Search

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 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.

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.

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.

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.

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.

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 (

We studied six different parameters derived from three different radiophysical techniques used to monitor area over the central Italy around the time of L'Aquila M6.3 earthquake of 6 April 2009. Namely VLF monitoring, ground based ionospheric sounding and GPS vertical TEC. The parameters derived from these data are: variations of amplitude of VLF signal on different propagation passes, critical frequency foF2 variations, cross-correlation coefficient for different pairs of ground based ionosondes, vertical GPS TEC time series, local variability index for the network of GPS receivers, and GPS TEC map. High congruence of techniques in time and space is found.

[1] 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 7 yr 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 >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 (

The results of the monitoring of three VLF/LF signals collected in Petropavlovsk station (Kamchatka, Russia) and one VLF signal collected on board of the DEMETER French satellite are presented. Two periods of the seismic activity occurred in the Japan-Kamchatka area during November–December 2004 and July–September 2005 were investigated and the earthquakes with M≥6.0 in the Japan-Kamchatka area, located inside one or more of the third Fresnel zones of the three radio paths were considered. The ground data were analysed using residual signal of phase dP or of amplitude dA, defined as the difference between the signal and the average of few quiet days (±5 days) immediately preceding or following the current day. Also the satellite data were processed by a method based on the difference between the real signal and the reference one, but in order to obtain this last signal it was necessary to construct previously a model of the signal distribution over the selected area. The method consists: (a) in averaging all the data available in the considered region over a period characterized by low level seismicity, regardless of the global disturbances, in particular, of the magnetic activity; (b) in computing a polynomial expression for the surface as a function of the longitude and the latitude. The model well describes the real data in condition of their completeness and in absence of magnetic storms or seismic forcing. In the quoted periods of seismic activity clear anomalies both in the ground and in satellite data were revealed. The influence of the geomagnetic activity cannot to be excluded, but the seismic forcing seems more probable.

By using network observation of subionospheric VLF (very low frequency)/LF (low frequency) signals in Japan and in Russia, we have found a significant ionospheric perturbation prior to the recent 2011 March 11 Japan earthquake (EQ) which occurred at sea proximate to the Tohoku area on the main island (Honshu) of Japan was an exceptionally huge plate-type EQ. A remarkable anomaly (with a decrease in the nighttime amplitude and also with enhancement in dispersion) was detected on March 5 and 6 along the propagation path from the NLK (Seattle, USA) transmitter to Chofu (together with Kochi and Kasugai). We also have observed the corresponding VLF anomaly during a prolonged period of March 1 - 6, with minima in the nighttime amplitude on March 3 and 4 along the path from JJI (Miyazaki, Kyushu) to Kamchatka, Russia. This ionospheric perturbation has been discussed extensively with respect to its reliability. (1) How abnormal is this VLF/LF propagation anomaly? (2) What was the temporal evolution of terminator times? (3) Were there any solar-terrestrial effects (especially the effect from geomagnetic storms) on the VLF/LF propagation anomaly? (4) The effect of any other EQs and foreshock activities on the VLF/LF anomaly? (5) Were there any correlations with other related phenomena? Finally, (6) are there any other examples of a VLF/LF propagation anomaly for oceanic EQs? We then compared the temporal properties of ionospheric perturbations for this EQ with those of a huge number of inland EQs and compared the corresponding spatial scale with the former result of the same oceanic 2004 Sumatra EQ with nearly the same magnitude. Finally, the generation mechanism of those seismo-ionospheric perturbations is briefly discussed.