Showing papers on "Very low frequency published in 2015"

Long-term determination of energetic electron precipitation into the atmosphere from AARDDVARK subionospheric VLF observations

Abstract: We analyze observations of subionospherically propagating very low frequency (VLF) radio waves to determine outer radiation belt energetic electron precipitation (EEP) flux magnitudes. The radio wave receiver in Sodankyla, Finland (Sodankyla Geophysical Observatory) observes signals from the transmitter with call sign NAA (Cutler, Maine). The receiver is part of the Antarctic-Arctic Radiation-belt Dynamic Deposition VLF Atmospheric Research Konsortia (AARDDVARK). We use a near-continuous data set spanning November 2004 until December 2013 to determine the long time period EEP variations. We determine quiet day curves over the entire period and use these to identify propagation disturbances caused by EEP. Long Wave Propagation Code radio wave propagation modeling is used to estimate the precipitating electron flux magnitudes from the observed amplitude disturbances, allowing for solar cycle changes in the ambient D region and dynamic variations in the EEP energy spectra. Our method performs well during the summer months when the daylit ionosphere is most stable but fails during the winter. From the summer observations, we have obtained 693 days worth of hourly EEP flux magnitudes over the 2004–2013 period. These AARDDVARK-based fluxes agree well with independent satellite precipitation measurements during high-intensity events. However, our method of EEP detection is 10–50 times more sensitive to low flux levels than the satellite measurements. Our EEP variations also show good agreement with the variation in lower band chorus wave powers, providing some confidence that chorus is the primary driver for the outer belt precipitation we are monitoring.

Van Allen Probes observations of unusually low frequency whistler mode waves observed in association with moderate magnetic storms: Statistical study.

Abstract: We show the first evidence for locally excited chorus at frequencies below 0.1 fce (electron cyclotron frequency) in the outer radiation belt. A statistical study of chorus during geomagnetic storms observed by the Van Allen Probes found that frequencies are often dramatically lower than expected. The frequency at peak power suddenly stops tracking the equatorial 0.5 fce and f/fce decreases rapidly, often to frequencies well below 0.1 fce (in situ and mapped to equator). These very low frequency waves are observed both when the satellites are close to the equatorial plane and at higher magnetic latitudes. Poynting flux is consistent with generation at the equator. Wave amplitudes can be up to 20 to 40 mV/m and 2 to 4 nT. We conclude that conditions during moderate to large storms can excite unusually low frequency chorus, which is resonant with more energetic electrons than typical chorus, with critical implications for understanding radiation belt evolution.

Theoretical study of lower ionospheric response to solar flares: sluggishness of D-region and peak time delay

Abstract: The rates of ion production and loss processes in the lower ionosphere during solar and other astronomical ionizing events vary with height. This variations influence the time lags of the response in different ionospheric layers. Very Low Frequency (VLF) signals reflected from any of these layers follow this time lag or delay during a transient cosmic events. One of the easiest ways to study this property is to observe the shift in the peak of VLF signal amplitude with respect to the peak of solar flares. We numerically model to find ion densities and resulting VLF signal perturbations during some solar flares. We clearly find from the model that the delay in the peak of the electron densities (with respect to peak of the ionizing event) in the lower ionosphere varies from height to height. The result also depends on the properties of events, such as peak intensity and sharpness. We investigate analytically how the delay of electron density peak should depend on height varying chemical rate parameters as well as the nature of transient events. Our capability is demonstrated using three classes (namely, X, M and C) of solar flares. The work is a step forward in our goal to employ ionosphere as a natural detector for astronomical observations.

Using time domain waveforms of return strokes to retrieve the daytime fluctuation of ionospheric D layer

Abstract: It suffers great difficulties in remote sensing of the lower ionosphere by using conventional probing measurements during past 50 years. By far a new sounding technique basing on the powerful very low frequency(VLF) radiation emitted by lightning discharges is under developing. In this study, we introduced and established a lightning based detection network to locate the lightning events and record lightning waveforms during 2012. Basing on acquired waveforms occurring in a life cycle of thunderstorm and calculation of the virtual ionosphere heights, fluctuations of ionospheric D layer can be retrieved. One case study in 8:00-18:00(local time) July 7th, 2012 is performed in this article. The results indicate that in this case study the virtual D layer reflection height of VLF radiation emitted by lightning discharges is in the range of 60-75 km and its variation trend is consistent with changing of solar zenith angle. Further correlation analysis between D layer reflection heights and solar X-ray flux densities recorded by GOES satellite reveals that solar flares will cause significant modification of the lower ionosphere in 3-4 min.

Ionospheric d-region temperature relaxation and its influences on radio signal propagation after solar x-flares occurrence

Abstract: In this paper our attention is focused on relations between radio signal propagation characteristics and temperature changes in D-region after solar X-flare occurrence. We present temperature dependencies of electron plasma frequency, the parameter that describes medium conditions for propagation of an electromagnetic wave, and the refractive index which describes how this wave propagates. As an example for quantitative calculations based on obtained theoretical equations we choose the reaction of the D-region to the solar X-flare occurred on May 5th, 2010. The ionospheric modelling is based on the experimental data obtained by low ionosphere observations using very low frequency radio signal.

Resonance structure and mode transition of quarter‐wave ULF pulsations around the dawn terminator

Abstract: Quarter-wave modes are standing shear Alfven waves supported along geomagnetic field lines in space. They are predicted to be generated when the ionosphere has very different conductance between the north compared with the south ionosphere. Our previous observation reported that the resonant frequency is sometimes very low around the dawn terminator and suggested these were due to quarter-wave modes. In this paper, we examine the resonance structure that provides further evidence of the presence of quarter-wave modes. Data from three magnetometers in New Zealand were analyzed. Four events are discussed which show extraordinarily low eigenfrequencies, wide resonance widths, and strong damping when the ionosphere above New Zealand was in darkness while the conjugate northern hemisphere ionosphere was sunlit. Later in the morning, the eigenfrequencies and resonance widths changed to normal daytime values. The wide resonance width and the strong damping of the quarter-wave modes arise from strong energy dissipation in the dark side ionosphere. One event exhibited field line resonance structure continuously through a transition from very low frequency to the normal daytime values. The frequency change began when the dawn terminator passed over New Zealand and finished 1 h later when the ratio of the interhemispheric ionospheric conductances decreased and reached ~5. These observations are strong evidence of the presence of quarter-wave modes and mode conversion from quarter- to half-wave resonances. These experimental results were compared with the ULF wave fields obtained from a 2.5-dimensional simulation model.

Multistation observations of the azimuth, polarization, and frequency dependence of ELF/VLF waves generated by electrojet modulation

Abstract: Modulated ionospheric heating experiments are performed with the High Frequency Active Auroral Research Program facility in Gakona, Alaska, for the purpose of generating extremely low frequency (ELF) and very low frequency (VLF) waves. Observations are made at three different azimuths from the heating facility and at distances from 37 km to 99 km. The polarization of the observed waves is analyzed in addition to amplitude as a function of modulation frequency and azimuth. Amplitude and eccentricity are observed to vary with both azimuth and distance from the heating facility. It is found that waves radiated at azimuths northwest of the facility are generated by a combination of modulated Hall and Pedersen currents, while waves observed at other azimuths are dominated by modulated Hall currents. We find no evidence for vertical currents contributing to ground observations of ELF/VLF waves. Observed amplitude peaks near multiples of 2 kHz are shown to result from vertical resonances in the Earth-ionosphere waveguide, and variations of the frequency of these resonances can be used to determine the D region ionosphere electron density profile in the vicinity of the HF heater.

Unexpected Very Low Frequency (VLF) Radio Events Recorded by the Ionospheric Satellite DEMETER

Abstract: DEMETER was a low Earth orbiting microsatellite in operation between July 2004 and December 2010. Its scientific objective was the study of ionospheric perturbations in relation to seismic activity and man-made activities. Its payload was designed to measure electromagnetic waves over a large frequency range as well as ionospheric plasma parameters (electron and ion densities, fluxes of energetic charged particles). This paper will show both expected and unusual events recorded by the satellite when it was in operation. These latter events have been selected from the DEMETER database because they are rare or even have never been observed before, because they have a very high intensity, or because they are related to abnormalities of the experiments under particular plasma conditions. Some events are related to man-made radio waves emitted by VLF ground-based transmitters or power line harmonic radiation. Natural waves, such as atypical quasi-periodic emissions or uncommon whistlers, are also shown.

Sub-ionospheric VLF signal anomaly due to geomagnetic storms: a statistical study

Abstract: . We investigate quantitatively the effect of geomagnetic storms on the sub-ionospheric VLF/LF (Very Low Frequency/Low Frequency) propagations for different latitudes based on 2-year nighttime data from Japanese VLF/LF observation network. Three statistical parameters such as average signal amplitude, variability of the signal amplitude, and nighttime fluctuation were calculated daily for 2 years for 16–21 independent VLF/LF transmitter–receiver propagation paths consisting of three transmitters and seven receiving stations. These propagation paths are suitable to simultaneously study high-latitude, low-mid-latitude and mid-latitude D/E-region ionospheric properties. We found that these three statistical parameters indicate significant anomalies exceeding at least 2 times of their standard deviation from the mean value during the geomagnetic storm time period in the high-latitude paths with an occurrence rate of anomaly between 40 and 50 % presumably due to the auroral energetic electron precipitation. The mid-latitude and low-mid-latitude paths have a smaller influence from the geomagnetic activity because of a lower occurrence rate of anomalies even during the geomagnetically active time period (from 20 to 30 %). The anomalies except geomagnetic storm periods may be caused by atmospheric and/or lithospheric origins. The statistical occurrence rates of ionospheric anomalies for different latitudinal paths during geomagnetic storm and non-storm time periods are basic and important information not only to identify the space weather effects toward the lower ionosphere depending on the latitudes but also to separate various external physical causes of lower ionospheric disturbances.

Residual vasomotor activity assessed by heart rate variability in a brain-dead case.

Abstract: A patient assessed by heart rate variability (HRV) methodology, beginning just after the completion of brain death (BD) diagnosis, showed remaining very low frequency (VLF) waves for approximately 10 min. A time-varying spectral analysis showed that during the first 550 s, a significant power spectral density remained in the high-frequency (HF), low-frequency (LF) and VLF bands. From 550 to 675 s, the HF oscillations totally vanished, and a marked progressive decay of the LF and VLF power density occurred. After 700 s the VLF undulations stopped and remaining small amplitude oscillations at 0.2 Hz coincided with the ventilator frequency. The VLF oscillations recorded in our case might be related to residual sympathetic vasomotor activity that progressively disappeared due to the extension of necrosis affecting the nervous centres of the lower part of the medulla and the first 2–3 cervical spine segments.

Anomalously strong vertical magnetic fields from distant ELF/VLF sources

Abstract: There are many sources of very low frequency (VLF—3–30 kHz) and extremely low frequency (ELF—3–3000 Hz) radiation in the Earth-ionosphere waveguide (e.g., lightning and ELF/VLF communication transmitters). At distances of thousands of kilometers from these sources, the vertical component of the ELF/VLF AC magnetic fields is expected to be very weak and several orders of magnitude lower than the horizontal magnetic components. However, measurements in Israel show a relatively strong vertical magnetic component in both the ELF and VLF bands, at the same order of magnitude as the horizontal components. Our measurements suggest that the real Earth-ionosphere waveguide might often be very different from the theoretical waveguide used in model calculations. In addition, our results imply that using only the horizontal components for direction finding or the absolute magnetic field strength may result in errors, since often a significant fraction of the magnetic field energy hides in the vertical component.

Stripping very low frequency communication signals with minimum shift keying encoding from streamed time-domain electromagnetic data

Abstract: The objective of this research is to eliminate very low frequency (VLF) communication signals with frequencies in the range of 15–25 kHz from streamed time-domain electromagnetic data. The frequency-domain approach of notch filtering or bandwidth limitation is unsatisfactory when early delay-time data are required in a time-domain system. United States military VLF transmitters presently communicate 200 bits/s using minimum shift keying modulation, and it is possible to derive the encrypted bitstream from the data sampled at typical geophysical streaming rates. The method involves convolving the data with waveforms of frequency different by one-quarter of the bit rate above and below the carrier frequency and using the difference between the convolutions to predict the transmitted bits. The transmitted signal is then exactly recreated from the decoded bits, and the predicted signal at the receiver is subtracted from the data stream. It is possible to predict bit rates and encoding methods from ot...

Analysis of the effectiveness of ground‐based VLF wave observations for predicting or nowcasting relativistic electron flux at geostationary orbit

Abstract: Poststorm relativistic electron flux enhancement at geosynchronous orbit has shown correlation with very low frequency (VLF) waves measured by satellite in situ. However, our previous study found little correlation between electron flux and VLF measured by a ground-based instrument at Halley, Antarctica. Here we explore several possible explanations for this low correlation. Using 220 storms (1992–2002), our previous work developed a predictive model of the poststorm flux at geosynchronous orbit based on explanatory variables measured a day or two before the flux increase. In a nowcast model, we use averages of variables from the time period when flux is rising during the recovery phase of geomagnetic storms and limit the VLF (1.0 kHz) measure to the dawn period at Halley (09:00–12:00 UT). This improves the simple correlation of VLF wave intensity with flux, although the VLF effect in an overall multiple regression is still much less than that of other factors. When analyses are performed separately for season and interplanetary magnetic field (IMF) Bz orientation, VLF outweighs the influence of other factors only during winter months when IMF Bz is in an average northward orientation.

A log-likelihood ratio for improved receiver performance for VLF/LF communication in atmospheric noise

Abstract: Receiver-improving techniques are important for submarine communication at the LF/VLF bands, due to very demanding reception conditions. At the surface, the interference environment is dominated by the atmospheric noise, which is highly impulsive in character and may impede the reception of the radio signals. To handle the demanding channel and the impulsive interference environment, error correction and an adapted receiver are necessary. In this paper, we propose a log-likelihood ratio (LLR) as soft output from the demodulator suitable for atmospheric noise. The radio system is assumed to use minimum shift keying (MSK) and a low parity density check (LDPC) code. It is shown that the proposed interference-adapted LLR improves the performance substantially in atmospheric noise compared to when the LLR is designed for AWGN. The performance is also compared to a solution, where the soft output from the demodulator is simplified to a limiter, and to a solution when a larger system bandwidth is used in combination with a limiter. It is concluded that the proposed interference-adapted LLR achieves the best performance in the comparison, although the performance could probably be further improved, when compared to the obtained Shannon capacity of this particular interference.

Multipath propagation of low-frequency radio waves inferred from high-resolution array analysis

Abstract: The low frequency radio sky shows the locations of electromagnetic radio sources with a characteristic dilution of precision. Here we report a thorough high resolution analysis of radio waves from low frequency (∼20-150 kHz) radio communication transmitters which are recorded with a small aperture array of radio receivers during the day. It is found that the observed dilution of precision results from the array geometry of the radio receivers, a birefringent wave propagation and the correlated multipath propagation of low frequency radio waves. The influence of the array geometry on the dilution of precision is reduced by taking into account the impulse response of the array. This procedure reveals for the very first time the splitting of one single radio source into two distinct source locations separated by ∼0.2°-1.9° which result from a birefringent wave propagation. The two locations are yet more clearly identified by using the polarity of the modulated wave number vectors of the radio waves. This polarity is also used to quantify the dilution of precision arising from correlated multipath propagation which is discriminated against wave number fluctuations arising from the timing accuracy of the radio receivers. It is found that ∼69% of the wave number variability is of natural origin and ∼31% originates from the timing accuracy of the receivers. The wave number variability from correlated multipath propagation results in a standard deviation ∼2-8% relative to the source location. This compact measurement of correlated multipath propagation is used to characterize the uncertainty of source locations in the radio sky. The identification of correlated multipath propagation strongly suggests the existence of very fast processes acting on time scales <1 ms in the D-region ionosphere with physically meaningful effects on low frequency radio wave propagation. This important result has implications for practical applications in that the observed multipath propagation enables the determination of natural limits for the accuracy of navigation and lightning location methods using low frequency radio waves.

Numerical modelling of VLF radio wave propagation through earth-ionosphere waveguide and its application to sudden ionospheric disturbances

Abstract: In this thesis, we theoretically predict the normal characteristics of Very Low Frequency (3~30 kHz) radio wave propagation through Earth-ionosphere waveguide corresponding to normal behavior of the D-region ionosphere. We took the VLF narrow band data from the receivers of Indian Centre for Space Physics (ICSP) to validate our models. Detection of sudden ionospheric disturbances (SIDs) are common to all the measurements. We apply our theoretical models to infer the D-region characteristics and to reproduce the observed VLF signal behavior corresponding to such SIDs. We develop a code based on ray theory to simulate the diurnal behavior of VLF signals over short propagation paths (2000~3000 km). The diurnal variation from this code are comparable to the variation obtained from a more general Long Wave Propagation Capability (LWPC) code which is based on mode theory approach. We simulate the observational results obtained during the Total Solar Eclipse of July 22, 2009 in India. We also report and simulate a historic event, namely, the lunar occultation of a solar flare during the annular solar eclipse of 15th January, 2010 and find the effects on the D-region electron density profiles.

Inverse problem in Ionospheric Science: Prediction of solar soft-X-ray spectrum from Very Low Frequency Radiosonde results

Abstract: X-rays and gamma-rays from astronomical sources such as solar flares are mostly absorbed by the Earth's atmosphere. Resulting electron-ion production rate as a function of height depends on the intensity and wavelength of the injected spectrum and therefore the effects vary from one source to another. In other words, the ion density vs. altitude profile has the imprint of the incident photon spectrum. In this paper, we investigate whether we can invert the problem uniquely by deconvolution of the VLF amplitude signal to obtain the details of the injected spectrum. We find that it is possible to do this up to a certain accuracy. Our method is useful to carry out a similar exercise to infer the spectra of more energetic events such as the Gamma Ray Bursts (GRBs), Soft Gamma Ray Repeaters (SGRs) etc. by probing even the lower part of the atmosphere. We thus show that to certain extent, the Earth's atmosphere could be used as a gigantic detector of relatively strong events.

Techniques to determine the quiet day curve for a long period of subionospheric VLF observations

Abstract: Very low frequency (VLF) transmissions propagating between the conducting Earth's surface and lower edge of the ionosphere have been used for decades to study the effect of space weather events on the upper atmosphere. The VLF response to these events can only be quantified by comparison of the observed signal to the estimated quiet time or undisturbed signal levels, known as the quiet day curve (QDC). A common QDC calculation approach for periods of investigation of up to several weeks is to use observations made on quiet days close to the days of interest. This approach is invalid when conditions are not quiet around the days of interest. Longer-term QDCs have also been created from specifically identified quiet days within the period and knowledge of propagation characteristics. This approach is time consuming and can be subjective. We present three algorithmic techniques, which are based on either (1) a mean of previous days' observations, (2) principal component analysis, or (3) the fast Fourier transform (FFT), to calculate the QDC for a long-period VLF data set without identification of specific quiet days as a basis. We demonstrate the effectiveness of the techniques at identifying the true QDCs of synthetic data sets created to mimic patterns seen in actual VLF data including responses to space weather events. We find that the most successful technique is to use a smoothing method, developed within the study, on the data set and then use the developed FFT algorithm. This technique is then applied to multiyear data sets of actual VLF observations.

Modeling of long-path propagation characteristics of VLF radio waves as observed from Indian Antarctic station Maitri

Abstract: Propagation of very low frequency (VLF) radio signal through the Earth-ionosphere waveguide depends strongly on the plasma properties of the ionospheric D layer. Solar extreme ultraviolet radiation plays the central role in controlling physical and chemical properties of the lower ionospheric layers and hence determining the propagation characteristics of a VLF signal. The nature of interference among different propagating modes varies widely with the length of the propagation path. For a very long path, exposure of solar radiation and thus the degree of ionization vary by a large amount along the path. This influences the VLF signal profile by modulating the sky wave propagation. To understand the propagation characteristics over such a long path, we need a thorough investigation of the chemical reactions of the lower ionosphere which is lacking in the literature. Study of radio signal characteristics in the Antarctic region during summer period in the Southern Hemisphere gives us a unique opportunity to explore such a possibility. In addition, there is an extra feature in this path—the presence of solar radiation and hence the D region for the whole day during summer in at least some sections of the path. In this paper, we present long-distance propagation characteristics of VLF signals transmitted from VTX (18.2 kHz) and NWC (19.8 kHz) transmitters recorded at the Indian permanent station Maitri (latitude 70° 45′S, longitude 114° 40′E) in 2007–2008. A very stable diurnal variation of the signal has been obtained with no signature of nighttime fluctuation due the presence of 24 h of sunlight. Using ion production and recombination profiles by solar irradiance and incorporating D region ion chemistry processes, we calculate the electron density profile at different heights. Using this profile in the Long Wavelength Propagation Capability code, we are able to reproduce the amplitude of VLF signal.

Development of high sensitivity amplifier for VLF receiver application

Abstract: In this paper, the development of amplifier based on AD744 monolithic Op-Amp is proposed for VLF receiver application. Due to the wide usage of very low frequency (VLF) wave for research on natural phenomena, a VLF receiver application particularly with high sensitivity is absolutely required. The proposed VLF amplifier which is intended to produce high sensitivity for the frequency range of 10–30kHz is designed to have an output connectable to the soundcard of personal computer for further data processing. After achieving the optimum design, the hardware realization is then implemented on a printed circuit board (PCB). From the measurement result, it shows that the sensitivity is almost linear for the input signal lower than −12dBm and becoming quadratic for the input signal higher than −12dBm.

Ionospheric very low frequency transmitter

Abstract: The theme of this paper is to establish a reliable ionospheric very low frequency (VLF) transmitter, which is also broad band. Two approaches are studied that generate VLF waves in the ionosphere. The first, classic approach employs a ground-based HF heater to directly modulate the high latitude ionospheric, or auroral electrojet. In the classic approach, the intensity-modulated HF heater induces an alternating current in the electrojet, which serves as a virtual antenna to transmit VLF waves. The spatial and temporal variations of the electrojet impact the reliability of the classic approach. The second, beat-wave approach also employs a ground-based HF heater; however, in this approach, the heater operates in a continuous wave mode at two HF frequencies separated by the desired VLF frequency. Theories for both approaches are formulated, calculations performed with numerical model simulations, and the calculations are compared to experimental results. Theory for the classic approach shows that an HF heater wave, intensity-modulated at VLF, modulates the electron temperature dependent electrical conductivity of the ionospheric electrojet, which, in turn, induces an ac electrojet current. Thus, the electrojet becomes a virtual VLF antenna. The numerical results show that the radiation intensity of the modulated electrojet decreases with an increase in VLF radiation frequency. Theory for the beat wave approach shows that the VLF radiation intensity depends upon the HF heater intensity rather than the electrojet strength, and yet this approach can also modulate the electrojet when present. HF heater experiments were conducted for both the intensity modulated and beat wave approaches. VLF radiations were generated and the experimental results confirm the numerical simulations. Theory and experimental results both show that in the absence of the electrojet, VLF radiation from the F-region is generated via the beat wave approach. Additionally, the beat wave approach generates VLF radiations over a larger frequency band than by the modulated electrojet.

Transmitter harmonic cancellation for carrier aggregation/multiband operation

Abstract: A method and apparatus for cancelling, from signals received by a radio device in a first frequency band, interference generated by the radio device in the first frequency band when the radio device transmits simultaneously radio signals on at least a second frequency band.

Southern boundaries of ultraenergetic relativistic electron precipitations in several cases from 1982 to 1986 years

Abstract: We analyze the rare phenomenon of ultraenergetic relativistic electron (∼100 MeV) precipitation into the middle polar atmosphere prevalent under quiescent geophysical conditions. Such events have been established previously from ground-based radio wave measurements for two radio paths—one is purely auroral and the other is partly auroral—that have a mutual point of signal reception. We solve an inverse very low frequency wave problem of the second type using these particular paths. The solution gives a linear scale of the northern part of radio path, which has been disturbed by the ultraenergetic relativistic electron precipitations. By such a way the effective latitudes of southern boundaries for several precipitation events, published earlier, were determined. Nearly circular shape of equatorward cutoff at ∼61°magnetic latitude without day-night asymmetry supports that ultraenergetic relativistic electron precipitation is due to precipitation of very high energy electrons coming from outer space.

Strange VLF bursts in northern Scandinavia: case study of the afternoon "mushroom-like" hiss on 8 December 2013

Abstract: . 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, v = 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 Tromso 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.

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

Abstract: 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.

System for very low-frequency and extremely low-frequency communication with deep-sunk and remote objects - 1

Abstract: FIELD: radio engineering, communicationSUBSTANCE: invention relates to very low-frequency (VLF) and extremely low-frequency (ELF) communication with deep-sunk and remote underwater objects Disclosed system for very low-frequency and extremely low-frequency communication with deep-sunk and remote objects comprises a transmitting system, which consists of: a driving oscillator; a modulator; a control, protection and automation system; a power amplifier; a matching device; an antenna current indicator and a current source, wherein reception and detection of radiation generated by VLF-ELF generators is carried out using a trailing cable antenna, an antenna amplifier and a VLF-ELF receiver on-board the underwater object, characterised by that it further includes: N converters, N earthing devices for the antenna system, which is in the form of an extended straight line consisting of N sections, underground unshielded cable sections, an antenna system with a length l equal to several tens of hundreds of kilometresEFFECT: use of the disclosed device provides electromagnetic compatibility of the transmitting antenna with radio-electronic stations and engineering structures and ensures environmental safety of a VLF-ELF radio station3 cl, 3 dwg, 1 tbl

Sunrise effect on 40 kHz signal amplitude and its characteristic variation with respect to geomagnetic storms

Abstract: The sunrise effect is a characteristic feature of very low frequency and low frequency radio waves propagated over a large distance. The 40 kHz signal level, transmitted from Miyakoji station (37.4...

Effect of temperatures on very low frequency partial discharge diagnostics

Abstract: Effects of temperatures on corona discharges developed by needle/bowl electrode configuration at very low frequency are investigated. Partial discharge measurements are carried out under enclosed controlled air temperature range between 20°C and 40°C. Sinusoidal voltage waveform is applied to the test sample under very low frequency and power frequency. Experiment results show that electrical discharge behaviours at 0.1 Hz and power frequency are dependent on ambient temperature in a similar manner. Nevertheless, there are still noticeable differences in partial discharge characteristics at very low frequency at various temperatures, especially in the phase position of discharge events. This could be explained from more availability of free electrons emitted from the negative electrode under higher temperature. Experiment results suggest that, under very low frequency excitation, there might be a critical temperature that triggers change in PD behaviour at a certain applied voltage level.