This paper is related to the use of ionospheric density variations to tentatively predict earthquakes. The results of this statistical analysis are presented as a function of various parameters. The ion density was recorded by the low‐altitude satellite DEMETER during more than 6 years, and a search for anomalies was automatically conducted with the complete data set. In a second time, a software checked if each anomaly could correspond to an earthquake. The search was conducted at less than 1500 km from the anomaly positions, and until 15 days after the anomaly time. The earthquakes have been classified depending on their magnitude, depth, and position (below the sea or inland). This attempt to predict earthquakes of course generates a lot of false alarms and wrong detections. Nevertheless, it is shown that the number of good detections increases with the magnitude of the earthquakes. In average the number of perturbations is higher the day of the earthquake, and then smoothly decreases for the days before. Earthquakes below the sea are better detected. There are seismic areas close to the South Atlantic Magnetic Anomaly and at high latitudes where the number of natural perturbations is too important to expect a high number of good detections. Finally, when there are several perturbations corresponding to a single earthquake, it is possible to combine their positions to have a better estimation of the location of the future epicenter. However, uncertainties about the time and the magnitude are large.

Oppositely to a previous statistical work using a single time resolution of the total ion density measured onboard the Detection of Electro‐Magnetic Emissions Transmitted from Earthquake Regions (DEMETER) satellite, this work deals with statistical seismo‐ionospheric influences by comparing different parameters and various time resolutions. The O⁺ density and electron density recorded by the China Seismo‐Electromagnetic Satellite (CSES) for more than one year and by the DEMETER satellite for about 6.5 years have been utilized to globally search ionospheric perturbations with different time resolutions. A comparison is automatically done by software between the occurrence of these ionospheric perturbations determined by different data sets, and the occurrence of earthquakes under the conditions that these perturbations occur at less than 1,500 km and up to 15 days before the earthquakes. Combined with statistical results given by both satellites, it is shown that the detection rate r of earthquakes increases as the data time resolution and the earthquake magnitude increase and as the focal depth decreases. On average, the number of perturbations is higher the day of the earthquake, and then smoothly decreases the days before, which is independent of either ionospheric parameters or time resolutions. The number of right alarms is high near the South Atlantic Magnetic Anomaly area but its relationship with seismic activities is weak. The ion density tends to be more sensitive to seismic activities than the electron density but this needs further investigations. This study shows that the CSES could effectively register ionospheric perturbations due to strong EQs as the DEMETER satellite does.

A large number of various precursors have been reported since the Wenchuan M_S 8.0 earthquake (EQ) took place on 12 May 2008 in China. In this work, previous investigations of both ground-based electromagnetic (EM) parameters and spatial ionospheric parameters were first examined. The statistical results showed that various anomalies presented different time-scale variations but tended to be characterized by a common feature – reaching their climax on 9 May, three days before the Wenchuan event, which indicates a lithosphere–atmosphere–ionosphere (LAI) electromagnetic coupling. Second, the fluctuations on 9 May based on the observational ground-based ultra low frequency (ULF) electrical field at the Gaobeidian (GBD) station and the direct current/ultra low frequency (DC–ULF) geomagnetic vertical Z field at the Chengdu (CD) station were comparably analyzed with those of ionospheric disturbances reported previously. The results showed that distinct electromagnetic changes, geomagnetic “double low-point” phenomena, and ionospheric disturbances above both sides of the Earth started in turn, respectively, but reached their climax simultaneously within dozens of hours on 9 May. This evolutionary process increases the probability that electromagnetic energy propagates from the epicentral area, via the atmosphere and ionosphere, to the equatorial plane, and through this plane finally to its magnetically conjugated area in the opposite hemisphere, causing electromagnetic disturbances on the Earth’s surface, in the atmosphere, and in the ionosphere and its conjugate point, in that order.

This paper is related to the study of the ion density recorded by the low altitude satellite DEMETER. In a first time there is an automatic search for ionospheric perturbations in the complete satellite data set of ion densities. Then perturbations due to known ionospheric phenomena (for example, solar activity) are eliminated as well as perturbations not above a seismic zone. In a second time, there is a search to know if each selected perturbation corresponds to a future earthquake. The earthquakes have been classified depending on their magnitude and depth. This attempt to predict earthquakes of course generates false alarms and wrong detections. The results of this statistical analysis are presented as function of various parameters. It is shown that the number of false alarms is very important, because the ionosphere has variations not only linked to the seismic activity. The number of wrong detections is also important and can be explained by the fact that the satellite is above a seismic area only a few minutes per day and we do not expect continuous perturbations from a given earthquake. The more important results of this study is that the ratio between detected earthquakes and earthquakes to be detected increases with the magnitude of the earthquakes which intuitively makes sense.

This work presents ground based ultra low frequency (ULF) electromagnetic field measurements in the frequency range 0.1–10 Hz from January 2007 to December 2008. In this time period a strong earthquake series hits the Wenchuan region with a main shock of magnitude M_S = 8.0 on 12 May 2008. The Hebei ULF electromagnetic observation network includes eight observation stations in north China and the observation system named E-EM is employed to record the electric potential difference between two electrodes with an analog automatic real-time continuous pen recorder. First, weak electric signals appeared on 11 October 2007 at Ningjin station, most of which are with relative long periods ~0.4–3 s and unequal amplitudes ~0.5–20 mm. Then, similar signals appeared at Gaobeidian station at the end of October. Abnormal behavior with various time intervals appeared randomly and not every day. At the beginning of April 2008, one and a half months before the Wenchuan M_S = 8.0 earthquake, the anomalies were gradually subject to an intensive increase mainly in Gaobeidian SN direction and Ningjin EW direction. The abnormal behavior appeared almost every day and the amplitudes of electric signals, with short periods of ~0.1–0.3 s, enhanced to ~3–30 mm. Qingxian station started to record marginally high frequency signals in SN and EW components in the middle of April. On 9 May, 3 days before the main shock, the amplitude of high frequency information increased sharply at the same time in two components at Gaobeidian station and the maximum amplitude was up to 70 mm, i.e. 1.3 mV m⁻¹ for the electric field. This situation did not stop until 17 May, 5 days after the main event. However, this kind of climax phenomena did not happen at Ningjin station and Qingxian station. Then weak anomalous information lasted about four months again, and strong signals appeared again for a short time before several powerful aftershocks. It is the first time that an abnormity with so large an amplitude and so long a duration time in the observation history of this network though several strong earthquakes were recorded. Furthermore, no obvious interferences have been found during this period. So this event is possibly related to this shock although all these three stations are more than 1300 km away from the Wenchuan earthquake epicenter.

A three-layer (Earth–air–ionosphere) physical model, as well as a two-layer (Earth–air) model, is employed in this paper to investigate the ionospheric effect on the wave fields for a finite length dipole current source co-located at a hypocenter depth and along the main fault of an earthquake when the distance between the epicenter and an observing station is up to 1000 km or even more. The results show that all electrical fields are free of ionospheric effects for different frequencies in a relative short range, e.g., ∼ 300 km for f = 1 Hz, implying the ionospheric influence on electromagnetic fields can be neglected within this range, which becomes smaller as the frequency increases. However, the ionosphere can give a constructive interference to the waves passing through and make them decay slowly when an observation is out of this range; moreover, the ionospheric effect can be up to 1–2 orders of magnitude of the electrical fields. For a ground-based observable 1.3 mV m⁻¹ electric signal at f = 1 Hz 1440 km away from the Wenchuan M_S 8.0 earthquake, the expected seismo-telluric current magnitude for the Earth–air–ionosphere model is of 5.0 × 10⁷A, 1 magnitude smaller than the current value of 3.7 × 10⁸A obtained by the Earth–air model free of ionospheric effects. This indicates that the ionosphere facilitates the electromagnetic wave propagation, as if the detectability of the system were improved effectively and it is easier to record a signal even for stations located at distances beyond their detectability thresholds. Furthermore, the radiating patterns of the electrical field components |E_x| and |E_y| are complementary to each other, although any two-dimensional (2-D) power distribution of these components shows strong power areas as well as weak ones, which is advantageous to register a signal if the observing system is designed to measure both of them instead of only one.