Probing the lower ionospheric perturbations associated with earthquakes by means of subionospheric VLF/LF propagation

There have been reports for many years that the ionosphere is very sensitive to seismic effects, and the detection of ionospheric perturbations associated with earthquakes (EQs) attracts a lot of attention as a very promising candidate for short-term EQ prediction. In this review we present a possible use of VLF/LF (very low frequency (3–30 kHz)/low frequency (30–300 kHz)) radio sounding of seismo-ionospheric perturbations. In order to avoid the overlapping with my own previous reviews, we first show some pioneering results for the Kobe EQ and we try to present the latest results including the statistical evidence on the correlation between the VLF/LF propagation anomalies (ionospheric perturbations) and EQs (especially with large magnitude and with shallow depth), medium-distance (6-8 Mm) propagation anomalies, the fluctuation spectra of subionospheric VLF/LF data (the effect of atmospheric gravity waves, the effect of Earth's tides, etc.), and the mechanism of lithosphere-atmosphere-ionosphere coupling. Finally, we indicate the present situation of this kind of VLF/LF activities going on in different parts of the globe and we suggest the importance of international collaboration in this seismo-electromagnetic study.     

Neural network technique for identifying prognostic anomalies from low-frequency electromagnetic signals in the Kuril–Kamchatka region

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.     

Subionospheric VLF/LF monitoring of ionospheric perturbations for the 2004 Mid-Niigata earthquake and their structure and dynamics

This paper reports on the detailed spatial structure and temporal dynamics of the ionospheric perturbations associated with the huge 2004 Mid-Niigata earthquake in Japan. The terminator time method in VLF/LF subionospheric propagation has been used to deduce the presence and dynamics of the seismo-ionospheric perturbations by making full use of our Japanese VLF/LF network observations. It is found that significant shifts were observed in terminator time for some selected paths, a few to several days before the earthquake, and we obtained the anisotropic shape of the inferred ionospheric perturbation likely elongated along the fault lines there. The temporal dynamics of inhomogeneity of the perturbations is suggested on the basis of a comparison of the observed terminator time shift and theoretical full-wave computations (2D FDTD and 3D full-wave scattering).     

The wave-like structures of ionospheric perturbation associated with Sumatra earthquake of 26 December 2004, as revealed from VLF observation in Japan of NWC signals

The observations of subionospheric VLF waves from the Australian VLF transmitter NWC (frequency=19.8 kHz) at the Japanese receiving stations Chofu, Chiba and Kochi have been utilized to identify a possible precursor of ionospheric perturbations to the huge Sumatra earthquake of 26 December 2004. The VLF amplitude data at Japanese stations have indicated the depression in amplitude and also the enhancement in nighttime amplitude fluctuation before the earthquake. The nighttime fluctuation is composed of wave-like structures, and the wavelet analysis and cross-correlation analyses have been performed for those fluctuations. A significant enhancement in the fluctuation spectra in the period 20–30 min to ∼100 min (the frequency range of atmospheric gravity waves) is observed only before the earthquake. Then, the wave-like structures tend to propagate from the NWC–Kochi path to NWC–Chiba path with the time delay of ∼2 h, and so the wave propagation speed is estimated as ∼20 m/s. This finding might be important when we think of lithosphere–ionosphere coupling mechanism.     

Numerical simulation of LEP Trimpis observed at Poitiers,France, on signals from VLF transmitters

Transient amplitude and phase perturbations on subionospheric VLF signals, known as the Trimpi effect, are caused by the scattering of VLF radiation from localized ionization enhancements in the nighttime D-region. The patches of ionization are due to precipitation from the radiation belts of keV electrons, that is induced by lightning-generated whistlers.This work is concerned with the numerical simulation of such VLF perturbations, termed LEP (lightning-induced electron precipitation) or classic Trimpis. Two different codes are used to compute the VLF propagation in the Earth-ionosphere waveguide in the presence of a D-region inhomogeneity. The first is based on mode theory, and the second on the FDTD (finite-difference time-domain) method. Both codes are two-dimensional and, therefore, relevant only to LEP events lying on the transmitter-receiver great circle path (TRGCP). A method of simulation is proposed to interpret quantitatively VLF amplitude and phase changes in terms of the approximate location and size of the associated ionospheric perturbation along the TRGCP. The method is applied to LEP Trimpis observed at Poitiers (L = 2) on signals from the NAA and GQD transmitters. Results are discussed in the light of the information that may be deduced from the high-resolution analysis of VLF temporal signatures. A sequence of simultaneous LEP Trimpis observed at Poitiers on three widely separated transmission paths is studied; two alternative interpretations are proposed.     

A qualitative study of seismic-ionospheric precursor phenomena monitored by a very close to the epicenter VLF and LF receiver

This work investigates the occurrence of disturbances across a wide range of VLF and LF frequencies received prior to a seismic event (Mw = 4), that took place on May 12th 2012, the epicenter of which was very close (14 km) to the VLF/LF station. The signals analyzed were emitted from five VLF and five LF European transmitters. This seismic event produced precursory ionospheric disturbances, identified as spectral distortion, three days before its occurrence, providing a distinct pattern open to further investigation. Although the basis of the ionosphere interaction with seismic phenomena has been well documented in previous studies, the close proximity of the receiver to the seismic event provides a new perspective to this study. The monitored signals have undergone normalization and then they have been processed by means of the Hilbert-Huang Transform. Diagrams of the signals relevant to the phenomena are presented and the disturbances that are present in the raw data are accentuated through further processing.     

VLF/LF signal studies of the ionospheric response to strong seismic activity in the Far Eastern region combining the DEMETER and ground-based observations

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.     

2005年3月28日苏门答腊8.6级地震前的电离层扰动

2005年3月28日苏门答腊发生8.6级地震,通过分析处在地磁活动平静期的震前15天的数据,发现3月20、22、23和28日在赤道区域多个参量上出现电离层扰动,如电场和等离子体参量,认为这些电离层扰动可能与本次强震有关.震前电子密度异常有两种形态,一种是在原来的峰值变化区电子密度单调增加,幅度超过1σ,如3月20日和28日的异常;另外一种是改变了原来在赤道区的单峰变化形态,而呈现了双峰值和赤道谷值的变化形态,如3月22日和23日的扰动.分析结果显示,这两种形态的电子密度扰动都分布在一个比较大的范围内,经度上跨越100°,纬度范围30°,显示了比较长的异常持续时间和全球变化特性.VLF频段电场频谱的数据分析表明双峰形态的扰动不是磁共轭效应.同时低频电场扰动相对电子密度而言集中在一个相对小范围内,当N_e单调上升时,电场扰动却只出现在离震中最近的轨道上.分析认为震源产生的电场信号直接传播进入电离层是非常困难的,但是它可以通过岩石层-大气层-电离层圈层耦合机制造成电离层扰动.结合离子成分及其变化形态的分析,震前氧离子变化形态与氢、氦离子的形态相反,这与其他地震前的电离层异常扰动形态是类似的.综合分析认为,对于与地震相关的电离层异常的识别需要结合多个观测参量的联合分析.     

Spatiotemporal distributions of the electron density in the ionosphere by records of the total electron content and phase of VLF radio signals

Features of the structure and dynamics of the ionospheric plasma are studied in a comparison the ionospheric total electron content measurements with the phase and amplitude measurements of VLF–LF radio signals on global and regional paths. The ionospheric structure over Europe is reconstructed. The spatiotemporal dynamics of moving ionospheric disturbances under conditions of a powerful geomagnetic storm of March 17, 2015, is examined based on the reconstruction results. Analysis of the phase variation of VLF radio signals, together with the TEC measurement data, is not only an additional tool in the study of the dynamics of ionospheric disturbances; it also makes it possible to estimate electron density disturbances in different ionospheric layers.     

Relation between daytime and nighttime values of the critical frequency foF2

The relation between the daytime in the nighttime values of the critical frequencies (foF2) of the ionospheric F ₂ layer is considered. The correlation coefficient of foF2 measured at 1400 and 0200 LT of the same day is considered in various seasons of years with different solar activity (during the complete cycle of solar activity in 1979–1989). Special accent is made on the dependencies of the above mentioned correlation on a choice of magnetically quiet days with various limitations on maximal values of geomagnetic index Ap. It has been obtained that a statistically significant negative correlation between the foF2(1400) and foF2(0200) is more pronounced in the periods of high solar activity. The effect increases with increasing limitation of the considered days on value of Ap: the largest values of the correlation coefficient are observed if only very quiet days are considered (Ap < 6). There are preliminary indications that the considered relation between daytime and nighttime foF2 values depends on latitude.     

VLF phase and amplitude: daytime ionospheric parameters

Experimental observations of the daytime variations of VLF phase and amplitude over a variety of long subionospheric paths have been found to be satisfactorily modelled with a D-region ionosphere, described by the two traditional parameters, H′ and β (being measures of the ionospheric height and the rate of increase of electron density with height, respectively). This VLF radio modelling uses the NOSC Earth–ionosphere waveguide programs but with an experimentally deduced dependence of these two ionospheric parameters on solar zenith angle. Phase and amplitude measurements from several VLF Omega and MSK stations were compared with calculations from the programs LWPC and Modefinder using values of H′ and β determined previously from amplitude only data. This led to refined curves for the diurnal variations of H′ and β which, when used in these programs, give not only calculated amplitudes but also, for the first time, calculated phase variations that agree well with a series of observations at Dunedin, New Zealand, of VLF signals from Omega Japan, Omega Hawaii, NPM (Hawaii) and NLK (Seattle) covering a frequency range of 10–25 kHz.     

Time and spatial variations in the ratio of nighttime and daytime critical frequencies of the F 2 layer

The ratio of daytime and nighttime values of the foF2 critical frequency is analyzed on the basis of the data of 28 ionospheric stations in the Eastern Hemisphere. It is found that three types of time variations in this ratio are observed after 1980: an increase with time (a positive trend), a decrease with time (a negative trend), and the absence of pronounced changes (a zero trend). The sign of this trend is shown to be governed by the signs of the magnetic declination D and magnetic inclination I at the given ionospheric station. This fact makes it possible to assume that the above trend is caused by long-term variations in the zonal component V _ny of the horizontal wind in the thermosphere, the latter component contributing into the vertical drift velocity W. The causes of the systematic changes in the thermospheric circulation regime after 1980 are still unknown; however, it is quite probable that they are related to anthropogenic changes in the atmosphere.     

Aspects regarding the use of the INFREP network for identifying possible seismic precursors

In the last decades, one of the main research directions in identifying seismic precursors involved monitoring VLF (Very Low Frequency) and LF (Low Frequency) radio waves and analysing their propagation characteristics. Essentially this method consists of monitoring different available VLF and LF transmitters from long distance reception points. The received signal has two major components: the ground wave and the sky wave, where the sky wave propagates by reflection on the lower layers of the ionosphere. It is assumed that before and during major earthquakes, unusual changes may occur in the lower layers of the ionosphere, such as the modification of the charged particles number density and the altitude of the reflection zone. Therefore, these unusual changes in the ionosphere may generate unusual variations in the received signal level.The International Network for Frontier Research on Earthquake Precursors (INFREP) was developed starting with 2009 and consists of several dedicated VLF and LF radio receivers used for monitoring various radio transmitters located throughout Europe. The receivers’ locations were chosen so that the propagation path from these VLF/LF stations would pass over high seismicity regions while others were chosen to obtain different control paths.The monitoring receivers are capable of continuously measuring the received signal amplitude from the VLF/LF stations of interest. The recorded data is then stored and sent to an INFREP database, which is available on the Internet for scientific researchers. By processing and analysing VLF and LF data samples, collected at different reception points and at different periods of the year, one may be able to identify some distinct patterns in the envelope of the received signal level over time. Significant deviations from these patterns may have local causes such as the electromagnetic pollution at the monitoring point, regional causes like existing electrical storms over the propagation path or even global causes generated by high-intensity solar flares. As a consequence, classifying these perturbations and minimizing them (when possible) would represent an important step towards identifying significant pattern deviations caused by seismic activities.Taken into consideration some of the issues mentioned above, this paper intends to present some aspects meant to improve the overall performance of the existing INFREP network. The signal-to-noise ratio improvement of the monitoring receiver may be achieved by relocating the antenna (or even the entire monitoring system if possible) in areas with less electromagnetic pollution within the VLF and LF bands. Other solution may involve replacing the existing electric “whip” antennas with magnetic loop antennas.Regarding the measuring method, long-term averaging of the received signal to reduce the electromagnetic noise should be carefully applied. If the averaging time is too long, there is a risk that, during a seismic event, the details of the received signal envelope would be lost. Moreover, this may reduce the possibility of making correlations between the monitored stations and INFREP receivers in case of sudden ERP (Effective Radiated Power) variations of the VLF/LF stations. For the same reason, the time synchronization of the recorded data using (for instance) GPS technology is highly recommended.Other aspects related to the overall performance improvement of the INFREP network consist of monitoring other VLF/LF stations such as the Krasnodar station (south of Russia), part of the ALPHA/RSDN-20 VLF navigation system, or the 77.5 kHz DCF77 time signal transmitter (near Frankfurt am Main, Germany). Moreover, the installation of a new reception point in Romania (near Cluj-Napoca) for monitoring the Vrancea area (within the Carpathians Mountains) and the Adriatic region will provide complementary scientific data within the network.     

Anomalous Variations of Ionosphere Associated with the Strong Earthquake at Pakistan-Iran Border at a Low Latitude Station Agra,India

In this paper, we analyze the TEC data for April 2013 observed at Agra station, India (geogr. lat. 27.2° N, long. 78° E) to examine the effect of earthquake of magnitude M = 7.8 which occurred on 16 April 2013 at Pakistan–Iran border region. We process the TEC data using the s statistical criterion to find out anomalous variation in TEC data. We also study the VLF propagation signal from NPM, Hawaii (21.42° N, 158° W), which is monitored at the same station (Agra station) in the light of this earthquake as well as solar flares. The nighttime fluctuation method is used to analyze the VLF data for the period of ±5 days from the day of earthquake (11–21 April 2013). The anomalous enhancements and depletions are found in TEC data on 1–9 days before the occurrence of event.     

Spectral characteristics of magnetogravity waves generated by high energy mass source in the equatorial region of the atmosphere,Part II

In the first part of study the possibility of high frequency (HF) and low frequency (LF) modes of magnetogravity waves (MGW) propagation in the equatorial atmosphere in conditions of ionospheric F2 layer is demonstrated taking into account the finite conductivity. Here the spectral features of the excited HF MGW mode is studied. At the second part of study the dispersion relations for LF MGW in the ionospheric equatorial region is received. The spectral features of the disturbed components of medium displacement for LF MGW generated by a mass source are investigated. The results are applied to calculate the values of the spectral components of magnetogravity density, pressure and magnetic field disturbances caused by horizontal meteor sweep. The estimates may be useful in the analysis of ionospheric disturbances excited by high-energy geophysical sources.     

甚低频/低频人工源电波信号地震应用研究进展

甚低频/低频人工源电波作为一种主动源发射的通讯导航信号,因其长距离波导传播的特性在地-电离层观测及应用中表现出极大的优势。本文总结了二十世纪末以来基于甚低频/低频（VLF/LF）电波观测技术及其数据分析方法、典型震例及统计研究成果、圈层耦合机理等方面的研究进展,并通过探讨其应用成果的快速积累及前兆扰动起源等研究难点,展望了未来在我国多种地基和卫星电磁配合下的综合立体体系建设以及该技术可能发挥的最大贡献。      

Anomalous astronomical time-latitude residuals: a potential earthquake precursor

The geophysical mechanism behind astronomical time–latitude residuals (ATLR) are discussed. The photoelectric astrolabe at Yunnan Observatory (YO) observed apparent synchronous anomalous ATLR before the Wenchuan M8.0 earthquake (EQ) in May 12, 2008 and the Lushan M7.0 EQ n April 20, 2013. We compared the ATLR from the YO photoelectric astrolabe and EQ data since 1976. Anomalous ATLR was observed before several strong EQs in the Yunnan Province. We believe the photoelectric astrolabe can be used to predict strong EQs and the anomalous ATLR are a potential EQ precursor.     

Comparison of Ionospheric Parameters during Similar Geomagnetic Storms

The degree of closeness of ionospheric parameters during one magnetic storm and of the same parameters during another, similar, storm is estimated. Overall, four storms—two pairs of storms close in structure and appearance according to recording of the magnetic field Х-component—were analyzed. The examination was based on data from Sodankyla observatory (Finland). The f-graphs of the ionospheric vertical sounding, magnetometer data, and riometer data on absorption were used. The main results are as follows. The values of the critical frequencies foF2, foF1, and foE for different but similar magnetic storms differ insignificantly. In the daytime, the difference is on average 6% (from 0 to 11.1%) for all ionospheric layers. In the nighttime conditions, the difference for foF2 is 4%. The nighttime values of foEs differ on average by 20%. These estimates potentially make it possible to forecast ionospheric parameters for a particular storm.     

Variability of total electron content near the crest of the equatorial anomaly during moderate geomagnetic storms

The ionospheric responses to a large number (116) of moderate (?50≥D_st>?100 nT) geomagnetic storms distributed over the period (1980–1990) are investigated using total electron content (TEC) data recorded at Calcutta (88.38°E, 22.58°N geographic, dip: 32°N). TEC perturbations exhibit a prominent dependence on the local times of main phase occurrence (MPO). The storms with MPO during daytime hours are more effective in producing larger deviations and smaller time delays for maximum positive deviations compared to those with nighttime MPO. Though the perturbations in the equinoctial and winter solstitial months more or less follow the reported climatology, remarkable deviations are detected for the summer solstitial storms. Depending on the local times of MPO, the sunrise enhancement in TEC is greatly perturbed. The TEC variability patterns are interpreted in terms of the storm time modifications of equatorial electric field, wind system and neutral composition.