Showing papers on "Very low frequency published in 2020"

A Portable Very Low Frequency (VLF) Communication System Based on Acoustically Actuated Magnetoelectric Antennas

Abstract: A novel very low frequency (VLF) communication system using one pair of magnetoelectric (ME) antennas has been proposed. The ME antennas are strain-mediated acoustic resonators operating at their electromechanical resonance in the VLF band. The measured near-field radiation pattern reveals ME antennas are equivalent to dipole antennas. The magnetic field radiated by the ME transmitter has been predicted along with distance ranging from 1 mm to 100 km. The measured magnetic field distribution coincided well with the prediction, and the maximum communication distance of 120 m has been achieved. With 80 V driving voltage, the power consumption of the ME transmitter has been measured as 400 mW. Furthermore, the direct antenna modulation (DAM) has also been successfully demonstrated on the ME antennas.

Very-Low-Frequency transmitters bifurcate energetic electron belt in near-earth space.

Abstract: Very-Low-Frequency (VLF) transmitters operate worldwide mostly at frequencies of 10-30 kilohertz for submarine communications. While it has been of intense scientific interest and practical importance to understand whether VLF transmitters can affect the natural environment of charged energetic particles, for decades there remained little direct observational evidence that revealed the effects of these VLF transmitters in geospace. Here we report a radially bifurcated electron belt formation at energies of tens of kiloelectron volts (keV) at altitudes of ~0.8-1.5 Earth radii on timescales over 10 days. Using Fokker-Planck diffusion simulations, we provide quantitative evidence that VLF transmitter emissions that leak from the Earth-ionosphere waveguide are primarily responsible for bifurcating the energetic electron belt, which typically exhibits a single-peak radial structure in near-Earth space. Since energetic electrons pose a potential danger to satellite operations, our findings demonstrate the feasibility of mitigation of natural particle radiation environment.

Acoustically driven electromagnetic radiating elements.

Abstract: The low propagation loss of electromagnetic radiation below 1 MHz offers significant opportunities for low power, long range communication systems to meet growing demand for Internet of Things applications. However, the fundamental reduction in efficiency as antenna size decreases below a wavelength (30 m at 1 MHz) has made portable communication systems in the very low frequency (VLF: 3-30 kHz) and low frequency (30-300 kHz) ranges impractical for decades. A paradigm shift to piezoelectric antennas utilizing strain-driven currents at resonant wavelengths up to five orders of magnitude smaller than electrical antennas offers the promise for orders of magnitude efficiency improvement over the electrical state-of-the-art. This work demonstrates a lead zirconate titanate transmitter > 6000 times more efficient than a comparably sized electrical antenna and capable of bit rates up to 60 bit/s. Detailed analysis of design parameters offers a roadmap for significant future improvement in both radiation efficiency and data rate.

Modelling of supply voltage frequency effect on partial discharge repetition rate and charge amplitude from AC to DC at room temperature

Abstract: This paper has the purpose to derive an analytical model ("continuum" model) able to describe the behavior of partial discharge (PD) repetition rate and amplitude, occurring in a cavity embedded in polymeric insulation, as a function of the frequency of the supply voltage, going from AC power supply frequency, 50–60 Hz, to DC. In the range between DC and 50–60 Hz focus is made on data coming from tests under AC sinewave with very low frequency (VLF) such as 0.1 Hz and 0.01 Hz, which are commonly used for cable testing. It is shown that the proposed "continuum" model can provide reasonably good fit to the experimental results obtained in the range DC to 60 Hz, regarding PD repetition rate and amplitude. To reach such result, the equivalent circuit is modified from that commonly used and made by fully-capacitive or resistive components, in order to take into account the change of polarization mechanisms which, depending on dielectric material, may play a non-negligible role to establish the repetition rate from low frequency to DC power supply. In addition, the residual voltage after a PD event has to vary with frequency to reach good fitting. Also, it is shown that PD amplitude under DC and VLF can be lower than under AC 50–60 Hz due to the delay time of the firing electron, thus experimental PD amplitude varies with frequency depending on material and defect typology and location.

Outer Radiation Belt Electron Lifetime Model Based on Combined Van Allen Probes and Cluster VLF Measurements

Abstract: The flux of energetic electrons in the outer radiation belt shows a high variability. The interactions of electrons with very low frequency (VLF) chorus waves play a significant role in controlling the flux variation of these particles. Quantifying the effects of these interactions is crucially important for accurately modeling the global dynamics of the outer radiation belt and to provide a comprehensive description of electron flux variations over a wide energy range (from the source population of 30 keV electrons up to the relativistic core population of the outer radiation belt). Here, we use a synthetic chorus wave model based on a combined database compiled from the Van Allen Probes and Cluster spacecraft VLF measurements to develop a comprehensive parametric model of electron lifetimes as a function of L‐shell, electron energy, and geomagnetic activity. The wave model takes into account the wave amplitude dependence on geomagnetic latitude, wave normal angle distribution, and variations of wave frequency with latitude. We provide general analytical formulas to estimate electron lifetimes as a function of L‐shell (for L = 3.0 to L = 6.5), electron energy (from 30 keV to 2 MeV), and geomagnetic activity parameterized by the AE index. The present model lifetimes are compared to previous studies and analytical results and also show a good agreement with measured lifetimes of 30 to 300 keV electrons at geosynchronous orbit.

Multi-Experiment Observations of Ionospheric Disturbances as Precursory Effects of the Indonesian Ms6.9 Earthquake on August 05, 2018

Abstract: Taking the 2018 Ms6.9 Indonesia earthquake as a case study, the ionospheric perturbations in very low frequency (VLF) transmitters recorded by China Seismo-Electromagnetic Satellite (CSES) were mainly investigated, as well as the multi parameters of the plasma and electromagnetic field. The characteristics of electron density (Ne), GPS TEC, ULF electric field, ion drift velocity, and ionosphere height were extracted and compared with the features of the signal-noise ratio (SNR) from VLF transmitters of NWC at the southern hemisphere and JJI at the northern hemisphere. Most disturbances in VLF radio waves occurred along the orbits near the epicenter within 10 days before the earthquake. Along these orbits, we observed simultaneous modulations in the Ne and ULF electric field, as well as the changed ion drifting directions. There was also high spatial correspondence between both SNR and ionospheric height anomalies over the epicentral and its magnetic conjugate regions. Combined with the multi observations, these results suggest that the genesis of perturbations in signals emitted by VLF transmitters on satellite was more likely related to the overlapped electric field in the preparation area of the earthquake.

Magnetic Field Penetration Into a Metal Enclosure Using an ELF/VLF Loop Antenna

Abstract: The ability of extremely and very low frequency (ELF/VLF, 0–30 kHz) radio waves to penetrate conductive media is well established. Magnetic field penetration into a thin but highly conductive box using an ELF/VLF loop antenna transmitter is investigated. The work is relevant for electromagnetic shielding of ELF/VLF sensors, defect detection, inductive power transfer, and through container imaging. Analytical solutions are reviewed for related shielding problems, however, determining the penetration through a realistic shield geometry and finite sized near-field source requires a numerical approach. Surface integral equation (SIE) methods are well suited for shield modeling due to the low surface area to volume ratio of the shield. Method of moment techniques have been successfully applied to solving SIEs in the past, however, enforcing algorithm stability at low frequencies is known to require considerable effort. To alleviate the low-frequency concerns, a high-order locally corrected Nystrom (LCN) scheme is utilized to solve an SIE based on an augmented Muller formulation of Maxwell's equations. To validate the LCN simulations, an experiment is conducted using a loop antenna inside a 1.2 m aluminum cube of 2.7 mm thickness with an external ELF/VLF loop transmitter. Experimental results are shown to match within 3 dB of the LCN code predictions.

NWC Transmitter Effects on the Nightside Upper Ionosphere Observed by a Low-Altitude Satellite

Abstract: NWC is an extremely powerful very low frequency (19.8 kHz) transmitter located in the north‐west Australia. Although the transmitter typically operates continuously, it was off during the second half of 2007. This allows for a direct comparison of an ionospheric situation at the times when the transmitter is on with the times when the transmitter is off. We use electromagnetic wave and plasma measurements performed by the Detection of Electro‐Magnetic Emissions Transmitted from Earthquake Regions (DEMETER) spacecraft at an altitude of about 660 km. Given that the transmitter signal is significantly attenuated in lower ionospheric layers during the daytime and, moreover, the dayside ionosphere is controlled primarily by the solar radiation, concealing possible transmitter‐related effects, we focus exclusively on the nightside. We show that although the NWC transmitter signal does not significantly change the mean plasma density and only slightly increases the electron temperature, it causes significant perturbations of both these quantities at distances up to about 200 km. The wave intensity is considerably enhanced in the same spatial region close to the transmitter in a large range of frequencies above about 14 kHz. Finally, clear signatures of transmitter induced electron precipitation are detected to the east of the transmitter at somewhat larger L‐shells, consistent with a gyroresonance condition.

On the Accuracy of Ray-Theory Methods to Determine the Altitudes of Intracloud Electric Discharges and Ionospheric Reflections: Application to Narrow Bipolar Events.

Abstract: Narrow bipolar events (NBEs) (also called narrow bipolar pulses [NBPs] or compact intracloud discharges [CIDs]) are energetic intracloud discharges characterized by narrow bipolar electromagnetic waveforms identified from ground-based very low frequency (VLF)/low-frequency (LF) observations. The simplified ray-theory method proposed by Smith et al. (1999, https://doi.org/10.1029/1998JD200045; 2004, https://doi.org/10.1029/2002RS002790) is widely used to infer the altitude of intracloud lightning and the effective (or virtual) reflection height of the ionosphere from VLF/LF signals. However, due to the large amount of high-frequency components in NBEs, the propagation effect of the electromagnetic fields for NBEs at large distance depends nontrivially on the geometry and the effective conductivity of the Earth-ionosphere waveguide (EIWG). In this study, we investigate the propagation of NBEs by using a full-wave Finite-Difference Time-Domain (FDTD) approach. The simulated results are compared with ground-based measurements at different distances in Southern China, and we assess the accuracy of the simplified ray-theory method in estimating the altitude of the NBE source and the effective reflection height of the ionosphere. It is noted that the evaluated NBE altitudes have a slight difference of about ±1 km when compared with the full-wave FDTD results, while the evaluated ionospheric reflection heights are found to be bigger than those obtained from FDTD model by about 5 km.

The Electric Field Detector on Board the China Seismo Electromagnetic Satellite—In-Orbit Results and Validation

Abstract: The aim of this work is to validate the China Seismo-Electromagnetic Satellite 01 (CSES-01) Electric Field Detector (EFD) measurements through the analysis of the instrument response to various inputs: (a) geomagnetic field variations, (b) plasma density depletions, and (c) electromagnetic signals from natural and artificial sources such as Schumann resonance and VLF (Very Low Frequency) antennas. The knowledge of the geomagnetic induced electric field vs×B (where vs is the satellite speed and B and the local magnetic field), and the plasma variations effect, described by the Orbit Motion Limited (OML) theory, are key parameters to determine the expected theoretical values of the EFD sensors potentials data. Based on the CSES on-board measurements of plasma parameters and geomagnetic field, a direct quantitative validation is presented. In addition, the electromagnetic signals detection capability is checked but only qualitatively confirmed, since the ionospheric complexity does not allow an accurate theoretical computation of waves modulation. The quantitative comparison highlights the very good agreement between observed and theoretical potentials values during average condition. Conversely, in case of strong electric fields, the OML theory shows partial inability in reproducing the actual space plasma conditions resulting in a reduced theoretical values reliability. Finally, both natural and artificial electromagnetic signals are satisfactorily identified showing a reliable sensitivity in different frequency bands.

The very low-frequency transmitter radio wave anomalies related to the 2010 Ms 7.1 Yushu earthquake observed by the DEMETER satellite and the possible mechanism

Abstract: . Earthquakes may disturb the lower ionosphere through various coupling mechanisms during the seismogenic and coseismic periods. The VLF (very low-frequency) signal radiated from ground-based transmitters will be affected when it penetrates the disturbed ionosphere above the epicenter area, and this anomaly can be recorded by low-Earth orbit satellites under certain conditions. In this paper, the temporal and spatial variation of the signal-to-noise ratio (SNR) of the VLF transmitter signal in the ionosphere over the epicenter of 2010 Yushu Ms 7.1 earthquake in China is analyzed using DEMETER (Detection of Electro-Magnetic Emission Transmitted from Earthquake Regions) satellite observation. The results show that SNR over the epicenter of the Yushu earthquake especially in the southwestern region decreased (or dropped) before the main shock, and a GPS–TEC (Global Positioning System; total electron content) anomaly accompanied, which implies that the decrease in SNR might be caused by the enhancement of TEC. A full-wave method is used to study the mechanism of the change in SNR before the earthquake. The simulated results show SNR does not always decrease before an earthquake. When the electron density in the lower ionosphere increases by 3 times, the electric field will decrease about 2 dB, indicating that the disturbed-electric-field decrease of 20 % compared with the original electric field and vice versa. It can be concluded that the variation of electron density before earthquakes may be one of the important factors influencing the variation of SNR.

Contaminated Effect of Geomagnetic Storms on Pre-Seismic Atmospheric and Ionospheric Anomalies during Imphal Earthquake

Abstract: Study of seismo-ionospheric coupling mechanism demands the quiet geomagnetic condition to eliminate any kind of contamination in the lower atmospheric and ionospheric parameters. In this manuscript, we present the effect of back to back two geomagnetic storms before a strong earthquake happened in Imphal, India on January 4, 2016 (M = 6.7). We studied the lower ionospheric irregularities for the duration of 31 days by computing the nighttime fluctuations in Very Low Frequency (VLF) radio signal received transmitter JJI (22.2 kHz) in Japan at Ionospheric and Earthquake Research Centre & Optical Observatory (IERCOO), Sitapur, India. We also studied the presence of Atmospheric Gravity Wave (AGW) in nighttime VLF signal in lower ionospheric heights and the same computed that from SABER/TIMED satellite. Two geomagnetic storms occurred on December 21, and 31, 2015. By the conventional analysis, we found that there is a significant decrease in nighttime trend and an increase in nighttime fluctuations around 15 days before the earthquake and just on the first storm and thus the pre-seismic effects on VLF signal gets contaminated due to the presence of storms. The wave-like structure in VLF fluctuations shows significant increase in intensity by using Fourier and Wavelet analysis before the earthquake. By analysis of SABER data, we found significant enhancement in AGW around 10 days before the earthquake. As the wavelike structures are coming from neutral acoustics reasons from pressure or temperature variations, this paper exhibits a significant example of contamination in ionospheric parameters due to geomagnetic storm where the acoustics parameters remain un-contaminated.

Model of the propagation of very low-frequency beams in the Earth–ionosphere waveguide: principles of the tensor impedance method in multi-layered gyrotropic waveguides

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

Analysis of Partial Discharges in Electrical Tree Growth Under Very Low Frequency (VLF) Excitation Through Pulse Sequence and Nonlinear Time Series Analysis

Abstract: Electrical treeing is the main degradation mechanism in high voltage polymeric insulation, that leads to power system plant failure and the loss of electricity supply. Electrical trees grow under partial discharge (PD) activity, which can be measured and analyzed to understand and characterize electrical tree growth. In this work, PD measurements were analyzed for electrical trees grown in epoxy resin needle-plane samples under very low frequency (VLF, 0.1 Hz) voltage excitation. VLF is interesting as it is used for testing power cables and other high capacitance insulation loads. However, more experience and new methods are needed for PD interpretation. PDs were studied using two tools: pulse sequence analysis (PSA) and nonlinear time series analysis (NLTSA) from dynamic system theory. PSA was treated here as a particular case of NLTSA since their constructions are similar in their mathematical treatment. The experimental results showed that electrical trees grown at VLF had branch-type structure and times to breakdown about fifty times larger than samples aged at industrial frequency. PSA plots were compared with 2D projections of state-space trajectories that represent the dynamics of the nonlinear system (NLTSA approach). In terms of graphical representation, NLTSA 2D projections generated more clusters than the PSA plots, thus, it was interpreted that NLTSA revealed more details about the nonlinear dynamic system associated with electrical tree growth. On the other hand, using the NLTSA approach, the correlation dimension was estimated to characterize the electrical tree growth. The results showed a different evolution obtained for VLF excitation compared to the results reported for test samples aged at industrial frequency in other studies.

Imaging Conductive Objects Through Metal Enclosures Using ELF/VLF Magnetic Fields

Abstract: Imaging through conductive media is a pervasive problem in medical, industrial, and security applications. Several potential modalities such as X-ray, exotic particle beams, and related high resolution techniques have been employed in the past. However, the difficulty of production and safety of these technologies is a concern in practice. Of particular interest in this work are extremely and very low frequency (ELF/VLF, f<; 30 kHz) radio signals. We consider the feasibility of using ELF/VLF signals for detecting and imaging objects that are hidden inside thin-shelled conductive enclosures. It is shown that the hidden perfect electrical conductor (PEC) object partially blocks the incident magnetic field and results in a magnetic-field depletion that can be used to detect the object. Next, using FEKO simulations, a parametric study of through-conductor detection using a magnetic dipole is considered for a cubic aluminum shield. It is shown that signals above 1 kHz can be used to evaluate the outer shield properties, while signals at frequencies below 200 Hz can be used to effectively discern shapes of hidden objects by observing the magnetic distortion within 1 cm of the outer shield. To validate the results of theory and simulations, experiments matching the simulation conditions are conducted with a cubic aluminum container and hidden aluminum block. The experiments clearly demonstrate the presence of the magnetic field depletion as predicted by theory and simulations. The results of this work suggest that near-field ELF/VLF magnetic induction is an effective method for imaging through realistic metallic shields.

Measurement and modeling of partial discharge arising from different cavity geometries at very low frequency

Abstract: In this paper, a comparative study including measurement and modeling of partial discharge for various void geometries (cylinder, block, and prisms) at very low frequency (VLF) and power frequency (PF) voltage is presented. Results from experiments and simulations are presented with phase-resolved discharge patterns and integrated parameters (maximum/average discharge magnitudes, and repetition rates). Dynamic simulation, achieved using a combination of finite element analysis and Matlab, matches the experimental results. Differences of discharge characteristics between VLF and PF may be attributed to the physical mechanisms in the modeling including cavity surface conductivity and space charge decay rate. Compared with discharge activities at PF, the VLF excitation in general exhibits relatively lower discharge magnitude and repetition rate. Also, the inception voltage is found to be lower at VLF.

Simultaneous Study of VLF/ULF Anomalies Associated with Earthquakes in Japan

Abstract: We carried out a simultaneous study of ground-based magnetic field and lower ionospheric anomalies during major earthquakes occurring around Japan in 2010 and 2012. Ultra Low Frequency (ULF) geomagnetic field waveforms of Esashi station and Very Low Frequency (VLF) Japanese transmitter (JJY) electric signal amplitude received in Moshiri station Hokkaido during nighttime (22:00-02:00 LT) were used to minimize the local interference. Twenty earthquakes having magnitude greater than 5.5 were considered for the data analysis for two years. Nighttime amplitude fluctuations and polarization from the received VLF transmitter signal amplitude and ULF magnetic field respectively were calculated to identify anomalous signatures in relation to every earthquake. We found most earthquakes analyzed indicating VLF amplitude anomalies simultaneously occurred with ULF magnetic field anomalies within a week prior to the earthquakes. Stronger anomalies were observed for larger magnitude and shallower earthquakes. Focal mechanism of earthquakes was also examined to identify the effectiveness of generating anomaly. Both VLF and ULF anomalies were observed for reverse fault type earthquakes occurring under the strong pressure in the crust. Obtained results may indicate the common anomaly source both for VLF and ULF in the lithosphere and are consistent with currently proposed Lithosphere-Atmosphere-Ionosphere (LAI) coupling scenarios during the earthquake preparation period.

FDTD propagation of VLF-LF waves in the presence of ions in the earth-ionosphere waveguide

Abstract: The finite-difference time-domain (FDTD) method has been used for a long time to compute the propagation of very low frequency (VLF) and low frequency (LF) radio waves in the Earth-Ionosphere waveguide. In previously published FDTD schemes, only the electronic density of the ionosphere was accounted for, since in usual natural conditions the effect of the ion density can be neglected. In the present paper, the FDTD scheme is extended to the case where one or several ion species must be accounted for, which may occur in special natural conditions or in such artificial conditions as after high altitude nuclear bursts. The conditions that must hold for the effect of the ions not to be negligible are discussed, the FDTD scheme with ions is derived, and numerical experiments are provided to show that the effect of the ions may be significant when the ionosphere is disturbed by incident flows of γ or β rays.

Ionospheric D Region Parameters Obtained Using VLF Measurements in the South Pacific Region

Abstract: The diurnal variations in the phase and amplitude of NWC, NPM, and NLK very low frequency (VLF) transmissions received at Suva, Fiji, have been modeled using the Long Wave Propagation Capability (LWPC V2.1) code to determine the ionospheric D region parameters, H΄ (reference height) and β (rate of increase of electron density with height), for different daytime and nighttime conditions along the transmitter-receiver great circle paths (TRGCPs). Measured VLF signal amplitude and phase show explicit variation over the day and nighttime along the TRGCP, also revealing amplitude minima and phase steps during sunrise and sunset as the day/night terminator traverses the TRGCP. While the daytime signal strength is reasonably smooth, at nighttime, the signal exhibits a great deal of variability. For three signal paths, the mean daytime H΄ and β were found to be 70.7 km and 0.40 km-1, respectively, while nighttime mean values of these parameters were found to be 84.2 km and 0.68 km-1, respectively. The temporal and day-to-day variability of the nighttime D region parameters shows that H΄ and β ranges in between 83.0 – 85.0 km and 0.58 – 0.80 km-1, respectively. One of the possible sources of nighttime signal variability is increase in the number of modes propagating and relative complex interference between them along the TRGCPs, whereby the weaker modes also become significant at night due to reduced attenuation. In addition, the variations in the nighttime D region itself may be a cause of high signal variability.

Magnetic field propagation model of low frequency/very low communication based on mechanical antenna of electret

Abstract: Because of its stable propagation characteristics and small attenuation in the medium, low-frequency (LF) electromagnetic wave can penetrate into the sea and underground with small loss. Although its transmission bandwidth is narrow, which limits its application range, it has irreplaceable wide applications in long-distance navigation, communication and frequency release, especially in underwater communication. Therefore, the study of low frequency/very low frequency (LF/VLF) propagation is of great theoretical and military value. In the LF/VLF communication systems, the transmitting antenna is an extremely important part, and its performance has an important influence on the whole system. However, the wavelength of the LF electromagnetic wave is very long. In order to obtain the ideal radiation effect, the traditional method needs a huge transmitting antenna system, which is too large in size and power consumption. Therefore, it will be a disruptive innovation in the field to realize a technology that can significantly reduce the size the existing LF/VLF information network communication system. In view of this, in this paper we propose a kind of LF/VLF signal transmitting antenna in which an excitation device is used to drive the polarization charge of the electret to move mechanically. By accelerating the charge to form a conductive alternating electromagnetic field which can generate and radiate electromagnetic wave, under the excitation of the wave source, it carries the energy and information in the form of energy flow and propagates in a certain medium. Then, through using the magnetic field receiving system to measure the magnetic field vector in the electromagnetic wave, the effective LF/VLF signal can be obtained, thus achieving the high electromagnetic wave effective radiation which overturns the restriction that the antenna size needs to be comparable to the wave length of the radiation signal in the traditional LF navigation communication system. At the same time, an analytical model of magnetic field propagation is established based on this structure, and the influence of antenna size, shape and other relevant parameters on the performance of antenna communication are studied as well. In order to reduce the loss of accuracy and improve the calculation speed, it is necessary to choose the correct analytical model and the appropriate parameters of magnetic field generated by the mechanical antenna according to the actual situation. The research work is of great significance for designing and optimizing mechanical antennas.

Lightning Distance Estimation Using LF Lightning Radio Signals via Analytical and Machine-Learned Models

Abstract: Lightning geolocation is useful in a variety of applications, ranging from weather nowcasting to a better understanding of thunderstorm evolution processes and remote sensing of the ionosphere. Lightning-generated radio signals can be used in range estimation of lightning return strokes, for which the most commonly employed technique is the time difference of arrival in lightning detection networks. Though these instrument networks provide the most reliability and best accuracy, users without access to them can instead benefit from lightning geolocation using a standalone instrument. In this article, we present the framework for training fast models capable of estimating negative cloud-to-ground lightning location from single-instrument observations of very low frequency/low frequency (VLF/LF, 3–300 kHz) radio pulses or “sferics,” without knowledge of the ionosphere’s D-region state. The models are generated using an analytical method, based on the delay between ground and skywave, and a machine learning method. The training framework is applied to three different data sets to assess model accuracy. Validation of the machine-learned models for these data sets, which include both simulated and observed sferics, confirms this technique as a promising solution for lightning distance estimation using a single receiver. Distance estimates using a machine-learned model for observed sferics in Kansas yield an RMSE of 53 km with 68% of them being within 9.8 km. Estimates using the analytical method are found to have an RMSE of 54 km with 68% of them being within 32 km. Limitations of our methodology and potential improvements to be investigated are also discussed.

Radio aeronomy in Nigeria: First results from very low frequency (VLF) radio waves receiving station at Anchor University, Lagos

Abstract: The study of the Earth’s atmosphere and the space environment is important because of the role played by the medium in the activities that affect the Earth and its inhabitants directly or indirectly. A robust capability to monitor, model and predict the happenings in the atmospheric space through deployment of both space- and ground-based observational systems for data acquisition, is key to result-oriented scientific research in atmospheric and space sciences. In this paper, we highlight the importance of regional deployment of observational facilities for data acquisition to complement current observational tools. We briefly review the capabilities of very low frequency (VLF) radio waves (in monitoring and studying changes in the atmosphere and ionosphere), and also present data obtained from our newly deployed VLF radio waves receiver at Anchor University, Lagos (AUL). We show that the diurnal signature characterised by VLF radio signal reflected in the data of three of four propagation paths (AUL-HWU, AUL-JJI, AUL-NWC and AUL-VTX) received at AUL. This outcome shows that operational condition of the VLF radio wave receiver is good. We anticipate interesting findings as we exploit this institution-based dataset to probe ionospheric irregularities in our region for the first time. In the light of some identified ionospheric changes in the equatorial region, we anticipate that our research effort using this regional VLF dataset will yield new results that will be resourceful for better understanding and characterisation of the dynamics of the equatorial D-layer ionosphere, since our region of reception (Lagos) lies close to the equator.

Parametric interaction between ELF and VLF waves: 3D LSP simulation results

Abstract: We have investigated a non-linear parametric interaction between Extremely Low Frequency (ELF) and Very Low Frequency (VLF) waves using the Large Scale Plasma (LSP) simulation code in three dimensions. We have forced linear modes in the plasma at ELF and VLF frequencies through a direct excitation of the wave electric field. The modes are forced using traveling plane waves which propagate at specific values of the wave-vector and frequency consistent with the whistler and Fast Magnetosonic dispersion relations. We demonstrate, using a Lagrangian fluid model in LSP, that an electromagnetic ELF wave and a quasi-electrostatic VLF wave can parametrically couple to excite electromagnetic whistler waves. We also show the formation of multiple sidebands around the carrier VLF wave. This work serves to further validate a model developed in Sotnikov et al. [J. Geophys. Res. 99, 8917 (1994); J. Geophys. Res. 96, 11363 (1991)] and adapted by Mishin and Sotnikov [Plasma Phys. Controlled Fusion 59, 124003 (2017)] to explain the observations of whistler waves in the plasmasphere [E. Mishin and V. Sotnikov, Plasma Phys. Controlled Fusion 59, 124003 (2017) and Mishin et al., Geophys. Res. Lett. 38, L21101 (2011)].

Characterization of gravity waves in the lower ionosphere using very low frequency observations at Comandante Ferraz Brazilian Antarctic Station

Abstract: . The goal of this work is to investigate the gravity wave (GW) characteristics in the low ionosphere using very low frequency (VLF) radio signals. The spatial modulations produced by the GWs affect the conditions of the electron density at reflection height of the VLF signals, which produce fluctuations of the electrical conductivity in the D region that can be detected as variations in the amplitude and phase of VLF narrowband signals. The analysis considered the VLF signal transmitted from the US Cutler, Maine (NAA) station that was received at Comandante Ferraz Brazilian Antarctic Station (EACF, 62.1 ∘ S, 58.4 ∘ W), with its great circle path crossing the Drake Passage longitudinally. The wave periods of the GWs detected in the low ionosphere are obtained using the wavelet analysis applied to the VLF amplitude. Here the VLF technique was used as a new aspect for monitoring GW activity. It was validated comparing the wave period and duration properties of one GW event observed simultaneously with a co-located airglow all-sky imager both operating at EACF. The statistical analysis of the seasonal variation of the wave periods detected using VLF technique for 2007 showed that the GW events occurred all observed days, with the waves with a period between 5 and 10 min dominating during night hours from May to September, while during daytime hours the waves with a period between 0 and 5 min are predominant the whole year and dominate all days from November to April. These results show that VLF technique is a powerful tool to obtain the wave period and duration of GW events in the low ionosphere, with the advantage of being independent of sky conditions, and it can be used during the whole day and year-round.

Numerical Simulation of Very-Low-Frequency Waves Passing through the Magnetoactive Plane-Layered Plasma of Earth’s Lower Ionosphere

Abstract: —The article describes an algorithm for calculating the characteristics of the passage of low-frequency electromagnetic waves through the magnetoactive plane-stratified lower ionosphere. The algorithm is based on numerical integration of wave equations for the case of a plane wave incident on the ionosphere from below, taking into account the plasma’s ionic composition. As an illustration, the basic characteristics of passage of waves in the 0.05–1 kHz frequency range are calculated. It is shown that the power transmission coefficient of these waves is quite large, ranging from 0.2–0.4 for most of the frequencies used. It is shown that in the daytime near a frequency of ~300 Hz, the transmission coefficient in the daytime has a maximum and is approximately double the nighttime values.

Assimilating VLF Transmitter Observations With an LETKF for Spatial Estimates of the ${D}$ -Region Ionosphere

Abstract: The ${D}$ -region ionosphere plays an important role in coupling the neutral atmosphere to denser ionospheric plasma above. Practically, it impacts long-range radio communications between extremely low frequencies (ELFs), where communications are enabled by the layer, and high frequencies (HFs), which are attenuated by the layer. A combination of its altitude and extremely low electron density means that it is difficult to make measurements of the region using typical ionospheric remote sensing techniques, and our knowledge of the ${D}$ -region is limited as a result. This article presents the development of an ensemble Kalman filter method to spatially map ${D}$ -region electron density profiles over the continental United States using an array of very low frequency (VLF) radio transmitters and receivers. Data assimilation has previously been used to estimate higher altitude regions of the ionosphere, but its application to VLF radio measurements and the ${D}$ -region is new. The technique has several favorable features, including statistical confidence measures with every estimate, the ability to localize the influence of measurements, inclusion of physically realistic spatial correlations, relatively fast convergence, and the ability to add observations to the estimate as they become available. We describe the filter and present results for day, night, and terminator ionospheres using simulated data. We also present a study on the robustness of the filter when initialized with a prior that is far from the true ionosphere. The method shows promise for application to real data in the near future, especially for estimating the characteristics of large-scale features in the ${D}$ -region.

Single-Station Lightning Location Using Azimuth and Time of Arrival of Sferics

Abstract: Lightning strikes produce electromagnetic waves, now referred to as sferics, in the very low frequency (3 kHz–30 kHz) and the extremely low frequency (3 Hz–3 kHz) bands. Within these frequency bands, the Earth and ionosphere form a waveguide in which sferics propagate long distances with low attenuation. The structure of the received sferic waveform is mainly a function of propagation distance and the waveguide's parameters. This suggests that each observed sferic waveform contains information about the distance that this sferic has propagated which can be used to geolocate lightning. There are various approaches for analyzing received sferics, which mostly rely on measurements from multiple stations. However, in these methods, each station imposes an additional cost for building, maintenance, and synchronization. Here we present a novel method to estimate both the emission time and location of lightning, which works by measuring sferics recorded at a single station. We first process the sferic waveforms to obtain the arrival times of the very low frequency and extremely low frequency radiation components which propagate with different speeds. Once these two separate arrival times are determined, we use them to approximate the distance the sferic propagated in the Earth-ionosphere waveguide. We have used this novel method in combination with a method to find sferic direction to geolocate a significant number of lightning strikes for 4 July 2013. Using this proposed method, the distance of propagation estimates are accurate to within 6.7% of the National Lightning Detection Network-determined propagation distance, and the direction of propagation estimates are accurate to within ∼ 1.3% of the National Lightning Detection Network-determined direction.

Investigation of ionosphere response to geomagnetic storms over the propagation paths of very low frequency radio waves

Abstract: We analysed variations in signal metrics and the diurnal amplitude of VLF radiowaves from four propagation paths during intervals of 4 geomagnetic storms on 17, 26 September, 25 October and 1 Novem...