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. 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 MS = 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 MS = 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.

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Abstract. This case study developed a method for data processing over six years, from 2004 to 2010, of 70 keV-2.3 MeV electrons recorded by the DEMETER (Detection of Electro-Magnetic Emissions Transmitted from Earthquake Regions) satellite. Short time increases in electron counting rates, having 99 % probabilities of not being Poisson fluctuations, were statistically selected using geomagnetic invariant space and called electron bursts. Temporal series were analysed confirming the seasonal variations in low energy bands of 70-450 keV. Differently from previous results, the DEME-TER results exhibited two peaks of electron bursts: one in the period June-August and one in the period DecemberFebruary annually. Specifically, six earthquake cases are presented in detail having increases in electron burst number prior to events. Moreover, electron burst precipitation occurring before each strong earthquake of the entire period over the life of the satellite with M ≥ 7.0 was verified as having a probability greater than 97 % of not being of a statistical origin. Low energetic electrons in 70-330 keV resulted occurring more frequently near seismic activity than those observed in 330 keV-2.34 MeV energy bands at the satellite altitude in the ionosphere.

The spatial distribution features of electric field over three Alpha transmitters in Russia were analyzed based on the Demeter satellite records at local nighttime during the solar minimum in December of 2008, where the three transmitters are with the same emitted power of 500 kW and the same radio waves at 11.9 kHz, 12.6 kHz, and 14.9 kHz. The results of observations showed that the maximal electric field reached −80 to −70 dB (hereafter referred as to V/m) at 660 km altitude, and the horizontal covered area even exceeded 80° in longitude with electric field above −100 dB at 14.9 kHz. The lowest electric field and the smallest longitude scale were detected over Krasnodar (KRA), which is demonstrated that the lower ionosphere plays an important role in attenuating the energy as suggested by the simulation results from the full‐wave propagation model. Another feature over KRA was the significant decrease in electromagnetic field strength at 11.9 kHz and 12.6 kHz, being one order of magnitude lower than the other two transmitters, where the lower hybrid resonance waves affected severely the whistler mode wave mode propagation. Compared with the ground very low frequency observations at Tonghai and Ya'an in China, the most complex variations were observed from KRA, while the east transmitter Khabarovsk maintained high strength of electromagnetic power in a longer distance than the middle transmitter Novosibirsk in local nighttime, which is consistent with the large covering scale in the topside ionosphere due to the enhancement by wave‐particle interaction from the other transmitter.

Abstract. Ionospheric perturbations in plasma parameters have been observed before large earthquakes, but the correlation between different parameters has been less studied in previous research. The present study is focused on the relationship between electron density (N e ) and temperature (T e ) observed by the DEMETER (Detection of Electro-Magnetic Emissions Transmitted from Earthquake Regions) satellite during local nighttime, in which a positive correlation has been revealed near the equator and a weak correlation at mid-and low latitudes over both hemispheres. Based on this normal background analysis, the negative correlation with the lowest percent in all N e and T e points is studied before and after large earthquakes at mid-and low latitudes. The multiparameter observations exhibited typical synchronous disturbances before the Chile M8.8 earthquake in 2010 and the Pu'er M6.4 in 2007, and T e varied inversely with N e over the epicentral areas. Moreover, statistical analysis has been done by selecting the orbits at a distance of 1000 km and ±7 days before and after the global earthquakes. Enhanced negative correlation coefficients lower than −0.5 between N e and T e are found in 42 % of points to be connected with earthquakes. The correlation median values at different seismic levels show a clear decrease with earthquakes larger than 7. Finally, the electric-field-coupling model is discussed; furthermore, a digital simulation has been carried out by SAMI2 (Sami2 is Another Model of the Ionosphere), which illustrates that the external electric field in the ionosphere can strengthen the negative correlation in N e and T e at a lower latitude relative to the disturbed source due to the effects of the geomagnetic field. Although seismic activity is not the only source to cause the inverse N e -T e variations, the present results demonstrate one possibly useful tool in seismo-electromagnetic anomaly differentiation, and a comprehensive analysis with multiple parameters helps to further understand the seismo-ionospheric coupling mechanism.