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Using IAP (plasma analyzer) and ISL (Langmuir probe) experiments onboard DEMETER (Detection of Electromagnetic Emissions Transmitted from Earthquake Regions) satellite and GPS (Global Positioning System) measurements, we have statistically analyzed the variations of the electron and ion densities to search for disturbances in the vicinity of four large earthquakes prior to events. The indices Dst and Kp were used to distinguish pre-earthquake anomalies from the other anomalies related to the geomagnetic activities. For each studied case, a very good agreement was found between the different parameters estimated by DEMETER and GPS data in the detection of pre-seismic anomalies. Our statistics results show that the anomalous deviations prior to earthquakes have different sign from case to case, and that their amplitude depends on the magnitude of the earthquake. It has also been found that the electron density measured by the ISL experiment at night detects anomalous variations significantly before the earthquakes. The appearance of positive and negative anomalies in both of DEMETER and TEC (Total Electron Content) data during 1 to 5 days before all studied earthquakes during quiet geomagnetic conditions indicates that these anomalous behaviors are highly regarded as seismo-ionospheric precursors.

This paper explores multi-instrument space-borne observations in order to validate physical concepts of Lithosphere-Atmosphere-Ionosphere Coupling (LAIC) in relation to a selection of major seismic events. In this study we apply some validated techniques to observations in order to identify atmospheric and ionospheric precursors associated with some of recent most destructive earthquakes: M8.6 of March 28, 2005 and M8.5 of Sept. 12, 2007 in Sumatra, and M7.9 of May 12, 2008 in Wenchuan, China. New investigations are also presented concerning these three earthquakes and for the M7.2 of March 2008 in the Xinjiang-Xizang border region, China (the Yutian earthquake). It concerns the ionospheric density, the Global Ionospheric Maps (GIM) of the Total Electron Content (TEC), the Thermal Infra-Red (TIR) anomalies, and the Outgoing Longwave Radiation (OLR) data. It is shown that all these anomalies are identified as short-term precursors, which can be explained by the LAIC concept proposed in [S. Pulinets, D. Ouzounov, J. Asian Earth Sci. 41, 371 (2011)].

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.

We present results of a systematic study of intensity of VLF electromagnetic waves observed by the DEMETER spacecraft in the upper ionosphere (altitude 700 km). We focus on the detailed analysis of the previously reported decrease of wave intensity shortly before the main shock during the nighttime. Using a larger set of data (more than 3.5 years of measurements) and a newly developed data processing method, we confirm the existence of a very small but statistically significant decrease of wave intensity 0–4 hours before the time of the main shock at frequencies of about 1.7 kHz. It is shown that the decrease does not occur directly above the earthquake epicenter but is shifted about 2° in the westward direction. Moreover, it is demonstrated that the decrease occurs more often close to shallower earthquakes and close to earthquakes with larger magnitudes, as it is “intuitively” expected, representing an additional proof of validity of the obtained results. Finally, no dependence has been found on the occurrence of the earthquake below the ocean or below the continents.

We present results of a statistical study of a possible influence of the seismic activity on the intensity of very low frequency (VLF) transmitter signals observed by a low-altitude satellite. Electric field measurements performed by the Detection of Electro-Magnetic Emissions Transmitted from Earthquake Regions (DEMETER) satellite during its entire mission spanning almost 6.5 years were used. Among various VLF transmitter signals detected, we focused particularly on the NWC and JJI transmitters, because of their favorable locations close to seismically active areas. We evaluate the intensities of the detected transmitter signals at the times when they passed in the vicinity of an imminent earthquake during the propagation in the Earth-ionosphere waveguide, and we statistically compare them with the intensities measured at the times when there was no earthquake present. Only earthquakes with magnitudes larger than or equal to 5 and depths shallower than or equal to 40 km were considered in the analysis. Moreover, due to the low intensity of detected transmitter signals during the day, the analysis is limited exclusively to the nightside. Although the amount of relevant data is rather low, the obtained results show that there is a decrease of the detected intensity shortly (0–3 hours) after the times of the main shocks observed both for the NWC and JJI transmitter signals. The effect is spatially rather limited, observed when the signal passes within about 4 degrees from the earthquake epicenter. The intensity decrease appears to be consistent with acoustic-gravity waves propagating from the earthquake region and influencing the bottom of the ionosphere.

More than four years of VLF electric field data recorded by DEMETER have been analyzed, in order to monitor the first cut‐off frequency (QTM1) of the Earth‐ionosphere waveguide, at around 1.6–1.8 kHz. Since losses in a waveguide are maximized right at the cut‐off frequency, DEMETER (∼700 km orbit) can detect the minimum of energy of the leaking fields coming from the waveguide. This measurement permits to draw a global map of its value (f1), which is directly related to the effective height of the ionosphere (h) by the relation f1 = c/2h (c is the speed of light). It enables the remote sensing of the D region, which is one of the less known layers of the ionosphere, because it is too low for satellites to orbit inside it and too high for balloons to reach it. The effective height depends mainly on the electron density (Ne) and neutral density (Nn) profiles, which determine the plasma frequency and the electron mobility. The effective height shifts downward 5–10 km in southern warm season in the South Pacific Ocean. Another effect is observed in the Indian and Atlantic Oceans; the effective height decreases its value twice a year, in the area of roughly ±15° from the geomagnetic equator. The main causes for the changes on the effective reflection height are the solar radiation and the thunderstorm activity. However, the observed shifts are more prominent over the oceans, and a possible explanation for this difference could be attributed to i) less polluted conditions above the oceans (aerosols change the atmospheric conductivity and then the global atmospheric electric circuit), ii) the effect of the current associated to the thunderclouds on the bottom of the ionosphere because thunderstorms are much more numerous above land, or iii) ionization by elves because their occurrence is larger above oceans. Key Points D‐region remote sensing from space Seasonal changes on D‐region electron density D‐region changes connected to oceans

Daytime longitudinal structures of the electron density (Ne) and temperature (Te) in the topside ionosphere observed by Hinotori and DEMETER are examined under various conditions of solar flux, local time, and seasons. Results from both satellites show a similar longitudinal Ne structure in the morning from July to October, although the value of Ne observed by Hinotori is higher than that of DEMETER owing to higher solar flux. This result implies that the longitudinal structure of Ne may appear in any solar cycle. Further, a negative correlation between Ne and Te in the longitudinal structures appears in the morning when Ne is low, while a positive correlation appears around the magnetic equator when Ne is sufficiently enhanced during noontime in the high solar flux. A spectrum analysis performed on the DEMETER data reveals that wave numbers 1–2 for Ne and Te are dominant and nondominant. The observed wave numbers 3–4 for Ne are dominant during November–May and June–October, while they are dominant for Te during October–June and July–September. Both Ne and Te show the largest power of wave number 3 in December and wave number 4 in September. Further, observed annual variations of wave numbers 3–4 for Ne and Te also differ from wave numbers 3–4 generated by waves in the lower thermosphere. It can be interpreted as discrepancies between the longitudinal distributions of Ne and Te caused by difference in the condition of zonal winds driving E region dynamo and meridional winds modulating the ionospheric plasma structures. Key Points Annual variations of longitudinal structure of Ne and Te in the ionosphere Wave‐4 and wave‐3 dominate in December and September Zonal and medrional wind affect longitudinal structure of Ne and Te

The electron density data recorded by the Langmuir Probe Instrument (ISL, Instrument Sonde de Langmuir) onboard the DEMETER satellite have been collected for nearly 4 yr (during 2006–2009) to perform a statistical analysis. During this time, more than 7000 earthquakes with a magnitude larger than or equal to 5.0 occurred all over the world. For the statistical studies, all these events have been divided into various categories on the basis of the seismic information, including Southern or Northern Hemisphere earthquakes, inland or sea earthquakes, earthquakes at different magnitude levels, earthquakes at different depth levels, isolated events and all events. To distinguish the pre-earthquake anomalies from the possible ionospheric anomalies related to the geomagnetic activity, the data were filtered with the Kp index. The statistical results obviously show that the electron density increases close to the epicentres both in the Northern and the Southern Hemisphere, but the position of the anomaly is slightly shifted to the north in the Northern Hemisphere and to the south in the Southern Hemisphere. The electron density related to both inland and sea earthquakes presents an anomaly approximately close to the epicentres, but the anomaly for sea earthquakes is more significant than for inland earthquakes. The intensity of the anomalies is enhanced when the magnitude increases and is reduced when the depth increases. A similar anomaly can also be seen in the statistical results concerning the isolated earthquakes. All these statistical results can help to better understand the preparation process of the earthquakes and their influence up to the ionospheric levels.

Wave‐particle interactions can play a key role in the process of transfer of energy between different electron populations in the outer Van Allen radiation belt. We present a case study of wave‐particle interactions in the equatorial source region of whistler‐mode emissions. We select measurements of the Cluster spacecraft when these emissions are observed in the form of random hiss with only occasional discrete chorus wave packets, and where the wave propagation properties are very similar to previously analyzed cases of whistler‐mode chorus. We observe a positive divergence of the Poynting flux at minima of the magnetic field modulus along the magnetic field lines, indicating the central position of the source. In this region we perform a linear stability analysis based on the locally measured electron phase space densities. We find two unstable electron populations. The first of them consists of energy‐dispersed and highly anisotropic injected electrons at energies of a few hundreds eV to a few keV, with the perpendicular temperature more than 10 times higher than the parallel temperature with respect to the magnetic field line. Another unstable population is formed by trapped electrons at energies above 10 keV. We show that the injected electrons at lower energies can be responsible for a part of the waves that propagate obliquely at frequencies above one half of the electron cyclotron frequency. Our model of the trapped electrons at higher energies gives insufficient growth of the waves below one half of the electron cyclotron frequency and a nonlinear generation mechanism might be necessary to explain their presence even in this simple case.

We present a study of plasma density variations observed by the DEMETER spacecraft in the vicinity of a very powerful earthquake in Chile. This earthquake of moment magnitude 8.8 occurred on 27 February 2010 with an epicenter located at 35.85°S, 72.72°W. Data recorded 10–20 days before the main shock along orbits close to the future epicenter show increasing plasma densities. In a second step, a statistical analysis with DEMETER data has been performed using the first 3 months of the years 2007–2010 to monitor density variations in the vicinity of the epicenter at the same local time and seasonal conditions. This study shows that a large increase of the plasma density is very uncommon at this location and at this time and that the increases observed during the days before the main shock could be considered as possible short‐term precursors of this powerful earthquake. Key Points A very powerful event Ionospheric variations observed a few days before Unique ionospheric variations during the 4 years before