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.
Based on the ionospheric electromagnetic data observed on DEMETER satellite of France, the ionospheric electromagnetic signals were analyzed within 10 days before Chile M7.9 earthquake on November 14, 2007. It is found that, low frequency electromagnetic disturbances began to increase in a large scale of latitude, and reached to a maximum one week prior to the earthquake, and at about three days before the quake, the peak values shifted to lower latitude. Taking three days as a group, spatial images of a few parameters were analyzed, from which it can be seen during the five days prior to this earthquake, the amplitude and scale of anomalies are enlarged, while the epicenter is located at the boundary of anomalous region. The anomalous tempo-spatial variation in electron density prior to the earthquake were also obtained in terms of tracing the data from revisited orbits in half a year prior to the quake.
Based on electron density data recorded by the Detection of Electro-Magnetic Emissions Transmitted from Earthquake Regions (DEMETER) satellite of France, the phenomena of disturbed electron density in the ionosphere before the Wenchuan Earthquake of magnitude 8.0 on 12 May 2008, were analysed. The results showed that the anomalies concentrated 3 days prior to the earthquake, especially the variation on 9 May, when the electron density reduced above the northeast epicentral area, and the peak values moved to the equator. Compared with the variations in the geomagnetic indexes and other parameters related to solar activity, the geomagnetic condition and solar activity were all quiet from 7 to 12 May, so the variation on 9 May might be related to this strong earthquake. In addition, the geomagnetic field observed at the surface was analysed, and the Z component data at stations near the epicentral area showed apparent aberrations of daily variation compared with those at stations in eastern China. All these phenomena illustrate that obvious perturbations in the ionosphere and at the surface might be related to the preparation process of the Wenchuan Earthquake.
Based on the data of France DEMETER (Detection of Electro-Magnetic Emissions Transmitted from Earthquake Regions) satellite, we have made identification to the level 1 data using level 2 picture, moving median processing, analyses of latitude changes and spatial differences. We found some anomalies in the ionosphere above the epicenter of the 27 February 2010 Chile M 8.8 earthquake. In an area 1500 km apart from the epicenter, electron density rose on 20 February 2010. The electron density declined on 24 th . The electric, magnetic fields and some plasma parameters disturbed at the same time on 25 th and 26 th . Besides, on 26 th , electron density of 2 orbits exceeded the background values. The electric field spectrum around the magnetic equator on 27 th began to drop 4 hours before the great earthquake, and meanwhile strong perturbations of some plasma parameters occurred.
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