Abstract
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 (Ne) and temperature (Te) 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 Ne and Te 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 Te varied inversely with Ne 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 Ne and Te 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 Ne and Te 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 Ne–Te 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. \\keywords{Ionosphere (plasma temperature and density)}
Highlights
The DEMETER (Detection of Electro-Magnetic Emissions Transmitted from Earthquake Regions) satellite was launched on 29 June 2004 in France and operated until 10 December 2010, with an inclination of 98◦ (Cussac et al, 2006)
From the observations of ionospheric perturbations associated with large earthquakes, several models have been proposed to explain the seismo–ionospheric coupling mechanism; these include an electromagnetic-wave-penetrating model from the lithosphere to the ionosphere (Molchanov et al, 1995), an acoustic-wave-propagating model (Hegai et al, 1997) and electrical-field-coupling models related to radon emission, aerosol accumulation, rock current and surface charge (Pulinets, 2004, 2009; Sorokin et al, 2007; Kuo et al, 2011)
The last model plays an important role in explaining the ionospheric disturbances related to seismic activities in plasma parameters, such as the perturbations in GPS TEC, foF2 and electron density; Figure 7
Summary
The DEMETER (Detection of Electro-Magnetic Emissions Transmitted from Earthquake Regions) satellite was launched on 29 June 2004 in France and operated until 10 December 2010, with an inclination of 98◦ (Cussac et al, 2006). It was a solar-synchronous-orbit satellite, with the local time of each orbit passing the equator being the same: 10:30 in the daytime and 22:30 in the nighttime. Many ionospheric perturbations related to seismic cases have been detected (Parrot et al, 2006; Sarkar et al, 2007, 2012; Zhang et al, 2009, 2010a, b, 2012a; Zeng et al, 2009; Pisa et al, 2011; Liu et al, 2011). Discussion and conclusions are provided in the last section of the paper
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