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Previous research on the interference of very low frequency (VLF) signals in the equator region was inadequate and largely concentrated in the middle and high latitude regions. Therefore, this research aims to determine the disruption of VLF waves in the ionospheric D-region above Malaysia, which is in the equator area. This paper presents observations of early/fast, early/slow, and lightning-induced electron precipitation (LEP) events in January 2010. Broadband and narrowband data are monitored and investigated using Japan’s JJI Ebino transmitter (32°40' N, 130°81' E) to the receiver at the Universiti Kebangsaan Malaysia (2°55' N, 101°46' E). Broadband and narrowband data are analyzed with theoretical considerations and linked to events from interference in the ionospheric D-region. Many early/fast, early/slow, and LEP events are found to originate from the lightning release activity emitted and may alter the amplitude and VLF signal phase in the lower layer ionosphere over Malaysia. [ABSTRACT FROM AUTHOR]

A threshold condition has been established for excitation of VLF electromagnetic radiation with a chorus structure of the dynamic spectrum in the daytime magnetosphere using the BPA (Beam Pulse Amplifier) mechanism for amplifying short noise electromagnetic pulses. The kappa distribution was used as a model function of the electron velocity distribution in the magnetosphere. Calculations performed for this distribution have shown that the threshold for excitation of chorus largely depends on the electron plasma parameter equal to the ratio of gas-kinetic pressure of electrons to magnetic pressure. This pattern is not contradicted by the dependence of the probability of excitation of chorus on the degree of magnetic field irregularity, which we derived from observations made by the Van Allen Probe spacecraft. It is sharp fluctuations in the magnetic field strength near its local minima outside the plasmasphere where the radiation under study can be excited. If there is an irregularity, the probability of detecting chorus is >70 %; and if there is no or very low irregularity, the probability of the absence of any emissions is ~80 %. The results indicate a common reason for the excitation of chorus and the magnetic field irregularity — a small but finite value of the plasma parameter.

We present a new four‐parameter model of the D‐region (60–90 km) ionospheric electron density, useful in very low frequency (VLF, 3–30 kHz) remote sensing. VLF waves have a long history of use to indirectly infer D‐region conditions, as they reflect efficiently and thus are sensitive to small changes in the electron density. Most historical efforts use VLF observations along with a forward model of the D‐region and VLF propagation. The ionospheric assumptions in the forward model are altered until the output matches the observation. The most common D‐region model, known as the Wait‐Spies ionosphere, takes the electron density as exponentially increasing with altitude and specifies a height and steepness. This model was designed to capture the VLF propagation variations evident at a single frequency. The real D‐region is likely more complex. The limited number of D‐region rocket passes that have previously been compiled tend to show the existence of a "ledge" somewhere between 70 and 90 km. Broadband VLF signals emitted from lightning allows a more sophisticated parametrization. Using carefully averaged amplitudes and phases of VLF sferics, we formulate a more general four‐parameter D‐region model that includes a ledge discontinuity. Using lightning‐emitted VLF observations along with a theoretical model, we find that this model better describes the ionosphere during the daytime. During the ambient nighttime and during a solar flare the two‐parameter ionosphere may be sufficient, at least for the purposes of calculating broadband VLF propagation, since the ledge either weakens or moves outside the altitude range of VLF sensitivity. Plain Language Summary: The Earth's ionosphere can be thought of as the border zone between Earth's atmosphere and space environment, between 60 and 1,000 km altitude. In this region, the thinning air is electrically charged by radiation from the sun, among other things, creating many free electrons. The ionosphere is important because, for example, it impacts several important forms of communication, both ground‐to‐ground and satellite‐to‐ground. The lowest part of the ionosphere is called the D‐region 60–90 km, and it is particularly tough to measure. For decades, researchers have fit a line to describe the number of electrons in the D‐region as a function of altitude. Fitting a line is not so good but it is often the best we can do since we have so little information. It is probably not very close to reality. By using untapped information contained in the radio emissions from lightning, which propagate in the VLF band (3–30 kHz) many thousands of kilomerter, we are able to describe the D‐region conditions with a more complicated shape that is closer to the physical reality. Key Points: A new four‐parameter model of D‐region ionospheric electron density is introduced to better capture the D‐region ledgeWith this model, VLF remote sensing with broadband lightning sferics detect the depth and location of the ledge during the daytimeDuring a solar flares and at nighttime, the ledge either dissipates or falls outside the altitude range of VLF sensitivity [ABSTRACT FROM AUTHOR]

Measurements of amplitude of vertical components of very low frequency (VLF) electric field emissions have been in progress at Chaumuhan, Mathura (27.5°N, 77.68°E), Uttar Pradesh, India, at the frequency of 3.012 kHz using a borehole antenna since 24 March 2011. In the present study, we have statistically analysed the VLF data from 1 January to 31 July 2015 using the mean (m) and standard deviation (s) around the mean (m ± 2s) criterion with respect to two major earthquakes (M = 7.8, 7.3) and their aftershocks that occurred on 25 April and 12 May 2015 in Khudi and Kodari regions of Nepal respectively. Results of the analysis show a spiky enhancement above the m + 2s line in the daily variation of the normalized VLF data, 7-14 days before the onset of main shocks of these earthquakes. These anomalous enhancements in the normalized values of amplitude are examined in light of various other possible causative sources such as magnetic storms, local lightning, solar flares and solar eclipse, rainfall, wind speed, temperature and pressure of the atmosphere. It was found that the VLF amplitude anomalies are not related to these sources. Further, we have developed a theoretical model for generation and propagation mechanisms of VLF emissions associated with the said earthquakes and their aftershocks by assuming an ensemble of elementary radiators randomly oriented and distributed in space and time in the preparation zones of these earthquakes. The computed electric field from the model is in good agreement with the observations. [ABSTRACT FROM AUTHOR]

We constructed a program, we call "Iriatten," to calculate using parameters from the International Reference Ionosphere, attenuation of Very Low Frequency (VLF) radiation transiting the ionosphere. The calculations are performed as function of latitude, longitude, year, month, day, and hour and based upon the quasi‐longitudinal approximation put forth by Helliwell, where the wave vector k, is in the direction of propagation, and follows the Earth's magnetic field lines. The best values of various parameters are taken from the literature. Attenuation occurs through electron neutral collisions. In his seminal 1965 text, Helliwell presented atmospheric attenuation curves for day and night, 2 and 20 kHz, as a function of latitude. Program Iriatten is used to re‐calculate Helliwell's curves and then to fully exercise access to seasonality. High power VLF transmitters are utilized as a reference signal and two physical models are used to represent them. In the Vertically Inclined model, the attenuation is assumed to occur "straight up" from the location of the VLF transmitter. In the Crary model waves are launched from a dipole source and ionospheric attenuation is applied where the wave is turned up in the Earth‐ionospheric waveguide. Both representations reveal a seasonality that is unmistakable. Using the two models the ionospheric attenuation is calculated for four VLF transmitters and compared to observations by the DEMETER. Results from the Crary transmitter model more realistically portray seasonal variations exemplified by DEMETER satellite data. Crary model errors, which are within 11 dB, are lower than or comparable to Vertically Inclined errors. Key Points: There is a seasonality in VLF attenuation for waves transiting the ionosphereCrary model most accurately portrays the seasonality, as verified by DEMETER DataCrary model errors, within 11 dB, are lower than, or comparable to Vertically Inclined model errors [ABSTRACT FROM AUTHOR]

The new type of daytime natural very low frequency (VLF) whistler mode emissions of the magnetospheric origin was found in the VLF observations at Kannuslehto station (L ~ 5.5) in Northern Finland. The events occurred at the frequencies above 4-5 kHz even up to 15 kHz. These emissions have never been observed earlier in the spectrograms because they were hidden by strong impulsive atmospherics (sferics) in the same frequency range originating in lightning discharges. After filtering out the sferics, we surprisingly discovered completely new types of high frequency right-hand polarized peculiar VLF emissions with various unusual dynamic spectra. These new VLF emissions typically occur as a sequence of a number of short (~1-3 min) burst-like structures or single short patches with the total duration up to a few hours. The spectral peculiarities of several long-lasting VLF events as well as individual short VLF patches are discussed. The dynamic spectra of these new VLF patches raise the question of the temporal and spatial details of the wave-particle interactions in the magnetospheric plasma. These emissions are observed only under quiet or weakly disturbed space weather conditions, but during small negative values of the Dst index, indicating the presence of a certain excess of the radiation belt electrons. Note, these high frequency VLF patches are observed at the frequencies much higher than the half of the equatorial electron gyrofrequency which is equal to ~2.7 kHz at L ~ 5.5. It seems that these emissions are generated deep in the magnetosphere, but the detailed nature, generation region, and propagation behavior of these newly discovered VLF emissions remain unknown. An appearance of high frequency VLF patches could be an indirect indicator of a local enhancement of electron fluxes in the radiation belt that are not directly measured and may occur even in the absence of visible ground-based geomagnetic disturbances. Further researches may shed new light on wave-particle interactions occurring in the Earth's radiation belts. [ABSTRACT FROM AUTHOR]

We study quasiperiodic very low frequency (VLF) emissions observed simultaneously by Van Allen Probes spacecraft and Kannuslehto and Lovozero ground‐based stations on 25 December 2015. Both Van Allen Probes A and B detected quasiperiodic emissions, probably originated from a common source, and observed on the ground. In order to locate possible regions of wave generation, we analyze wave‐normal angles with respect to the geomagnetic field, Poynting flux direction, and cyclotron instability growth rate calculated by using the measured phase space density of energetic electrons. We demonstrate that even parallel wave propagation and proper (downward) Poynting flux direction are not sufficient for claiming observations to be in the source region. Agreement between the growth rate and emission bands was obtained for a restricted part of Van Allen Probe A trajectory corresponding to localized enhancement of plasma density with scale of 700 km. We employ spacecraft density data to build a model plasma profile and to calculate ray trajectories from the point of wave detection in space to the ionosphere and examine the possibility of their propagation toward the ground. For the considered event, the wave could propagate toward the ground in the geomagnetic flux tube with enhanced plasma density, which ensured ducted propagation. The region of wave exit was confirmed by the analysis of wave propagation direction at the ground detection point. Key Points: Complex analysis of generation region of quasiperiodic VLF emissions were observed simultaneously by two Van Allen Probes and on the groundWave growth rate frequency band matched the observed emission band only in a localized (∼0.3 Earth radii) region along Van Allen Probe A trajectoryRay tracing demonstrated the importance of a density duct for wave propagation toward the ground [ABSTRACT FROM AUTHOR]

Unquestionably, current density pseudo-section derived from in-phase very low frequency (VLF) data, developed by Karous and Hjelt, has been one of the most popular preliminary schemes for the interpretation of VLF data due to its straightforwardness and applicability. In practice, however, the Karous and Hjelt filtering method does not behave accurately and has some weaknesses: (i) the depth estimation is erratic, even in shallow cases, (ii) to a far lesser extent, the horizontal estimation is inaccurate, in particular when the sources interfere with each other, and (iii) under circumstances where multiple bodies are in close proximity, spurious artifacts appear. These factors could be misleading and result in a wrong interpretation. Changing the Biot–Savart law into a linear equation and employing inverse theory in this paper, the precision of spatial estimations, in the case of superficial objects, becomes far more precise. More importantly, independent of the number of neighboring targets and their closeness, no spurious artifacts are observed. This methodology is tested on synthetic isolated and interfering various models with different sizes, depths, shapes and resistivities. It is also applied to real data over the 'Ghoori Ghale' cave in the vicinity of Kermanshah, Iran, and its satisfying and noteworthy solutions are compared with the Karous and Hjelt method. [ABSTRACT FROM AUTHOR]

We present perturbations in very low frequency (VLF) signals received at Ionospheric & Earthquake Research Centre (IERC) (Lat. 22.50 ° N, Long. 87.48 ° E) during and prior to two earthquakes, one on 11 March 2011 at 11:16:24 LT (M = 9) in Honshu, Japan and another on 12 May 2015 at 12:35:19 LT (M = 7.3) in Kodari, Nepal. The VLF signal transmitted from JJI (22.2 kHz) in Japan (Lat. 32.05 ° N, Long. 131.51 ° E) showed strong shift in VLF-sunrise terminator times towards night-time few days prior to both the earthquakes. These two earthquakes took place near the transmitter JJI and receiver IERC respectively. We simulated the VLF sunrise terminator time shifts using the long wavelength propagation capability (LWPC) code. To effectively represent the D-region ionospheric variabilities, we assumed a mean dynamic perturbation over the path and presented them with a set of effective Wait's parameters (βeff, h eff ′ ). We have successfully reproduced the temporal trend of the normalized VLF signal amplitude at VLF sunrise terminators for a few days around both the earthquakes. We used Wait's exponential model for estimating the altitude profile of D-region electron density ( N e (h) ) at VLF sunrise terminator times around both the earthquakes and quantified the changes of those N e (h) profiles. [ABSTRACT FROM AUTHOR]

In this study, we investigate the very low frequency (VLF) signals in the ionosphere propagating from ground‐based transmitters by using the data from the newly launched China Seismo‐Electromagnetic Satellite (CSES), which is also called ZH‐1. The simultaneous ionospheric perturbations and electron precipitations induced by the NWC transmitter are also analyzed, and the possible mechanisms are discussed. The ZH‐1 and DEMETER observations of VLF transmitters are compared morphologically, which are generally consistent with the full‐wave simulations. The results show that the electromagnetic waves excited by the VLF transmitters penetrate into the ionosphere and propagate in the magnetosphere to its conjugate hemisphere as a ducted or nonducted whistler. When the VLF transmitter is located at L < 1.5, the wave propagates more easily as a nonducted whistler. When the wave propagates as a ducted whistler, the point of maximum observed transmitter‐induced electric field in the conjugate hemisphere will move poleward in the northern hemisphere and equatorward in the southern hemisphere. The results also show that the data stability of electromagnetic field recorded by ZH‐1 satellite is almost the same as DEMETER satellite, and there is a good correlation between the ZH‐1 observation and simulated results. These conclusions demonstrate that the nightside VLF band electromagnetic observation of ZH‐1 satellite is stable and reliable. Key Points: ZH‐1 reveals that VLF transmitters could induce significant enhancement in the electromagnetic field in the ionosphereIonospheric heating and energetic electron precipitation induced by NWC transmitter are confirmed by ZH‐1 observationZH‐1 observations of electromagnetic field radiated from VLF transmitters are consistent with the full‐wave simulation results [ABSTRACT FROM AUTHOR]

Abstract Beat wave (BW) high frequency (HF) ionospheric heating experiments were conducted to generate very low frequency (VLF) waves. The VLF waves were registered with a VLF receiver located ∼15 km east of the European Incoherent Scatter (EISCAT) heating facility in Tromsø, Norway. A fluxgate magnetometer was used to monitor auroral electrojet current, and ionospheric conditions were measured using a Dynasonde. Correlation coefficients between VLF amplitudes and the deviation of geomagnetic north–south components were calculated. Experimental results show that strong and positive correlation exists the majority of the time, but sometimes no correlation or even a negative correlation occurred. This is consistent with similar past experiments that took place with exclusively AM generation. These results therefore support the conclusion that BW generation of VLF waves is no different than with AM, likely occurring in the D or lower E ionospheric region. [ABSTRACT FROM AUTHOR]

Ground observations of VLF (very low frequency) waves have often been used to infer VLF activity in the magnetosphere; however, they are not an unbiased measure of activity at satellite altitudes due to transionospheric absorption and subionospheric attenuation. We propose several empirical models that control for these effects. VLF power spectral density (PSD) from the VLF/ELF Logger Experiment (VELOX, L=4.6, Halley, Antarctica) is used to predict DEMETER low Earth orbit VLF PSD. Validation correlations of these models are as high as 0.764; thus, ground VLF receivers spaced around the Earth could provide coverage of outer radiation belt lower band chorus over the latitudinal limits of this model (±45–75°). Correlations of four frequency bands (centered at 0.5, 1.0, 2.0, and 4.25 kHz) are compared. The simple linear correlation between ground and satellite VLF PSD in the 1.0‐kHz channel was 0.606 (at dawn). A cubic model resulted in higher correlation (0.638). VLF penetration to the ground is reduced by ionospheric absorption during solar illumination and by disruption of ducting field lines during disturbed conditions. Subionospheric attenuation also reduces VLF observations from distant field lines. Addition of these covariates improved predictions. Both solar illumination and disturbed conditions reduced ground observation of VLF PSD, with higher power waves penetrating to the ground proportionately less than lower power waves. The effect of illumination in reducing wave penetration was more pronounced at higher frequency (4.25 kHz), with the effect at a midrange frequency (2.0 kHz) falling between these two extremes. Key Points: Models accounting for transionospheric absorption and subionospheric attenuation improve satellite‐ground VLF PSD correlationsValidation of these empirical models resulted in correlations between predicted and observed satellite VLF PSD of up to 0.764Ground VLF receivers spaced around the Earth could provide longitudinal coverage of outer radiation belt chorus over ±45–75° latitude [ABSTRACT FROM AUTHOR]

Ground‐based Pc5 ultralow frequency (ULF) wave power in multiple ground‐based meridians is compared to the very low frequency (VLF) wave amplitude proxy, derived from Polar Operational Environmental Satellite (POES) precipitation, for the 33 storms studied by Li et al. (2015, https://doi.org/10.1002/2015GL065342). The results reveal common L‐shell and time profiles for the ULF waves and VLF proxy for every single storm, especially at L ≤ 6, and identical discrimination between efficient and inefficient radiation belt electron acceleration. The observations imply either ULF waves play a role in driving precipitation, which is falsely interpreted as VLF wave power in the proxy, ULF waves drive VLF waves (the reverse being energetically unfeasible), or both have a common driver with nearly identical L‐shell and time dependence. Global ground‐based ULF wave power coherence implies that a small number of meridians can be used to estimate storm time radial diffusion coefficients. However, the strong correspondence between ULF wave power and VLF wave proxy complicates causative assessments of electron acceleration. Plain Language Summary: Several studies have been using satellites to indirectly infer the presence of a type of very low frequency (VLF) waves by measuring the precipitation of electrons down to Earth. By using ground instruments to measure the power of ultralow frequency (ULF) waves during a set of geomagnetic storms, we find that their morphology almost perfectly matches that of the inferred VLF ones. This suggests either that the ULF waves cause precipitation themselves, thereby polluting the indirect method of inferring VLF waves, or that they produce VLF waves, or that both ULF and VLF waves are produced at the same time and place by a common cause. Since the presence of ULF or VLF waves is needed for different methods of electron acceleration in space, this observation affects the discussion on which method is more important during geomagnetic storms. We also find that those ULF wave measurements are very similar for different terrestrial longitudes. Key Points: There is a common L‐shell and time profile of storm time ground‐based ULF wave power and the POES proxy for VLF wave amplitudeGlobal ground‐based ULF wave power coherence implies that a small number of meridians can be used to characterize radial diffusion coefficientsULF wave power and the POES VLF wave proxy discriminate identically between efficient and inefficient radiation belt acceleration events [ABSTRACT FROM AUTHOR]

In the tourism area of Goa Kiskendo, Jatimulyo Village, district Girimulyo of Kulonprogo, Yogyakarta province which is quite popular for naturally cave tourist destination. In November 2017, there was a landslide that broke the transportation route to the Goa Kiskendo, Yogyakarta-Central Java province tourism area. Based on the Center of Volcanology for Disaster and Geological Mitigation map of soil movement in Girimulyo district, Jatimulyo Village is in the medium to high-risk movement zone. To anticipate aftershocks of a landslide in the same area, it is necessary to map areas that are potentially landslide occurs. For mapping, this area, the geophysical surveys using resistivity method with dipole-dipole configuration and electromagnetic very low frequency (VLF) resistivity mode were conducted in Goa Kiskendo area to identify the slip surface based on the contrast resistivity value. From the results of resistivity data processing, and 3D visualization it is obtained a contrast of different resistance values consist of two layers. The first layer, which is in the range of 10 Ωm-79 Ωm, is identified as soil and the second layer, which is in the range of 0,15 Ωm- 10 Ωm is interpreted as water saturated limestone. Water saturated limestone is identified as the slip surface or bedrock in the depth of 3 m - 25 m, and the soil movement when the landslide occurs, the materials will move from the north to southeastward. [ABSTRACT FROM AUTHOR]

Indonesia is a country with the largest geothermal reserves in the world, but exploration and utilization of geothermal energy are still low. As an effort in increasing exploration process, an integrated geophysical method to image the subsurface structure of the geothermal system has been done at Seulawah Agam Geothermal Field, Aceh Province, Indonesia. Very low frequency (VLF-R) method was used for shallow and magnetotellurics method for deep structures studies in the area. The magnetic method was used for a complementary of the electromagnetic methods. Magnetotelluric data were acquired at ten stations along a 30 km distance profile crossing the Seulawah Agam volcano in a north-south direction. The VLV-R and Magnetic stations were measured on the Ie Ju manifestation area only, located at the northern slope of the volcano. The magnetic and VLF-R methods have been able to investigate the local structure, while the regional structure of subsurface is described by the magnetotelluric method. 2D resistivity model of magnetotelluric data shows a conceptual model of the geothermal system based on resistivity variation. The model also reveals two local faults crossing the volcano in a northwest-southeast direction. Most of the geothermal manifestations in Seulawah Agam are controlled these faults. Application of VLF-R and magnetic methods on the Ie Ju manifestation area has shown clearly near surface structures of the area. The inverted models have strong agreement with a reduction to the pole of total magnetic field anomalies. Presence of the conductors in the resistivity models coincides with low total magnetic field anomalies. In addition, the total magnetic field anomalies map can be also used to locate the presence of geothermal manifestation with or without their appearance on the surface [ABSTRACT FROM AUTHOR]

We present experimental results on the detection of artificial irregularities produced in the ionospheric plasma by a high-power VLF transmitter. The analysis of TEC variations and ionosonde data was used for the diagnostics of ionospheric turbulence. Spatial distribution of the electric field amplitude of the VLF transmitter was calculated for selection of the NAVSTAR and GPS satellites the signal from which passes through the perturbed region. A quite different TEC variation curve was observed in the reference measurement when the transmitter was out of operation. Operation of the VLF transmitter resulted in the appearance of F spread in the ionosonde data (the effect took place at night). The experiment showed for the first time that with the help of ground-based measurements of TEC it is possible to study the region of turbulence in the outer ionosphere induced by the operation of a VLF radio station. [ABSTRACT FROM AUTHOR]

The main objective of the current work is to develop a Very Low Frequency (VLF) monitoring system for characterizing low layer ionosphere. Low layer ionosphere is important for communication, and some researchers stressed that lower layer ionosphere can be a precursor to earthquake event. The VLF monitoring system composed of a 1-m antenna, a preamplifier to amplify the signal, an ADC converter and a data acquisition system. The optimization of 1-m antenna received dual frequencies 19.2 kHz and 19.8 kHz from South Vijayanarayanam, India and North West Cape, Australia, respectively. Subsequently, a low pass filter was designed in the preamplifier to pass VLF signals from 0{30 kHz. The results revealed that the system was able to characterize diurnal variation: sunrise and sunset and detect changes in the lower layer ionosphere region due to Solar activity. [ABSTRACT FROM AUTHOR]

Very Low Frequency (VLF) transmitter signals propagate in guided modes through the earthionosphere waveguide (EIWG) with very little attenuation and can be received literally around the globe. The measurements of the amplitudes of VLF signals is the most cost effective method for studying the propagational features of VLF waves in the EIWG and probe the transient ionospheric disturbances in the D-region. Our space weather monitors measure the effects on Earth which produce by solar flares by tracking the changes in VLF radio transmissions as itreflected by ionosphere. Solar Center at Stanford University's has developed an inexpensive space weather monitors that students can install and use . The instruments detect changes to the earth’s ionosphere caused by solar flares and other disturbances. A Super SIDs receiver was installed in the location of the Faculty of Science - University of Thi-Qar , to start the project through the design of our antenna. Data collection and analysis is handled by a local PC . VLF signals from the five navigational transmitters; 1) TBB , Turkey (26.7 KHZ), 2) DHO38 , Germany (23.4 KHZ) , 3) HWU , France (21.75 KHZ), 4) GQD , UK (22.1 KHZ) , 5) NON , Occupied Palestine (29.9 KHZ) continuously from May 2017 to 15 September 2017 and analyzed to accomplish the scientific objectives of this work. The signal strengths vary over the 24-hour period due to varying illumination of the transmitter receiver great circle paths (TRGCPs) by the sun. The night and day time signal strengths are different from each other due to the change in the reflection conditions at the lower ionosphere. Besides this, the signal strengths are largely affected by the discontinuity over the day and night propagation paths at the terminator which results in signal fading (minima) during sunrise and sunset transition of the terminator. Effects of solar flares that occurred during the monitoring period on the amplitudes of five VLF transmitter signals as mentioned earlier have also been studied. The amplitude enhancements of 1.3-15 dB for the solar flares of classes B9.2 - X9.3 respectively have been observed. [ABSTRACT FROM AUTHOR]

We reveal previously unknown quasi-periodic (QP) very low frequency (VLF) emissions at the unusually high-frequency band of ~7-12 kHz by applying the digital filtering of strong atmospherics to the ground-based VLF data recorded at Kannuslehto station (KAN). It is located in northern Finland at L ~ 5:5. The frequencies of QP emissions are much higher than the equatorial electron gyrofrequency at L ~ 5:5. Thus, these emissions must have been generated at much lower L shells than KAN. Two highfrequency QP emission events have been studied in detail. The emissions were right-hand polarized waves indicating an overhead location of the exit area of waves in the ionosphere. In one event, the spectral-temporal forms of the emissions looked like a series of giant "bullets" due to the very abrupt cessation. Unfortunately, we could not explain such a strange dynamic spectral shape of the waves. In the second event, the modulation period was about 3 min under the absence of simultaneous geomagnetic pulsations. The studied emissions lasted about 4 h and were observed under the very quiet geomagnetic activity. The adequate mechanisms of the generation and propagation of the revealed highfrequency QP emissions have not yet been established. We speculate that studied QP emissions can be attributed to the auto-oscillations of the cyclotron instability in the magnetospheric plasma maser. [ABSTRACT FROM AUTHOR]

This paper is a review of manmade waves observed by the ionospheric satellite DEMETER. It concerns waves emitted by the ground-based VLF and ELF transmitters, by broadcasting stations, by the power line harmonic radiation, by industrial noise, and by active experiments. Examples are shown including, for the first time, the record of a wave coming from an ELF transmitter. These waves propagate upwards in the magnetosphere and they can be observed in the magnetically conjugated region of emission. Depending on their frequencies, they perturb the ionosphere and the particles in the radiation belts, and additional emissions are triggered. [ABSTRACT FROM AUTHOR]

We propose a model of forming the signals and interference in the very low frequency wave range. Using this model, we determine the potentials of the space-polarization interference compensators in a communication channel with natural interference and jamming. [ABSTRACT FROM AUTHOR]

This paper summarizes a unique observation of storm-time VLF chorus emissions obtained during investigation of VLF data collected over a period of about one year at our low latitude ground-based Indian station Jammu (geomag. lat., 22 deg 26 min North, $L=1.17$ ) in order to connect the peculiarities of this ground-observed VLF chorus with the plasma processes in the magnetosphere during geomagnetic disturbances. The upper boundary frequency method (UBF-method) based on the measurement of the upper boundary frequency of the ground-observed chorus is used for the determination of the location of the source of storm-time chorus emissions observed at our ground station Jammu. Dynamics of chorus spectra of the emissions observed at Jammu on 18 February 1998 in daytime during magnetic storm period are investigated in detail in order to solve the physics of the ground-observed chorus emissions at low latitudes. To explain various temporal and spectral features of storm-time VLF chorus emissions observed at Jammu, a generation mechanism based on the backward wave oscillator (BWO) regime of the magnetospheric cyclotron maser has been presented. Using BWO model, the calculation of some magnetospheric parameters such as plasma density, large-scale electric field etc. are carried out. The conformity between the calculated and typical measured parameters is shown and discussed. Finally the conclusion of this study is reported. [ABSTRACT FROM AUTHOR]

The ionospheric D-region (~60 km up to ~95 km) and the corresponding neutral atmosphere, often referred to as the mesosphere-lower thermosphere (MLT), are challenging and costly to probe in situ. Therefore, remote sensing techniques have been developed over the years. One of these is based on very low frequency (VLF, 3-30 kHz) electromagnetic waves generated by various natural and man-made sources. VLF waves propagate within the Earth-ionosphere waveguide and are extremely sensitive to perturbations occurring in the D-region along their propagation path. Hence, measurements of these signals serve as an inexpensive remote sensing technique for probing the lower ionosphere and the MLT region. This paper reviews the use of VLF narrowband (NB) signals (generated by man-made transmitters) in the study of the D-region and the MLT for over 90 years. The fields of research span time scales from microseconds to decadal variability and incorporate lightning-induced short-term perturbations; extraterrestrial radiation bursts; energetic particle precipitation events; solar eclipses; lower atmospheric waves penetrating into the D-region; sudden stratospheric warming events; the annual oscillation; the solar cycle; and, finally, the potential use of VLF NB measurements as an anthropogenic climate change monitoring technique. [ABSTRACT FROM AUTHOR]

Objective: It is thought that abnormal cardiac impulses of the autonomic nervous system during sleep are responsible for sleep-related bradyarrhythmias. Despite a proposed common etiopathogenesis and having common name of “sleep-related bradyarrhythmias,” precise importance of sinoatrial or atrioventricular (AV) node involvement remains elusive. This study aimed to determine whether there is a difference in sleeprelated bradyarrhythmias from the point of view of heart rate variability (HRV). Methods: Patients were evaluated using 24-hour Holter electrocardiogram monitor. After careful medical evaluation, apparently healthy individuals with sleep-related sinus pauses ≥2 seconds on at least 1 occasion or those in whom Mobitz type I AV block occurred were included. Frequency and time domain analyses were conducted for daytime, nighttime, and 24-hour period. Results: Total of 37 patients with sinus pause(s), 40 patients with Mobitz type I AV block(s), and 40 healthy controls were included. On HRV analyses, all time and frequency domain parameters were better in sinus pause group for daytime, nighttime, and 24-hour average (p<0.05 for all). Results of heart rate-corrected HRV analyses still showed significantly better total power (TP) and very low frequency (VLF) in the sinus pause group compared with AV block group (TP: 7.1×10-3 vs. 5.4×10-3, p=0.011; VLF: 4.9×10-3 vs. 3.7×10-3, p=0.007). Conclusion: Despite proposed common autonomic mechanisms, sleep-related sinus pause cases demonstrated better HRV profile in comparison with Mobitz type I AV block. [ABSTRACT FROM AUTHOR]

This paper is concerned with a new, previously unknown type of high-frequency (above 4 kHz) VLF emissions that were detected during winter VLF campaigns in Kannuslehto ( L ~ 5.5), Finland. These previously unknown emissions have been discovered as a result of the application of special digital filtering: it clears the VLF records from pulse signals of intensive atmospherics, which prevent other kinds of VLF emissions in the same frequency range from being seen on spectrograms. As it appears, aside from wellknown bursts of auroral hisses and discrete quasiperiodic emissions, a previously unknown type of daytime right-hand polarized VLF waves is also present at frequencies above 4 kHz. These emissions can persist for several hours as series of separate short discrete wideband (from 4 to 10 kHz and higher) signals, each with a duration between one and several minutes. It has been found that such signals can be observed almost daily in winter. These emissions sound like bird's chirping to a human ear; for that reason, they were called 'bird emissions.' The dynamic spectra of individual signals often resemble flying birds. The signals are observed during daytime, more often in magnetically quiet conditions preceded by geomagnetic disturbances. As a rule, the occurrence of these bird emissions is accompanied by a slight increase in electron density in the lower ionosphere, which is evidence of the precipitation of energetic (>30 keV) electrons. This raises a number of questions as to where and how the VLF bird emissions are generated and how such emissions, at frequencies greatly exceeding half the electron equatorial gyrofrequency at L ~ 5.5, can reach the Earth's surface. [ABSTRACT FROM AUTHOR]

The lightning localization task is very interested for the meteorologist organizations, airlines, networks operators, military and etc. The electromagnetic properties of lightning can be measured using VLF sensor. The paper deals with some hardware and software of VLF sensors used in IKIR FEB RAS. [ABSTRACT FROM AUTHOR]

Ionosphere undergoes permanently solar flares that quickly change its properties inducing sometime unwanted effects. These changes, or events, are known as Sudden Ionospheric Disturbances (SIDs) and the knowledge of their magnitude may be of great interest to anticipate probable damages. Currently, there does not exist any classification of these ionospheric changes based on their amplitude due to the wide variability of its responses. The only way to surmise their importance is to study them indirectly, throughout the classification of the X-ray flux intensity recorded by satellites. An attempt of classification based on their duration was proposed by the American Association of Variable Star Observers (AAVSO) but it is not very accurate because SID's duration is measured directly from the raw signal of the Very Low Frequency (VLF) signal and/or the Low Frequency (LF) signal. The aim of this work is to investigate, through a set of simple mathematical techniques applied to VLF/LF signals recorded by ground based receivers, the best method to estimate SIDs durations and then propose a new classification based on these durations. [ABSTRACT FROM AUTHOR]

In this study, the heating of the nighttime lower ionosphere due to electromagnetic radiation in the very low frequency (VLF) band that are transmitted by cloud-to-ground (CG) lightning return strokes is investigated. For this purpose, temperature of electrons in lower ionosphere is calculated by using electron energy balance equation, which is obtained by using Maxwellian distribution. In the result of calculations, in 10 V/m of the electrical field for all modes of electromagnetic (EM) wave, it was observed that the electron temperatures increased by about 9000-11000 K at an altitude of about 85-90 km. With an increase in the electric field, it was observed that the altitude, where the maximum temperatures were reached, shifted higher. The Right-Handed mode of the EM wave, unlike other modes was not return based-state to an altitude of 95-100 km. To fully determine the effect of lightning induced electromagnetic waves on the lower ionosphere, considering the effects of polarized modes (Right and Left) can also provide more information. [ABSTRACT FROM AUTHOR]

A 2D inversion approach is developed to interpret VLF electromagnetic measurement recorded over variable topography. To depict the variable topography accurately, an octree mesh discretization is incorporated. Subsurface structure is modeled in terms of apparent current density distribution and compared with the inversion results for actual resistivity distribution obtained using numerical techniques. The study demonstrates that the results obtained using both approaches (current density and resistivity distribution) are comparable, but due to analytical expression, current density imaging is faster. The conjugate gradient method is used to reduce the computation time and storage space when solving the matrix equations, resulting in feasible and practical imaging inversion of VLF data. The preconditioned matrix, which is determined by the distances between the blocks and observation points, has an important function in improving the resolution. In case of flat earth, preconditioned conjugate gradient inversion of data results in images that are comparable to those obtained using resistivity inversion. We also test whether topography variation in the order of skin depth is significant to incorporate topography in the modeling. The example of a topographical field VLF data inversion shows the efficacy of the presented approach to appraise the subsurface structure in terms of current density. [ABSTRACT FROM AUTHOR]

Background: Research on functional water has revealed its physiological functions and health-beneficial effects. Accordingly, this study explored the effects of drinking an average dietary volume of functional coherent mineral water (FCMW) on blood pressure, short-term cardiovascular variables, fractal heart rhythm dynamics, and salivary immunoglobulin A. Methods: Fifteen healthy subjects were randomized in a pre-post crossover design, selected the morning after an overnight fast. Short-term electrocardiography (ECG) was measured before and after drinking either control mineral water (CMW) or FCMW on two separate occasions. Saliva was collected for four minutes before the start of each ECG measurement. Blood pressure was monitored in five-minute intervals for one hour. Results: Drinking 100 ml of FCMW temporarily increased arterial blood pressure at 20 to 25 minutes in the 60 minutes post-drinking. Drinking CMW led to a significant reduction in the heart rate, while all-time domain and Power Spectral Density parameters (PSD) were unaffected. Consumption of FCMW resulted in a highly significant difference and decrease in heart rate, and an increase in NN interbeat intervals, and in two of the PSD parameters. A large increase in total power had a significant increasing effect on 2-3-minute oscillations of the very low frequency (VLF) power, indicating a specific change in the heart's intrinsic cardiac rhythm that is fundamental to health and well-being. An extension in mono and multifractal scaling of heart rate dynamics defines a healthy function and non-local adaptability, indicating a higher capacity to respond to unpredictable stimuli and stresses, a function which improves autonomic stability. The effect on the heart rate and VLF power suggest an efficient cellular metabolism and a stabilizing effect on mucosal immunoglobulin A. Conclusions: Functional coherent mineral water with an apparent non-local electromagnetic identity triggered in healthy subjects a sustainable sympathetic response in cardiac VLF power, which is considered an intrinsic health-promoting rhythm produced by the heart itself that accompanies adaptive mono and multifractal heart rhythm dynamics. [ABSTRACT FROM AUTHOR]

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 σ 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. [ABSTRACT FROM AUTHOR]

We present here the results of sub-ionospheric VLF perturbations observed on NWC (19.8 kHz) transmitter signal propagating in the Earth-ionosphere waveguide, monitored at our low latitude station Agra. During the period of observation (June-December 2011), we found 89 cases of VLF perturbation, while only 73 cases showing early character associated with strong lightning discharges. Out of 73 events, 64 (~84%) of the early VLF perturbations are found to be early/slow in nature; the remaining 9 events are early/fast. The onset duration of these early/slow VLF perturbations is up to ~ 5 s. A total of 54 observed early events show amplitude change lying between ± 3.0 dB, and phase change ± 12 degree, respectively, and found to occur mainly during nighttime. One of the interesting results we found is that the events with larger recovery time lie far away from the VLF propagation path, while events with smaller duration of recovery are within the ± 50-100 km of signal path. The World Wide Lightning Location Network (WWLLN) data is analysed to find the location of causative lightning and temporal variation. The lightning discharge and associated processes that lead to early VLF events are discussed. [ABSTRACT FROM AUTHOR]

Seasonal shortages of groundwater are common in parts of the Arabian Shield, where complex basement hydrogeology can make siting of water wells difficult. To identify optimal production well locations, six 200-400 m-long Very Low Frequency (VLF) electromagnetic traverses and ten Vertical Electrical Soundings (VESes) were run at the western edge of the Arabian Shield near At-Taif town, Saudi Arabia. Here wadi sediments overlie fractured Precambrian basement, which in turn overlies unfractured basement. The fractured basement forms the water supply aquifer. Both VLF and VES data indicate significant lateral heterogeneity in the electrical conductivity of both wadi and basement deposits over lengths scales as small as ∼100 m. VES results correlate closely with data from two wells in the study area. The change in resistivity at the wadi-to-fractured basement contact is relatively subtle, but the transition from low resistivity fractured basement to high resistivity unfractured basement is well resolved. Inferred wadi thicknesses range from 0 to 14 m; the electrically conductive fractured basement extends from wadi down to 12–32 m depth. VES data indicate the fractured basement aquifer thickens progressively to the south in this area. A production well, sited on the basis of the VES analysis, successfully yielded 70m 3 /day. The relationship between VLF and VES data is complex, suggesting that the terrain is heterogeneous on the scale of the different effective sampling volumes of the two methods, and/or that fracture azimuth is locally heterogeneous. Overall resistivities in this study are similar to those observed at other locations in Saudi Arabia, suggesting these methods may be widely applicable for siting of groundwater wells in the complex basement of the Arabian Shield. [ABSTRACT FROM AUTHOR]

The Global Ionospheric Flare Detection System (GIFDS) is currently under development at the German Aerospace Center as a ground based detector for continuous monitoring of the solar flare activity in order to provide real time warnings on solar X-ray events. GIFDS is using Very Low Frequency (VLF) radio transmissions in the northern hemisphere which respond to enhanced ionization in the bottomside ionosphere caused by X-ray flares. Since solar flares can only be detected during daytime, VLF receivers have to be installed around the globe to guarantee continuous records at the dayside sector. GIFDS consists of a network of Perseus SDR (Software Defined Radio) receivers equipped with a MiniWhip antenna each. Reliable detection of solar flares is ensured by recording multiple frequency channels ranging from 0 to 500 kHz. The applicability of the system is demonstrated in a first analysis by comparing VLF measurements with GOES's (Geostationary Operational Environmental Satellite) X-ray flux data. The high potential of GIFDS for a permanent monitoring of solar flares in near real time is discussed. [ABSTRACT FROM AUTHOR]

The spatial-temporal variations in aurora and VLF emissions during an weak intensification in the auroral zone morning sector on December 30, 2011, have been analyzed. The event was accompanied by a negative bay (~70 nT) in the X component of the magnetic field at ground stations in northern Scandinavia. At the recovery phase of this bay, the precipitation zone moved and VLF emission frequency simultaneously increased over ten minutes, which may indicate that waves and precipitating electrons had a common source. VLF noise bursts in the 600-1000 Hz band with a characteristic modulation scale of ~10 s and the corresponding aurora intensifications localized in the ~100 km region were observed during the following ten minutes, which also confirms that recorded waves are related to electron precipitation. This correspondence of the pulsating aurora periods and VLF noise modulation has been revealed for the first time. The role of VLF wave generation processes during the cyclotron interaction with electrons in the magnetosphere and the propagation of these waves from the magnetosphere to the observation point are discussed. [ABSTRACT FROM AUTHOR]

It has been more than 20 years since the newer cable field-testing methods of very low frequency (VLF), tan delta, and field partial discharge testing were introduced to the field-testing arena. Over time, these technologies have matured, spawned a new testing industry, IEEE standards, technical committees, and a plethora of technical papers. How has this affected the cable manufacturer and what is its perspective on this area of field testing both as an acceptance and maintenance test method? This paper intends to discuss the guidelines established and some of the shortcomings that have been found. Although this paper may review the various standards and testing methods, its focus will be with the VLF test. [ABSTRACT FROM AUTHOR]

Comparative qualitative analysis of amplitude and phase delay variations was carried out along the trajectory of GQD/22.1 kHz and NAA/24.0 kHz VLF signal traces, propagating from Skelton (UK) and Maine (USA) toward Belgrade, induced by four isolated solar X-ray flare events occurred during the period from September 2005 to December 2006. For monitoring, recording and for storage of VLF data at the Institute of Physics in Belgrade, Serbia, the AbsPAL system was used. For modeling purposes of propagating conditions along GQD and NAA signal propagation paths, LWPCv21 program code was used. Occurred solar flare events induced lower ionosphere electron density height profile changes, causing perturbations in VLF wave propagation within Earth-ionosphere waveguides. As analyzed VLF signals characterize by different propagation parameters along trajectories from their transmitters to the Belgrade receiver site, their propagation is affected in different ways for different solar flare events and also for the same solar flare events. [ABSTRACT FROM AUTHOR]

Self-potential (SP) and VLF measurements were made in 1973, 1975, 1995, 1997 and 2012 across a basaltic dike that intruded into the Koae fault zone of Kilauea volcano, Hawaii in May 1973. The SP anomaly remained strong throughout. In 2012 it was at about 60% of the strength it had in 1973. In contrast, the VLF anomaly, though diminished, was still observable in 1995/1997, but by 2012 it had disappeared. A hydrothermal dike model, with parameters calibrated by modeling the solidification of Kilauea Iki lava lake, is used to calculate temperatures and conductivity variation. Following Jaeger's (1957) method, we find that the time in years for a dike of width W (m) to solidify is 0.0075 W 2 . Thus, a 1 m dike solidifies within the first few days, and after 39 years is only tens of degrees above ambient. Given the orders of magnitude difference between the conductivities of wet and dry basalt, we infer, that after solidification, the VLF anomalies were caused by induction in a localized veil of wet, hot basalt enveloping the dike, that was generated initially by condensation of steam, and subsequently by condensation of evaporated water as temperatures reduced. The conductivity anomaly persisted until the mid-nineties. By 2012, temperatures and condensation were too small for a VLF signal. The persistent SP anomaly is attributed to localized fluid disruption, with evaporation mainly at the water table and in the vadose zone. Streaming potentials are associated with evaporative circulation in the vadose zone. Next to the dike a positive potential is generated by upward flow of moisture-laden air, with a smaller negative potential on its flanks from downward infiltrating rainwater. The analysis indicates that the combination of SP and VLF measurements can characterize the evolving geothermal regime of intrusions above the water table. [ABSTRACT FROM AUTHOR]

We investigate a non-typical very low frequency (VLF) 1-4 kHz hiss representing a sequence of separated noise bursts with a strange "mushroom-like" shape in the frequency-time domain, each one lasting several minutes. These strange afternoon VLF emissions were recorded at Kannuslehto (KAN, ψ=67.74° N, λ=26.27°E; L ∼5.5) in northern Finland during the late recovery phase of the small magnetic storm on 8 December 2013. The left-hand (LH) polarized 2-3 kHz "mushroom caps" were clearly separated from the right-hand (RH) polarized "mushroom stems" at the frequency of about 1.8-1.9 kHz, which could match the lower ionosphere waveguide cutoff (the first transverse resonance of the Earth-ionosphere cavity). We hypothesize that this VLF burst sequence could be a result of the modulation of the VLF hiss electron-cyclotron instability from the strong Pc5 geomagnetic pulsations observed simultaneously at ground-based stations as well as in the inner magnetosphere by the Time History of Events and Macroscale Interactions during Substorms mission probe (THEMIS-E; ThE). This assumption is confirmed by a similar modulation of the intensity of the energetic (1-10 keV) electrons simultaneously observed by the same ThE spacecraft. In addition, the data of the European Incoherent Scatter Scientific Association (EISCAT) radar at Tromsø show a similar quasi-periodicity in the ratio of the Hall-to-Pedersen conductance, which may be used as a proxy for the energetic particle precipitation enhancement. Our findings suggest that this strange mushroom-like shape of the considered VLF hiss could be a combined mutual effect of the magnetospheric ULF-VLF (ultra low frequency-very low frequency) wave interaction and the ionosphere waveguide propagation. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The present work integrates ground-based ionosphere measurements using very-low-frequency radio transmissions with satellite measurements of the total electron content to draw common conclusions about the possible impact that the Mw6.1 earthquake that took place in Greece on January 26, 2014, had on the ionosphere. Very-low-frequency radio signals reveal the existence of an ∼4-day anomaly in the wavelet spectra of the signals received inside the earthquake preparation zone and a significant increase in the normalized variance of the signals prior to the earthquake (approximately 1 day before). Through total electron content analysis, it was possible to identify a clear anomaly from 15:00 until 20:00 UT on the day before the earthquake that appears again on the day of the earthquake between 07:00 UT and 08:00 UT. The anomalous values reach TEC∗Sigma ∼4.36 and 3.11, respectively. Their spatial and temporal distributions give grounds to assume a possible link with the earthquake preparation. The geomagnetic, solar and weather conditions during the considered period are presented and taken into account. This work is an initial and original step towards a multi-parameter approach to the problem of the possible earthquake-related effects on the ionosphere joining observations made from both ground stations and satellites. A well-founded knowledge of these phenomena is clearly necessary before dealing with their application to earthquake prediction purposes. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

During earthquake preparation geophysical processes occur over varying temporal and spatial scales, some leaving their mark on the surface environment, on various biota, and even affecting the ionosphere. Reports on pre-seismic changes in animal behaviour have been greeted with scepticism by the scientific community due to the necessarily anecdotal nature of much of the evidence and a lack of consensus over possible causal mechanisms. Here we present records of changes in the abundance of mammals and birds obtained over a 30 day period by motion-triggered cameras at the Yanachaga National Park, Peru, prior to the 2011 magnitude 7.0 Contamana earthquake. In addition we report on ionospheric perturbations derived from night-time very low frequency (VLF) phase data along a propagation paths passing over the epicentral region. Animal activity declined significantly over a 3-week period prior to the earthquake compared to periods of low seismic activity. Night-time ionospheric phase perturbations of the VLF signals above the epicentral area, fluctuating over the course of a few minutes, were observed, starting 2 weeks before the earthquake. The concurrent observation of two widely different and seemingly unconnected precursory phenomena is of interest because recently, it has been proposed that the multitude of reported pre-earthquake phenomena may arise from a single underlying physical process: the stress-activation of highly mobile electronic charge carriers in the Earth’s crust and their flow to the Earth’s surface. The flow of charge carriers through the rock column constitutes an electric current, which is expected to fluctuate and thereby emit electromagnetic radiation in the ultralow frequency (ULF) regime. The arrival of the charge carriers can lead to air ionization at the ground-to-air interface and the injection of massive amounts of positive airborne ions, known to be aversive to animals. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The Extremely Low Frequency (ELF) and Very Low frequency (VLF) technique are useful for studying the lower ionosphere of the Earth. Exploring this region has been begun in Viet Nam. We constructed the ELF/VLF remote sensing with low cost. This system includes a loop antenna, a pre-amplifier, a sound card, a PC with software and a GPS receiver. After surveying its characteristics, we found that the antenna has the high sensitivity and the pre-amplifier designed has the flat frequency response with frequency from 1 kHz to 20 kHz. It can record sferics generated by lightning discharges to estimate the parameters of the nighttime D-region ionosphere. It is also a teaching module to educate students in Space physics. [ABSTRACT FROM AUTHOR]