate the very low frequency (VLF) signals in the ionosphere propagating from ground‐based transmitters by using the data from the newly launched China Seismo‐Electromagnetic Satellite (CSES), which is also called ZH‐1. The simultaneous ionospheric perturbations and electron precipitations induced by the NWC transmitter are also analyzed, and the possible mechanisms are discussed. The ZH‐1 and DEMETER observations of VLF transmitters are compared morphologically, which are generally consistent with the full‐wave simulations. The results show that the electromagnetic waves excited by the VLF transmitters penetrate into the ionosphere and propagate in the magnetosphere to its conjugate hemisphere as a ducted or nonducted whistler. When the VLF transmitter is located at L < 1.5, the wave propagates more easily as a nonducted whistler. When the wave propagates as a ducted whistler, the point of maximum observed transmitter‐induced electric field in the conjugate hemisphere will move poleward in the northern hemisphere and equatorward in the southern hemisphere. The results also show that the data stability of electromagnetic field recorded by ZH‐1 satellite is almost the same as DEMETER satellite, and there is a good correlation between the ZH‐1 observation and simulated results. These conclusions demonstrate that the nightside VLF band electromagnetic observation of ZH‐1 satellite is stable and reliable.

The quasiperiodic (QP) waves accompanied by simultaneous energetic electron precipitations in the high‐latitude ionosphere were recorded by the sun‐synchronous circular orbit China Seismo‐Electromagnetic Satellite (CSES). The new features of QP waves observed by CSES are the well‐pronounced rising‐tone structures and very short repetition periods, which are not ofen reported by previous studies. The repetition period of QP waves varies from ~1 to over 20 s, with wave spectral properties displaying dynamic structures and clear cutoff frequencies. The majority of QP waves appear at geomagnetic latitudes from ~50° to 65°, and L shell from ~3.5 to 4, mostly inside the plasmapause. The QP waves obliquely propagate towards decreasing L shell directions with right‐handed polarization, with wave normal angles varying from ~30° to 50°. Out of the 68 events examined, 19 of them show synchronous variations of ultralow frequency (ULF) magnetic field pulsations in certain portions of the wave event. The energetic electrons predominately precipitate in a double‐peak pattern (~400 to 550 keV and ~700 to 800 keV). No clear periodic precipitating fluxes were found. The estimated time interval of energetic electrons driving free energy to modulate the whistler‐mode waves varies from 0.04 to 30°s, which is roughly consistent with the observed repetition periods (10 to over 20 s). For the fast repetition period (~1 to 2s) QP wave, it is likely caused by the bouncing of the extremely/very low frequency (ELF/VLF) whistler‐mode wave packets from the conjugate ionosphere.

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.

Earthquakes may disturb the lower ionosphere through various coupling mechanisms during the seismogenic and coseismic periods. The VLF (very low-frequency) signal radiated from ground-based transmitters will be affected when it penetrates the disturbed ionosphere above the epicenter area, and this anomaly can be recorded by low-Earth orbit satellites under certain conditions. In this paper, the temporal and spatial variation of the signal-to-noise ratio (SNR) of the VLF transmitter signal in the ionosphere over the epicenter of 2010 Yushu Ms 7.1 earthquake in China is analyzed using DEMETER (Detection of Electro-Magnetic Emission Transmitted from Earthquake Regions) satellite observation. The results show that SNR over the epicenter of the Yushu earthquake especially in the southwestern region decreased (or dropped) before the main shock, and a GPS–TEC (Global Positioning System; total electron content) anomaly accompanied, which implies that the decrease in SNR might be caused by the enhancement of TEC. A full-wave method is used to study the mechanism of the change in SNR before the earthquake. The simulated results show SNR does not always decrease before an earthquake. When the electron density in the lower ionosphere increases by 3 times, the electric field will decrease about 2 dB, indicating that the disturbed-electric-field decrease of 20 % compared with the original electric field and vice versa. It can be concluded that the variation of electron density before earthquakes may be one of the important factors influencing the variation of SNR.

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.