Abstract. Based on data recorded by the French DEME-TER satellite, plasma perturbations have been analyzed before the Wenchuan 8.0 earthquake which occurred on 12 May 2008. Using the revisited orbits, the seasonal variations of the O + density at daytime and nighttime were obtained respectively. There mainly exist two kinds of shapes of O + density, with peak values in the Northern Hemisphere during May to September and reversely in the Southern Hemisphere during other months. Analysis on local daytime O + density showed that it reached its lowest values 3 days prior to the earthquake, i.e. on 9 May. With studies in more than 3 month time series and comparison with those in 2006 and 2007, it was found that the significant decrease of the O + density on 9 May 2008 was different from the trend variations of 2007 and 2006 both with relatively higher O + density in May. In addition, the ion density and temperature was also analyzed before the earthquake at local nighttime. They showed smallest value in O + density and fast and short-term variations in ion temperature on 9 May above the epicenter area. All these plasma anomalies may be related to the Wenchuan earthquake. Finally, using our results and those published in other papers, the possible mechanisms of these ionospheric anomalies were discussed.
Abstract. During the DEMETER operating period in 2004–2010, many strong earthquakes took place in the world. 69 strong earthquakes with a magnitude above 7.0 during January 2005 to February 2010 were collected and analysed. The orbits, recorded in local nighttime by satellite, were chosen by a distance of 2000 km to the epicentres during the 9 days around these earthquakes, with 7 days before and 1 day after. The anomaly is defined when the disturbances in the electric field PSD increased to at least 1 order of magnitude relative to the normal median level about 10−2μV2/m2/Hz at 19.5–250 Hz frequency band, and the starting point of perturbations not exceeding 10° relsupative to the epicentral latitude. Among the 69 earthquakes, it is shown that electrostatic perturbations were detected at ULF-ultra low frequency and ELF-extremely low frequency band before the 32 earthquakes, nearly 46%. Furthermore, we extended the searching scale of these perturbations to the globe, and it can be found that before some earthquakes, the electrostatic anomalies were distributed in a much larger area a few days before, and then they concentrated to the closest orbit when the earthquake would happen one day or a few hours later, which reflects the spatial developing feature during the seismic preparation process. The results in this paper contribute to a better description of the electromagnetic (EM) disturbances at an altitude of 660–710 km in the ionosphere that can help towards a further understanding of the lithosphere-atmosphere-ionosphere (LAI) coupling mechanism.
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.
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