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In June 2014, a number of meteotsunamis were detected in the Mediterranean and Black Sea area. These meteotsunamis were initiated by a unique high-altitude dynamical system which was initially originated above Spain and traveled across the Mediterranean Sea towards Black Sea and Turkey. Meteotsunamis unlike tsunamis driven by strong earthquakes are local events, and their formation has different mechanism. Atmospheric internal gravity waves (IGWs) are one of the main known sources of meteotsunamis (e.g. Vilibic et al. in Pure Appl Geophys 165:2169–2195, 2008). The synoptic system produced short-lived and small-scale atmospheric pressure perturbations which drifted with the jet stream-like bubbles and generated tsunami-like waves in the open waters. The bubbles with typical dimensions 15–60 km continuously form and collapse in the atmosphere at altitudes of 3–6 km. Such a “boiled” atmosphere generated IGWs propagating both downward, where they produced meteotsunamis (presumably under Proudman resonance condition) and upward into the ionosphere, with following dissipation and excitation of plasma density perturbations. One of the few experimental techniques, which can monitor perturbations of the ionization within the lower ionosphere, uses long-wave probing by very low and low frequency (VLF/LF) radio signals. To study the ionospheric disturbances observed during the chain of meteotsunamis affecting the Mediterranean Sea, we used VLF/LF data collected in South Europe by “The International Network for Frontier Research on Earthquake Precursors”. By applying the spectral analysis method to the anomalous VLF/LF signals, it was found that revealed periods of the signal variations were from 10 to 40–70 min in different stations, which are in the range of the atmospheric pressure oscillations and the meteotsunami events. These periods also correspond to the periods of IGWs.

The modeling of very low-frequency (VLF) electromagnetic (EM) beam propagation in the Earth–ionosphere waveguide (WGEI) is considered. A new tensor impedance method for modeling the propagation of electromagnetic beams in a multi-layered and inhomogeneous waveguide is presented. The waveguide is assumed to possess the gyrotropy and inhomogeneity with a thick cover layer placed above the waveguide. The influence of geomagnetic field inclination and carrier beam frequency on the characteristics of the polarization transformation in the Earth–ionosphere waveguide is determined. The new method for modeling the propagation of electromagnetic beams allows us to study the (i) propagation of the very low-frequency modes in the Earth–ionosphere waveguide and, in perspective, their excitation by the typical Earth–ionosphere waveguide sources, such as radio wave transmitters and lightning discharges, and (ii) leakage of Earth–ionosphere waveguide waves into the upper ionosphere and magnetosphere. The proposed approach can be applied to the variety of problems related to the analysis of the propagation of electromagnetic waves in layered gyrotropic and anisotropic active media in a wide frequency range, e.g., from the Earth–ionosphere waveguide to the optical waveband, for artificial signal propagation such as metamaterial microwave or optical waveguides.

Diagnostics of the outer envelopes of the Earth through global satellite navigational systems and through the equally global network of ground receivers and very-low-frequency transmitters make it possible to explore fairly complex processes of lithosphere–ionosphere interactions. The effects of earthquakes and tsunamis on the ionosphere are an example of these processes. A brief review of the current advances in studying these processes is presented.

In this study we present measurements and simulations of mid-latitude sub-ionospheric propagation paths between several VLF/LF transmitters and the Graz seismo-electromagnetic receiver facility (Schwingenschuh etal, 2011) during the current solar minimum condition. The upper D/E-region boundary of the waveguide is stable during the low solar activity in the years 2018 and 2019, i.e. measured VLF/LF amplitude and phase variations are mainly due to natural excitations from the lithosphere, atmosphere, and man-made disturbances. In particular, this period gives a baseline to characterize VLF amplitude and phase modulations in the waveguide cavity related to seismic activity over Europe. In addition, this opportunity let us probe the signal threshold and feed-back into waveguide simulation models. We conclude, proven long-term VLF/LF measurements, the continuous monitoring of the cavity, could be valuable in the assessment of seismic hazard scenarios.Ref:Schwingenschuh, K., Prattes, G., Besser, B. P., Mocnik, K., Stachel, M., Aydogar, Ö., Jernej, I., Boudjada, M. Y., Stangl, G., Rozhnoi, A., Solovieva, M., Biagi, P. F., Hayakawa, M., and Eichelberger, H. U.: The Graz seismo-electromagnetic VLF facility, Nat. Hazards Earth Syst. Sci., 11, 1121–1127, https://doi.org/10.5194/nhess-11-1121-2011, 2011.

Modeling propagation of VLF electromagnetic beams in the waveguide earth-ionosphere (WGEI) is of a great importance because variation in the characteristics of these waves is an effective instrument for diagnostics the influences on the ionosphere from above (Sun-Solar Wind-Magnetosphere-Ionosphere), from below (the most powerful meteorological, seismogenic and other sources in the lower atmosphere and lithosphere/Earth, such as hurricanes, earthquakes, tsunamis etc.), from inside the ionosphere (strong thunderstorms and lightning discharges) and even from the far space (such as gamma-flashes, cosmic rays etc.). Thus, VLF became one of the most universal instrument for monitoringthe Space Weather in the direct sense of this term, i.e. the state of the Sun-Earth space and the ionosphere as it is particularly determined by all possible relatively powerful sources, wherever they are placed. This paper is devoted mostly to modelling VLF electromagnetic beam propagation in the WGEI. We present a new tensor impedance method for modelling propagation of electromagnetic beams (TIMEB) in a multi-layered/inhomogeneous waveguide. Suppose that such a waveguide, i.e. WGEI, possesses the gyrotropy and inhomogeneity with a thick cover layer placed above the waveguide. Note a very useful and attractive feature of the proposed TIMEB method: in spite of a large thickness of the waveguide cover layer, the proposed effective impedance approach reflects an impact of such a cover on the electromagnetic (EM) waves, which propagate in the waveguide. This impedance approach can be applied for EM waves/beams in layered gyrotropic/anisotropic active media in very wide frequency range, from VLF to optics. Moreover, this approach can be applied to calculations of EM waves/beams propagation in the media of an artificial origin such as metamaterial microwave or optical waveguides. The results of the modelling the propagation of VLF beams in the WGEI are included. The qualitative comparison between the theory and experimental observation of increasing losses of VLF waves in the WGEI is discussed. The new proposed method and its further development allows the comparison with the results of the future rocket experiment. This method allows to model (i) excitation of the VLF modes in the WGEI and their excitation by the typical VLF sources, such as radio wave transmitters and lightning discharges and (ii) leakage of VLF waves/beams into the upper ionosphere/magnetosphere.

An experimental study of the phase and amplitude observations of sub‐ionospheric very low and low frequency (VLF/LF) signals is performed to analyze the response of the lower ionosphere during the 21 August 2017 total solar eclipse in the United States of America. Three different sub‐ionospheric wave paths are investigated. The length of the paths varies from 2,200 to 6,400 km, and the signal frequencies are 21.4, 25.2, and 40.75 kHz. The two paths cross the region of the total eclipse, and the third path is in the region of 40–60% of obscuration. None of the signals reveal any noticeable amplitude changes during the eclipse, while negative phase anomalies (from −33° to −95°) are detected for all three paths. It is shown that the effective reflection height of the ionosphere in low and middle latitudes is increased by about 3–5 km during the eclipse. Estimation of the electron density change in the lower ionosphere caused by the eclipse, using linear recombination law, shows that the average decrease is by 2.1 to 4.5 times.

In this paper we analysed sudden phase anomalies (SPAs) of VLF–LF signals recorded at Graz (Austria), Birr (Ireland) and Moscow (Russia) stations during two strong solar flares in September 2017. The first X-class 9.3 flare occurred on 6 September at 12:02 UT, and the second X-class 8.2 flare was observed on 10 September 2017 at 16:06 UT. Data from seven transmitters in a frequency range between 20 and 45 kHz are used for the analysis. The SPAs were observed in all middle-latitude paths (differently orientated) with path lengths from 350 to 7000 km. Solar X-ray burst data were taken from GOES satellite observations in the wavelength band of 0.05–0.4 nm. It was found that (i) the amplitude of SPAs in different paths varies from 10 to 282∘, and (ii) the correlation between the amplitudes of SPAs, the lengths of paths and the signal frequency is weak. The change in effective height of reflection due to lowering of the reflecting layer during the flares was found to be about 12 km for the first event and about 9 km for the second event. Spectral analysis of the X-ray and LF data, filtered in the range between 5 s and 16 min, showed that the LF signal spectra are very similar to X-ray spectra. Maxima of both X-ray and LF spectra are in 2–16 min interval.

The analyzed amplitude and phase variations in electromagnetic VLF and LF signals at 20–45 kHz, received in Moscow, Graz (Austria), and Sheffield (UK) during the total solar eclipse of March 20, 2015, are considered. The 22 analyzed paths have lengths of 200—6100 km, are differently oriented, and cross 40–100% occultation regions. Fifteen paths crossed the region where the occultation varied from 40 to 90%. Solar eclipse effects were found only on one of these paths in the signal phase (–50°). Four long paths crossed the 90–100% occultation region, and signal amplitude and phase anomalies were detected for all four paths. Negative phase anomalies varied from–75° to–90°, and the amplitude anomalies were both positive and negative and were not larger than 5 dB. It was shown that the effective height of the ionosphere varied from 6.5 to 11 km during the eclipse.

In this paper, we suggest a technique for forecasting seismic events based on the very low and low frequency (VLF and LF) signals in the 10 to 50 Hz band using the neural network approach, specifically, the error back-propagation method (EBPM). In this method, the solution of the problem has two main stages: training and recognition (forecasting). The training set is constructed from the combined data, including the amplitudes and phases of the VLF/LF signals measured in the monitoring of the Kuril-Kamchatka region and the corresponding parameters of regional seismicity. Training the neural network establishes the internal relationship between the characteristic changes in the VLF/LF signals a few days before a seismic event and the corresponding level of seismicity. The trained neural network is then applied in a prognostic mode for automated detection of the anomalous changes in the signal which are associated with seismic activity exceeding the assumed threshold level. By the example of several time intervals in 2004, 2005, 2006, and 2007, we demonstrate the efficiency of the neural network approach in the short-term forecasting of earthquakes with magnitudes starting from M ≥ 5.5 from the nighttime variations in the amplitudes and phases of the LF signals on one radio path. We also discuss the results of the simultaneous analysis of the VLF/LF data measured on two partially overlapping paths aimed at revealing the correlations between the nighttime variations in the amplitude of the signal and seismic activity.

The disturbances of very low and low frequency signals in the lower mid-latitude ionosphere caused by magnetic storms, proton bursts and relativistic electron fluxes are investigated on the basis of VLF–LF measurements obtained in the Far East and European networks. We have found that magnetic storm (−150 < Dst < −100 nT) influence is not strong on variations of VLF–LF signals. The anomalies with negative amplitude were registered during the main and recovery phases for several magnetic storms (mainly for three northernmost paths). The correlation between VLF–LF signals and geomagnetic activity is rather weak even for these paths (≈ 12–18%). Also, the correlation between magnetic activity and VLF signal variations recorded onboard the DEMETER satellite is not found. The significant influence of outer-zone particles (energetic particle sensor on board/Geostationary Operational Environmental Satellite (GOES) measurements) on the VLF–LF signal variations is found for almost half of the sub-ionospheric paths.

Signals of two Japanese transmitters (22.2 kHz and 40 kHz) recorded on the ground VLF/LF station in Petropavlovsk-Kamchatsky and on board the DEMETER French satellite have been analyzed during a seismic activity in Japan in May–June 2008. The period of analysis was from 18 April to 27 June. During this time two rather large earthquakes occurred in the north part of Honshu Island – 7 May (M=6.8) and 13 June (M=6.9). The ground and satellite data were processed by a method based on the difference between the real signal in nighttime and the model one. For ground observations a clear decrease in both signals has been found several days before the first earthquake. For the second earthquake anomalies were detected only in JJI signal. The epicenters of earthquakes were in reliable reception zone of 40 kHz signal on board the DEMETER. Signal enhancement above the seismic active region and significant signal intensity depletion in the magnetically conjugate area has been found for satellite observation before the first earthquake. Anomalies in satellite data coincide in time with those in the ground-based observation.

International audience; The paper presents the results of a joint analysis of ground-based and satellite observations of very low-frequency and low-frequency (VLF/LF) signals during periods of strong seismic activity in the region of Kuril Islands and Japan in 2004–2010. Ground and satellite data was processed using a method based on the difference between the real signal in nighttime and that of a model. The results of the analysis show a good correlation between ground-based and satellite data for several cases of strong (M ⩾ 6.8) earthquakes.

The subionospheric data from a Japanese very low frequency/low frequency (VLF/LF) receiving station at Moshiri, Hokkaido, are used to detect the response of the lower ionosphere to the tsunami triggered by the 2011 Tohoku earthquake. Disturbances in the phase and amplitude of VLF signals propagating from the transmitter in Hawaiian Islands are observed during the tsunami wave passage, and these effects in the ionosphere are compared to the in situ sea-level global positioning system (GPS) measurements near Japan. The frequency of the maximum spectral amplitude both for the VLF and GPS data is found to be in the range of periods of 8–50 min, which is likely to correspond to the period of the internal gravity waves generated by the tsunami.

We report on specific fluctuations in phase and amplitude of VLF signals that correlate both spatially and temporally with the passage of the tsunamis recorded by the Deep-ocean Assessments and Reporting of Tsunamis bottom pressure stations. Measurements from the VLF/LF receiver sited in Petropavlovsk-Kamchatsky and sensor buoys placed throughout the Pacific Ocean at great distances (Hawaii and Japan) from the epicenter are consistent with the hypothesis that the ocean tsunami following the Chile earthquake on 27 February 2010 radiated internal gravity waves which propagated through the lower ionosphere.

In this paper we develop a new method for the analysis of excitation and propagation of planetary electromagnetic waves (PEMW) in the ionosphere of the Earth. The nonlinear system of equations for PEMW, valid for any height, from D to F regions, including intermediate altitudes between D and E and between E and F regions, is derived. In particular, we have found the system of nonlinear one-fluid MHD equations in the β-plane approximation valid for the ionospheric F region (Aburjania et al., 2003a, 2005). The series expansion in a "small" (relative to the local geomagnetic field) non-stationary magnetic field has been applied only at the last step of the derivation of the equations. The small mechanical vertical displacement of the media is taken into account. We have shown that obtained equations can be reduced to the well-known system with Larichev–Reznik vortex solution in the equatorial region (see e.g. Aburjania et al., 2002). The excitation of planetary electromagnetic waves by different initial perturbations has been investigated numerically. Some means for the PEMW detection and data processing are discussed.

Data from the Pacific network of VLF receivers have been used to study the response of the lower ionosphere to the January 2011 Mt. Kirishima (South Japan) volcanic eruption. A major explosive eruption occurred on January 27, which was preceded by several small eruptions. Perturbations of nighttime subionospheric VLF signals have been detected on the day of the first small eruption on January 18 (UT) with the maximum observed about 1.5 h after the eruption. The nighttime signal remained disturbed during the subsequent pre-eruptive and eruptive activity of Mt. Kirishima. The daytime perturbations were not observed. The frequency of the maximum spectral amplitude was found to be in the range of periods of 6–30 min, which corresponds to the periods of internal gravity waves. These results suggest that the observed VLF ionospheric effects can possibly be produced by the penetration of gravity waves caused by the volcanic activity into the ionosphere.

It is found that clear lower ionospheric perturbations appeared as a precursor to the 11 March 2011 Japan earthquake. This study is based on the observation of two completely different phenomena: (1) subionospheric very low frequency/low frequency propagation anomaly on the NLK (Seattle, USA) – Chofu propagation path, and (2) depression of magnetospheric ultra low frequency emissions observed on the ground (Kakioka, etc.). But, both effects are suggested to be interpreted by a unified phenomenon of seismo-lower ionospheric perturbation because they occurred on the days of 5 and 6 March, 2011.

The relative importance of magnitude and depth of an earthquake (EQ) in the generation of seismo-ionospheric perturbations at middle latitudes is investigated by using the EQs near the propagation path from the Japanese LF transmitter, JJY (at Fukushima) to a receiving station at Petropavsk-Kamchatsky (PTK) in Russia during a three-year period of 2005-2007. It is then found that the depth (down to 100km) is an extremely unimportant factor as compared with the magnitude in inducing seismo-ionospheric perturbations at middle latitudes. This result for sea EQs in the Izu-Bonin and Kurile-Kamchatka arcs is found to be in sharp contrast with our previous result for Japanese EQs mainly of the fault-type. We try to interpret this difference in the context of the lithosphere-atmosphere-ionosphere coupling mechanism.

Data from the European network of very low/ low frequency (VLF/LF) receivers has been used to study the response of the lower ionosphere to the earthquake of magnitude 5.5 in Vrancea area on November 22, 2014. Negative amplitude anomalies have been observed during 3 days before the earthquake and two days after, on the LF (45.9 kHz) signal passed above the seismic area. No perturbations have been found for the same signal in control paths during this period. Other possible influences both from above and below which can produce perturbations in the ionosphere have been taken into consideration.

In this paper we describe the Graz seismo-electromagnetic very low frequency (VLF) facility, as part of the European VLF receiver network, together with the scientific objectives and results from two years operation. After a brief technical summary of the present system – with heritage from a predecessor facility – i.e. hardware, software, operational modes and environmental influences, we discuss results from statistical data and scientific events related to terrestrial VLF propagation over Europe.

The VLF/LF radio signals method for studying preseimic activity is applied to the Abruzzo earthquake (M=6.3, 6 April 2009). The data collected by three receivers located in Moscow (Russia), Graz (Austria) and Bari (Italy) at about 3000 km, 1000 km and 500 km from the epicenter were used. The signals received from the Sardinia (20.27 kHz) and the Sicily (45.9 kHz) transmitters, both located in Italy, were compared with those received from the Iceland (37.5 kHz), the Great Britain (19.58 kHz) and the Germany (23.4 kHz) transmitters. The propagation paths of the two Italian transmitters cross the epicentral area (seismic paths) unlike the paths of the other three signals (control paths). Using two different analyses, that are the study of the night-time signal and the research of shifts in the evening terminator times, clear anomalies were revealed 2–8 days before the occurrence of the Abruzzo earthquake in the seismic paths, while no anomalies have been found in the control paths.

Two methods for diagnosing ionospheric disturbances using VLF signals received on the DEMETER satellite are considered in connection with two cases of high seismic activity: the method for analyzing a signal reception zone in order to find large-scale spatial variations and the residual signal method used to reveal time variations. Based on an analysis of the VLF signal reception zone, performed in connection with the catastrophic earthquake near Sumatra, it has been found out that the signal amplitude decreased during a month before the earthquake. As a result, it has been concluded that the zone of ionospheric disturbances extends for several thousand kilometers. It has been indicated that the residual signal method showed good agreement between the ground and satellite data when high seismic activity near Japan was analyzed.

As the target earthquake we have taken a huge earthquake (EQ) named Miyagi-oki earthquake on 16 August 2005 (with magnitude of 7.2) and we have analyzed the 4 month period including the date of this EQ. In addition to our previous analysis on the nighttime average amplitude (trend) and nighttime fluctuation, we have proposed the use of fluctuation power spectra in the frequency rage of atmospheric gravity waves (period=10 min to 100 min) as a third parameter of subionospheric VLF/LF propagation characteristics. Then it is found that this third parameter would be of additional importance in confirming the presence of seismo-ionospheric perturbations. Finally, we have discovered an important role of lunar tidal effect in the VLF/LF data, which appears one and two months before this large EQ.

A case study has been performed on the lower ionospheric perturbations as seen from the Japanese subionospheric VLF (very low frequency, 3–30 kHz) /LF (low frequency, 30–300 kHz) propagation network for a recent powerful earthquake, i.e., the Miyagi-oki earthquake on 16 August 2005 (magnitude 7.2, and depth 36 km). Different propagation paths were examined and the two paths from a transmitter with call sign of JJY ( f = 40 kHz, in Fukushima prefecture) to both receiving stations of Kamachatka, Russia and Moshiri, Hokkaido, exhibit clear signatures of ionospheric perturbations a few days before and a few days after the earthquake. We have detected a clear nighttime amplitude decrease and an enhancement in nighttime fluctuation, both exceeding the corresponding three standard deviations from the mean. We discuss other seismogenic phenomena in order to study the lithosphere–ionosphere coupling.

On 22 August 2005 an earthquake with magnitude M=4.7 occurred in the Anzio (central Italy) offshore area. From 2002, a VLF-LF radio receiver is into operation in Bari (southern Italy). The intensity and the phase of the signals transmitted by GB (f=16 kHz, United Kingdom), FR (f=20.9 kHz, France), GE (f=23.4 kHz, Germany), IC (f=37.5 kHz, Island) and IT (f=54 kHz, Sicily, Italy) has been monitored with a 5 s sampling rate. The previous epicenter is near enough to some of the radio paths and the data collected were studied in order to reveal possible seismic effects. The raw analysis revealed a clear drop in the intensity of the FR radio signal on 19 August. Then the wavelet analysis was applied to the intensity and the phase data of the different radio signals. In the mentioned day an increase in the band 60–120 min was revealed in the spectra of the FR signal. Then the principal component analysis was applied and again the 19 August stood up as an anomalous day for the FR radio signal. The path of this signal, among the paths of the five radio signals collected by the Bari receiver, is the nearest to the mentioned epicentre and the anomaly revealed on 19 August appears as a precursor of the earthquake. This result confirms the possible precursor revealed by other researchers in the air Rn content in a site located 5 miles far from the epicenter.

Relatively long-period (4 years) data on different propagation paths by means of Japanese-Pacific VLF/LF network observation, are used to obtain further statistical significance on the correlation of ionospheric perturbations as revealed by VLF/LF propagation anomalies with earthquakes. Earthquakes with magnitude greater than 6.0, taken place only within the fifth Fresnel zone of each great-circle path are selected for the correlation study. It is finally found based on the superimposed epoch analysis that the nighttime trend (average amplitude) exhibits a significant decrease exceeding 2σ (σ: standard deviation) several days before the earthquake and the nighttime fluctuation exceeds the corresponding 2σ again several days before the earthquake when the earthquake depth is smaller than 30 km (shallow earthquakes). However, when we treat all earthquakes including deep earthquakes, the trend shows a significant decrease (just approaching 2σ line), and the nighttime fluctuation shows a less significant broad enhancement before the EQ.

The analysis of the VLF signals radiated by ground transmitters and received on board of the French DEMETER satellite, reveals a drop of the signals (scattering spot) connected with the occurrence of large earthquakes. The extension of the "scattering spots" zone is large enough (1000–5000 km) and, probably, it increases with the magnitude of the "relative" earthquake. A possible model to explain the phenomenology, based on the acoustic gravity waves and the ionosphere turbulence, is proposed. The method of diagnostics applied to this study has the advantage to be a global one due to the world wide location of the powerful VLF transmitters and of the satellite reception. However, a specific disadvantage exists because the method requires rather a long time period of analysis due to the large longitudinal displacements among the successive satellite orbits. At the moment, at least, one month seems to be necessary.

Variations of the LF signal from observations in 6 receiving stations during 2000–2004 are analyzed. Five stations are located in Japan (Kochi, Moshiri, Kasugai, Shimizu and Chiba) and one station is in Russia (Petropavlovsk–Kamchatski). All the stations are equipped with the identical OMNIPAL receivers. JJY (40 kHz) transmitter is situated in Fukushima prefecture (Japan). Sensitivity of the phase and amplitude of the LF signal to ionosphere and atmosphere perturbations associated with variations of the magnetic and meteorological conditions and solar activity is studied. Correlation of phase and amplitude variations of the LF signal with D st -index, outer-zone particles (protons and electrons) with high-pitch angle is established for all the stations. It is found that LF anomalies are typical for the main phase of magnetic storm if the sudden commencement happens in the night time. The similarity of phase and amplitude anomalies to the structure of the simultaneously observed Pi3 geomagnetic pulsations is found for all the stations. Correlation between variations of atmosphere pressure and amplitude of the LF signal is established. Correlation between variations of humidity for the stations located on the coast of Japan and amplitude of the LF signal is found.

The variations of phase and amplitude of LF signal propagating from the transmitter JJY (40 kHz, Fukushima prefecture, Japan) are analyzed. The receiver is situated in Petropavlovsk-Kamchatski (Russia). The time interval from July1, 2004 till January 24, 2005 is included in the examination. This interval is characterized by quiet seismic conditions up to the beginning of November, but rather strong seismic activity occurs in November and December not far from Hokkaido (Japan) and in the region of northern Kuril Islands. Seismic activity in a zone of sensitivity of our wave paths is determined by three series of earthquakes with M = 5.6–7.1. Nighttime “bay-like” phase and amplitude anomalies of the LF signal begin some days before the first earthquake and continue till the last earthquake of this series ( M = 6.2, November 11, 2004). Most strong and obvious anomalies appear in the amplitude of the LF signal. After that, during about one fortnight a period of seismic calm is observed. Then there is a following anomaly of the LF signal before two earthquakes ( M = 7.1, 28.11.04 and M = 6.8, 6.12.04). The next LF anomaly is observed before the earthquakes on December, 18 and 21 ( M = 5.8 and M = 5.6). During the whole period of seismic activity a significant shift in terminator times is also evident. The spectrum of LF seismo-induced anomalies shows a clear increase for the periods of 10–25 min.

A signal transmitted by Japan Time Standard LF station (40 kHz, Fukushima prefecture) and recorded in Petropavlovsk-Kamchatski (Russia) is analyzed during a time interval from 1 July 2004 till 24 January 2005. This interval is characterized by quiet seismic conditions up to the beginning of November, but rather strong seismic activity occurs in November and December not far from Hokkaido (Japan) and in the region of northern Kuril Islands. There were three series of earthquakes with M=5.6–7.1 in a zone of sensitivity of our wave path during two months. Nighttime "bay-like" phase and amplitude anomalies of the LF signal are observed several days before and during every series of earthquakes. During the whole period of seismic activity a significant shift in terminator times is also evident. The spectrum of LF seismo-induced anomalies shows a clear increase for the period of about 25 min.

We present a general concept of mechanisms of preseismic phenomena in the atmosphere and ionosphere. After short review of observational results we conclude: 1. Upward migration of fluid substrate matter (bubble) can lead to ousting of the hot water/gas near the ground surface and cause an earthquake (EQ) itself in the strength-weakened area; 2. Thus, time and place of the bubble appearance could be random values, but EQ, geochemistry anomaly and foreshocks (seismic, SA and ULF electromagnetic ones) are casually connected; 3. Atmospheric perturbation of temperature and density could follow preseismic hot water/gas release resulting in generation of atmospheric gravity waves (AGW) with periods in a range of 6–60min; 4. Seismo-induced AGW could lead to modification of the ionospheric turbulence and to the change of over-horizon radio-wave propagation in the atmosphere, perturbation of LF waves in the lower ionosphere and ULF emission depression at the ground.

The phase (P) and amplitude (A) anomalies in subionospheric LF signal (40 kHz) along the path Japan–Kamchatka of 2300 km have been studied for the data observed by means of a digital OminiPAL receiver for 2 years. The empirical model of background P and A daily variations for quiet and disturbed geomagnetic conditions in the absence of seismic activity is developed. We pay special attention to the P and A features during large magnetic storms. A sensitivity threshold of LF signal to deforming influence of the geomagnetic and seismic factors is defined. Two cases of bay-like behavior of LF phase and amplitude in nighttime are described as a clear earthquake precursor of LF signal. We have found from the statistical study that LF signal effect is observed only for earthquakes with M⩾5.5 and we discuss the possible mechanisms of the effect.

Very low and low frequency (VLF/LF) data recorded in the Far Eastern stations Petropavlovsk-Kamchatsky (158.92° E, 53.15° N), Yuzhno-Sakhalinsk (142.75° E, 46.95° N) and Yuzhno-Kurilsk (145.861° E, 44.03° N) are investigated to study the meteorological effects in the lower ionosphere. The results demonstrate the sensitivity of the VLF/LF signals to the variations of atmospheric pressure, humidity, wind velocity and temperature, and the VLF/LF record at the station of Yuzhno-Kurilsk is found to be most sensitive to those variations of atmospheric parameters. The region under consideration is characterized by high winter cyclonic activity in mid-latitudes and strong summer and autumn typhoon activity in low latitudes. VLF/LF signal variations during eight tropical cyclones (TCs) with different intensity are considered. Negative nighttime anomalies in the signal amplitude that are most probably caused by TC activity are found for six events. Those anomalies are observed during 1–2 days when TCs move inside the sensitivity zones of the subionospheric paths. Perturbations of the VLF signal observed during two TCs can be caused by both the TC influence and seismic activity, but no correlation between TC intensity and magnitude of the signal anomalies is found. Spectral analysis of the typhoon-induced disturbed signals revealed the fluctuations with time periods in the range of 7–16 and 15–55 min that corresponds to the range of internal gravity waves periods.

We examined changes in the spectral composition of the low frequency (LF) subionospheric signals from the NRK transmitter (37.5 kHz) in Iceland that were received in Bari (Italy) relative to the earthquake that occurred in L’Aquila on April 6, 2009. In our previous studies, we have reported the occurrence of preseismic night-time anomalies using observations from three receivers located in Bari, Graz (Austria) and Moscow (Russia). The strongest anomalies in the signals were observed in the NRK-Bari propagation path during the period 5-6 days before the L’Aquila earthquake, as well as during the series of aftershocks. During this period, similar very low frequency (VLF)/LF amplitude anomalies were also observed along several other propagation paths that crossed the L’Aquila seismogenic zone. Spectral analysis of the LF signals filtered in the frequency range 0.28 mHz to 15 mHz shows differences in the spectra for seismo-disturbed days when compared to those for either quiet or geomagnetically disturbed days. These spectral anomalies, which are only observed in the propagation path between NRK and Bari, contain signals with periods of about 10 min to 20 min. These periodic signals are absent both in the spectra of the undisturbed signals for the control paths, and in the spectra of the signals received during geomagnetic storms. The same changes in the spectral composition were observed in the analysis of LF (40 kHz) signals from the JJY transmitter in Japan that were received in Petropavlovsk-Kamchatsky (Russia) during the occurrence of three strong earthquakes with M ≥7.0. The results of this study support the theoretical prediction that the possible mechanism for energy penetration from the origin of an earthquake through the atmosphere and into the ionosphere is based on the excitation and upward propagation of internal gravity waves.

International audience; Signals from very low frequency (VLF)/ low frequency (LF) transmitters recorded on the ground station at Petropavlovsk-Kamchatsky and on board the French DEMETER satellite were analyzed for the Simushir earthquake (M 8.3; November 15, 2006). The period of analysis was from October 1, 2006, to January 31, 2007. The ground and satellite data were processed by a method based on the difference between the real signal at night-time and the model signal. The model for the ground observations was the monthly averaged signal amplitudes and phases, as calculated for the quiet days of every month. For the satellite data, a two-dimensional model of the signal distribution over the selected area was constructed. Preseismic effects were found several days before the earthquake, in both the ground and satellite observations.Nombre de références

The presence of ionospheric perturbations in possible association with two huge earthquakes (Noto-hanto peninsula and Niigata-chuetu-oki earthquakes) in 2007 was studied on the basis of a conventional statistical study for a particular propagation path from the JJI transmitter in Miyazaki, Kyushu, to Moshiri in Hokkaido. This is based on automatic routine-based signal processing, in which the trend as the average nighttime amplitude is significantly decreased, with almost simultaneous significant enhancement in the night-time fluctuation as the night-time integration of negative fluctuation from the average. It is, however, shown that this routine-based signal analysis sometime suffers from artificial (or man-made) effects. Thus, in this study, we propose an additional use of principal component analysis (PCA) for simultaneous observation of a few VLF/LF propagation paths. With the application of this PCA method to multi-path data, the artificial effects can be reasonably removed, and also only the geophysical effects associated with earthquakes are detected, by focusing mainly on the third principal component. The satisfactory separation of the principal components is made possible by pre-analysis of the VLF data (extraction from the raw data of the average over a whole year). This PCA method enables us to identify the seismogenic effects in association with earthquakes with smaller magnitudes, down to M 5.5 or M 5.0.

In this paper we present results obtained from more than two years continuous very low frequency (VLF) measurements between a network of VLF transmitters and receivers. The focus is on VLF amplitude and phase variations in the Earth lithosphere-ionosphere cavity with the scientific objective to characterise long term trends possibly related to planetary waves and climatological factors. After considering the nominal diurnal and seasonal behaviour of the individual paths in the VLF waveguide, we distinguish between natural and artifical disturbances on different temporal and spatial scales. We conclude that it is too early to speak of a clear trend, but VLF investigations together with complementary measurements can be a useful tool in long term environmental and climatological studies on medium spatial scales.

The subionospheric VLF/LF propagation is extensively used to investigate the lower ionospheric perturbation in possible association with earthquakes. An extensive period of data over seven years from January 2001 to December 2007 and a combination of different propagation paths in and around Japan are used to examine the statistical correlation between the VLF/LF propagation anomaly (average nighttime amplitude, dispersion and nighttime fluctuation) and earthquakes with magnitude greater than 6.0. It is then found that the propagation anomaly exceeding the 2σ (standard deviation) criterion indicating the presence of ionospheric perturbation is significantly correlated with earthquakes with shallow depth (