Showing papers on "Very low frequency published in 2019"

A high Q piezoelectric resonator as a portable VLF transmitter.

Abstract: Very low frequency communication systems (3 kHz–30 kHz) enable applications not feasible at higher frequencies. However, the highest radiation efficiency antennas require size at the scale of the wavelength (here, >1 km), making portable transmitters extremely challenging. Facilitating transmitters at the 10 cm scale, we demonstrate an ultra-low loss lithium niobate piezoelectric electric dipole driven at acoustic resonance that radiates with greater than 300x higher efficiency compared to the previous state of the art at a comparable electrical size. A piezoelectric radiating element eliminates the need for large impedance matching networks as it self-resonates at the acoustic wavelength. Temporal modulation of this resonance demonstrates a device bandwidth greater than 83x beyond the conventional Bode-Fano limit, thus increasing the transmitter bitrate while still minimizing losses. These results will open new applications for portable, electrically small antennas. Designing high radiation efficiency antennas for portable transmitters in low frequency communication systems remains a challenge. Here, the authors report on using piezoelectricity to more efficiently radiate while achieving a bandwidth eighty three times higher than the passive Bode-Fano limit.

Sampling Rate Effects on Resting State fMRI Metrics.

Abstract: Low image sampling rates used in resting state functional magnetic resonance imaging (rs-fMRI) may cause aliasing of the cardiorespiratory pulsations over the very low frequency (VLF) BOLD signal fluctuations which reflects to functional connectivity (FC). In this study, we examine the effect of sampling rate on currently used rs-fMRI FC metrics. Ultra-fast fMRI magnetic resonance encephalography (MREG) data, sampled with TR 0.1 s, was downsampled to different subsampled repetition times (sTR, range 0.3-3 s) for comparisons. Echo planar k-space sampling (TR 2.15 s) and interleaved slice collection schemes were also compared against the 3D single shot trajectory at 2.2 s sTR. The quantified connectivity metrics included stationary spatial, time, and frequency domains, as well as dynamic analyses. Time domain methods included analyses of seed-based functional connectivity, regional homogeneity (ReHo), coefficient of variation, and spatial domain group level probabilistic independent component analysis (ICA). In frequency domain analyses, we examined fractional and amplitude of low frequency fluctuations. Aliasing effects were spatially and spectrally analyzed by comparing VLF (0.01-0.1 Hz), respiratory (0.12-0.35 Hz) and cardiac power (0.9-1.3 Hz) FFT maps at different sTRs. Quasi-periodic pattern (QPP) of VLF events were analyzed for effects on dynamic FC methods. The results in conventional time and spatial domain analyses remained virtually unchanged by the different sampling rates. In frequency domain, the aliasing occurred mainly in higher sTR (1-2 s) where cardiac power aliases over respiratory power. The VLF power maps suffered minimally from increasing sTRs. Interleaved data reconstruction induced lower ReHo compared to 3D sampling (p < 0.001). Gradient recalled echo-planar imaging (EPI BOLD) data produced both better and worse metrics. In QPP analyses, the repeatability of the VLF pulse detection becomes linearly reduced with increasing sTR. In conclusion, the conventional resting state metrics (e.g., FC, ICA) were not markedly affected by different TRs (0.1-3 s). However, cardiorespiratory signals showed strongest aliasing in central brain regions in sTR 1-2 s. Pulsatile QPP and other dynamic analyses benefit linearly from short TR scanning.

Comparative study of the possible lower ionospheric anomalies in very low frequency (VLF) signal during Honshu, 2011 and Nepal, 2015 earthquakes

Abstract: 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...

VLF Current Distribution and Input Impedance of an Arbitrarily Oriented Linear Antenna in a Cold Plasma

Abstract: In this paper, we proposed a semianalytical method for calculating the current distribution and input impedance of a very low frequency (VLF: 3-30 kHz) linear antenna of arbitrary orientation in a homogeneous anisotropic cold plasma. By considering the effect of the geomagnetic inclination angle, the kernel function, in this case, has a more complicated form and requires extra analytical techniques to deal with. The computations show that the amplitude coefficients for the ordinary wave are evidently greater than those for the extraordinary wave. We also found that the shape of the current distribution is not sensitive to the orientation of the antenna, but the total current moment on the antenna will be decreased when the inclination angle becomes larger. Moreover, due to the higher attenuation rates for both the ordinary and extraordinary waves at a propagation direction perpendicular to the magnetic field, the overall trend for the input impedance of the antenna is increasing with the geomagnetic inclination angle. It is then concluded that the optimal posture for a VLF space-borne linear antenna should be as parallel as possible to the direction of the geomagnetic field in order to achieve maximum antenna efficiency.

Near-Field of a VLF Electric Dipole in an Anisotropic Plasma

Abstract: In this paper, we proposed a semianalytical method for evaluating the near-zone field of a very low frequency (VLF) electric dipole in a homogeneous and anisotropic plasma. Due to the different attenuation properties for the ordinary wave (O-wave) and the extraordinary wave (E-wave), the integrals corresponding to their contributions should be truncated at different locations of the real axis and need to be separately treated. By applying the complex function theory and a speed-up convergence algorithm, the original oscillating integral for the near-field is transformed into two fast convergent integrals plus an analytical solution. Computations show that, in the near zone, the O-wave still has comparable amplitudes with the E-wave, and its influence on the total field should not be neglected. It is also observed from the radiation pattern that there exists a pronounced “cohesion effect” for the E-wave along the direction of the geomagnetic field, while the phase mutations of the O-wave exactly coincide with the minima of its field strength. Moreover, it is found that the radiation ability of a VLF electric dipole in an anisotropic plasma will increase with the operating frequency.

Dependence of properties of magnetospheric line radiation and quasiperiodic emissions on solar wind parameters and geomagnetic activity

Abstract: Very low frequency (VLF) electromagnetic waves in the inner magnetosphere sometimes exhibit either frequency or time modulation. These phenomena are called, respectively, magnetospheric line radiation (MLR) and quasiperiodic (QP) emissions. Data from Detection of Electro‐Magnetic Emissions Transmitted from Earthquake Regions (DEMETER) spacecraft were used to analyze their properties, such as MLR frequency spacing, QP modulation period, and QP intensity as functions of geomagnetic activity and solar wind parameters. Altogether, 1152 MLR events and 2172 QP emissions were analyzed. It is shown that the influence of the analyzed parameters on QP emissions is different for QP events with modulation periods shorter/longer than 20 s. While the properties of QP events with long modulation periods are significantly related to the geomagnetic activity and solar wind parameters, no such dependences are observed for events with short modulation periods. This suggests that there might be two types of QP emissions generated by two different mechanisms. It is further shown that there seems to be no relation between the properties of QP and MLR events observed at the same times. Finally, the event properties do not seem to be related to the whistler occurrence rate.

The 22 July 2009 Total Solar Eclipse: Modeling D Region Ionosphere Using Narrowband VLF Observations

Abstract: We present D region ionospheric response to 22 July 2009 total solar eclipse by modeling 19.8‐kHz signal from NWC very low frequency (VLF) navigational transmitter located in the Australia. NWC VLF signal was received at five stations located in and around eclipse totality path in the Indian, East Asian, and Pacific regions. NWC signal great circle paths to five stations are unique having eclipse coverage from no eclipse to partiality to totality regions, and the signal is exclusively confined in the low and equatorial regions. Eclipse‐induced modulations in NWC signal have been modeled by using long‐wave propagation capability code to obtain D region parameters of reflection height (H′) and sharpness factor (β). Long‐wave propagation capability modeling showed an increase in H′ of about 2.3 km near central line of totality, 3.0 km in the region near to totality fringe, and 2.4 to 3.0 km in the region under partial eclipse. Using H′ and β, Wait ionosphere electron density (Ne) profile at the daytime altitude of 75 km showed a decrease in Ne by about 58% at a station near totality central line, whereas at totality fringe and in partial eclipse region decrease in the Ne was between 63% and 71% with respect to normal time values. The eclipse associated variations in the H′, β, and Ne are less in low‐latitude region as compared to midlatitude. The study contributes to explain observations of wave‐like signature in the D region during an eclipse and difference in the eclipse effect in the different latitude‐longitude sectors.

Acoustically Driven and Modulation Inducible 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 demands for IoT applications. Especially in the very low frequency (VLF: 3-30 kHz) range and lower, propagation through tens of meters of seawater, hundreds of meters of earth, and hundreds of kilometers of air with only 2-3 dB of loss is possible. However, the fundamental reduction in efficiency as the size of electrical antennas decreases below a wavelength (30 m at 1 MHz) has made portable communication systems in the VLF and low frequency (LF: 30-300 kHz) ranges impractical for decades. A paradigm shift from electrical 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 using frequency-shift keying. Detailed analysis of design parameters offers a roadmap for significant future improvement in both radiation efficiency and data rate in the new field of acoustically driven antennas.

Experimental comparisons between AM and BW modulation heating excitation of ELF/VLF waves at EISCAT

Abstract: In this paper, the experimental results of the extremely/very low frequency (ELF/VLF) radiation from the ionosphere by amplitude modulation (AM) mode and dual-beam beat-wave (BW) mode modulation heating using the European Incoherent Scatter Scientific Association (EISCAT) heating facility are presented. By comparing the dependence of the ELF/VLF radiation sources formed by AM and BW models on the geomagnetic field disturbance and the differences in different periods, this paper aims to examine some issues such as source regions and mechanisms addressed in the theoretical study of ionospheric high frequency modulated heating. The results show that (1) in the AM mode, the intensity of the ELF/VLF radiation source depends on the intensity of geomagnetic disturbance and thus on the magnitude of natural currents in the ionosphere, while in the BW mode, the intensity of the ELF/VLF radiation source is less dependent on the intensity of geomagnetic disturbance; (2) during the relatively quiet period of geomagnetic disturbance, the intensity of the ELF/VLF radiation source formed by AM mode modulation heating decreases and by BW mode modulation, heating increases when the ionization of the lower ionosphere decreases. The results of our EISCAT modulation experiments presented in the current paper and the earlier one by Yang et al. [“The polarization characteristics of ELF/VLF waves generated via HF heating experiments of the ionosphere by EISCAT,” Phys. Plasmas 25, 092902 (2018)] support that the BW mode heating mechanism mainly results from the ponderomotive nonlinearity, and the modulation region is located in the F layer.

Study of relativistic beam of electron on whistler mode waves for subtracted distribution in Saturnian magnetosphere

Abstract: Very low frequency electromagnetic waves in the magnetosphere of Saturn are the focus of the present study. Hot injected beam effect on these waves like whistler mode waves for relativistic and non-relativistic subtracted bi-Maxwellian distribution function have been studied in the presence of perpendicular A.C electric field. By using the method of characteristics solutions and kinetic approach, expression for dispersion relation and growth rate has been derived via detailed calculations and derivations. By changing parameters of plasma like: ac frequency, thermal anisotropy, number density, parametric analysis has been done. The influence of AC frequency on Doppler shift is analyzed, and the comparative study of the effects of oblique and parallel propagating waves on the growth rate has been done. Some different results were found using the subtracted bi-Maxwellian distribution function discussed with the results of bi-Maxwellian distribution function. It can be seen that the effective parameters for generating whistler mode waves are not only temperature anisotropy, but also relativistic factor, alternating field frequency, subtracted distribution amplitude and the width of the loss cone distribution function, which have been discussed in the results and discussion section.

Optimal Window for the Estimation of Very Low Frequency Content in Heart Rate Variability Analysis

Abstract: The spectral analysis of heart rate variability (HRV) is an accepted method for assessing the autonomic control of the cardiovascular system. The electrocardiographic recordings used to extract the time interval between consecutive R waves (RR intervals or tachogram) should last 5 min or 24 h, according to the guidelines published in 1996 by the European Heart Journal (TASK FORCE). The three frequency bands recognized are the VLF (0.003–0.04 Hz), the LF (0.04–0.15 Hz) and the HF (0.15–0.4 Hz), which are associated with cardiovascular modulatory mechanisms. Given that the estimation of VLF in 5 min recordings is unreliable and that in some circumstances (orthostatism, controlled breathing, etc.) it is not possible to obtain 24 h recordings, it becomes necessary to consider other window sizes in order to estimate it with greater certainty. To show how the size of the window affects the estimation of the power spectral density, synthetic signals were evaluated using the Welch periodogram, comparing the power and spectral resolution obtained with conventional 5 min windows versus results obtained with windows that ranged from 300 to 3000 s in steps of 300 s, plus an additional one of 4000 s. Noise-free signals were generated, contaminated with White Gaussian Noise and mounted on a linear trend, to approximate the conditions of a real tachogram. The results suggest that the optimal size of the analysis window is 50 min, decreasing the power estimation error of the VLF band from 24.91 to 8.06%, increasing the spectral resolution and increasing the confidence in the evaluation of the three frequency bands defined for the HRV, especially for the VLF. These results suggest an alternative analysis for recordings with duration less than 24 h but that require evaluating the VLF, as in the HRV recordings of patients during their haemodialysis session.

Numerical Analysis of Electric Field Distribution at the Terminal of VLF Antenna

Abstract: When the high-power VLF (Very Low Frequency) communication system is in operation, dielectric loss and corresponding temperature rising will occur at the terminal of the antenna due to the alternating high electric field intensity environment. Based on the theoretical analysis of dielectric loss and temperature rise, the distribution of electric field intensity at the terminal of antenna and its influencing factors are numerically analyzed by establishing a three-dimensional simulation model. The results show that the electric field intensity varies linearly with the increase of the voltage of the antenna terminal; however, the working frequency has little effect on the electric field intensity. Finally, the reduction effect of a corona ring on the electric field intensity at the antenna terminal is analyzed.

Influence of Cavity Geometry on Partial Discharge Measurement at Very Low Frequency

Abstract: As a promising alternative to conventional high voltage AC testing at PF (power frequency 50/60 Hz), VLF (very-low-frequency, typically 0.1 Hz) testing is nowadays used in practice. The increasing application of VLF testing for condition assessment of power apparatus necessitates understanding the partial discharge (PD) behaviors at this frequency. In this paper, the influence of cavity geometry on PD activities is investigated. A comparative experimental study is carried out with PF and VLF voltage excitations for different void structures (cylindrical, block, prism). Measurement results are presented with phase-resolved discharge patterns and integrated parameters such as average void discharge, repetition rates, etc. Under the same applied overstress relative to inception voltage, measurement results show that void discharges are strongly dependent on the cavity geometry as well as excitation frequency. With increasing cavity volume, PDs are more likely to occur and in particular, VLF excitation yields lower discharge magnitude, repetition rate and the phase range of occurrence.

Methods for the Evaluation of the Stochastic Properties of the Ionosphere for Earthquake Prediction—Random Matrix Theory

Abstract: Seismo-ionospheric coupling is a field of great interest and is currently subject to rigorous study; using both ground and satellite data and many phenomenological features, the ionospheric precursors of earthquakes were identified. In this work, we present methods to study the stochastic properties of the lower ionosphere, derived from the data obtained with very low frequency (VLF) receivers at frequencies in the range of 19.6 to 37.5 kHz. Two main approaches are described: auto-correlation and random matrix theory treatments of amplitude time series data. It is shown that before shallow earthquakes with magnitudes greater than four, there are measurable changes that can be used in earthquake prediction. Although the exact form of the causal chain that leads to these changes are currently subject to diligent study, we believe that the investigations described herein are worth adding to the repertoire of ionospheric precursors.

Steady State Frequency Response Utilizing an Enhanced Chirp Test Signal

Abstract: One technique to obtain a rapid system frequency response is to excite the system with a chip signal where the frequency of the signal ramps up from a low start frequency to a final high frequency. With such a signal the system is never in steady state. If the ramp were a stair step with multiple periods for each frequency at each stair step, steady state could be achieved but the result will not be rapid. This is especially true where the desired system frequency response starts at a very low frequency where multiple periods of the first frequency could be many tens of seconds. A common technique is to configure the test signal as a Sum of Sines (SOS) where all the frequencies are summed together with the signal duration being a few periods of the lowest frequency. Algorithms to extract the frequency response from such a signal have been developed. A shortcoming of this approach is that this signal has a rogue wave that consumes measurement dynamic range. This limits the signal amplitude that can be allocated to each frequency and becomes a constraint on measurement resolution. With a chirp signal the full dynamic range is available at each frequency. A new variation of chirp along with an advanced algorithm has been developed whereby the rogue wave of the SOS signal is avoided and the signal duration is very close the SOS signal.

Evaluation of Actual Radio Noise of Pulsed Power Transmission Detected by Antenna

Abstract: Radio noise intensity caused by a series of electric pulses is evaluated. The pulses are utilized in pulsed power network already proposed as one of upcoming smart grids. In contrast to the power transmission scheme with continuous sinusoidal electric current of very low frequency, pulsed power transmission may affect surrounding field with high frequency radio noises. In this paper, based on the analysis of electric field strength close to the power line, the radio noises generated by electric pulses are roughly evaluated. In contrast to the previous works, where only the peak strength of each component in noise spectrum is considered, actually measured noise strength through a dipole antenna is roughly evaluated.

Modeling of VLF network observations due to lower ionospheric perturbation during a solar eclipse

Abstract: A large part of the path of the Annular Solar Eclipse of May 20,2012 (magnitude 0.9439) (ASE-2012) was over southern Japan. The D-region ionospheric changes associated with the ASE-2012, led to several degree of observable perturbations of sub-ionospheric very low frequency (VLF) radio signal. The solar eclipse associated signal changes were identified in VLF several receiving stations $(R_{x})$ simultaneously for the VLF signals coming from both Japanese and US VLF transmitters ($T_{x}$). In this work, we have analyzed temporal dependences of VLF electric amplitude perturbation $(\Delta A_{ecl,obs}(t))$ from two Japanese VLF transmitters (JJI (22.2kHz) and JJY (40.0kHz)), and the spatio-temporal characteristics of respective subionospheric perturbations are studied in detail. We consider the 2-parameter D-region ionospheric model with the exponential electron density profile. To model the shadow effect on the D-region ionosphere due to obscuration of solar disk, we assume a generalized space-time dependent 2-Dimensional Elliptical Gaussian distribution Model (2DEGM) for ionospheric parameters, such as, effective reflection height $(h')$ and sharpness factor $(\beta )$. In the vicinity of the eclipse zone, we compute the subionospheric VLF signal propagation for several signal propagation paths. In the simulation, we obtain the perturbation of VLF signal amplitude ($\Delta A_{ecl,LWPC}(t))$ at each station and compare with its observtaional counterpart $(\Delta A_{ecl,obs}(t))$.

A New Type of VLF Sinusoidal Waveform Generator Used for AC Withstand Voltage Testing of Power Cables

Abstract: Power cables play an important role in power grid, among which XLPE cables are widely used. Meanwhile, insulation failures of cables have become a severe safety hazard. AC withstand voltage testing is an effective method to assess the overall insulation level of power cables before they are put into operation. The very low frequency (VLF) testing equipment has been popular in recent years used for ac withstand voltage testing due to its portability. This paper presents a new type of VLF sinusoidal waveform generator which can generate a sinusoidal waveform at 0.1 Hz. Compared with other conventional VLF generators, the proposed design benefits at simple structure and novel control. The equipment consists of a bipolar charging circuit, a discharging circuit, a polarity conversion switch, a resistance voltage divider and the control system, and the key to generate VLF sinusoidal waveform is hysteresis control technology. This paper builds the simulation model of VLF generator using the SIMULINK software to verify the feasibility of system topology and control strategy. The simulation result shows that this design of VLF generator can generate a sinusoidal waveform at 0.1 Hz and its total harmonic distortion (THD) value is less than 5%. In the meantime, this paper studies the effect of some circuit parameters on the output waveform to get their optimal values by the SIMULINK software. In the end, an experiment platform was built in the laboratory to verify the simulation result, and the experimental result shows that this VLF generator can generate a sinusoidal waveform at 0.1 Hz and its THD value can meet the requirement of IEEE 400.2 standard, proving its feasibility and effectiveness.

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

Abstract: . The goal of this work is to investigate the gravity waves (GWs) 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/Marine (NAA) station that was received at Comandante Ferraz Brazilian Antarctic Station (EACF, 62.1° S, 58.4° W), which is a great circle path crossing longitudinally the Drake Passage. The wave periods of the GWs detected in the low ionosphere are obtained using the wavelet analysis applied to the VLF amplitude. The use of the VLF technique 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 wave periods detected using VLF technique for 2007 showed that the GW events occur almost all nights, with a higher frequency per month from March to October. The predominant wave periods are more frequent between 10 and 15 min occurring preferentially during the equinoxes, but there are some events with periods higher than 60 min appearing only in the solstices (January and July). 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 to be independent of sky conditions, and can be used during daytime and year-round.

Wave-induced particle precipitation into the ionosphere from the inner magnetosphere

Abstract: The Earth inner magnetosphere is a donut-shape large reservoir of magnetized plasma, energetic particles (keV – MeV), and various plasma waves. Plasma waves play important roles in the Earth’s magnetosphere. Plasma waves with very low frequency (VLF) of $\sim$ kHz and ultra low frequency (ULF) of $\sim$ Hz, can interact with energetic electrons and ions trapped in the magnetic field in the magnetosphere at approximately 3 – 10 Earth radii. Thus, these waves can change pitch-angle (angle to the magnetic field line in momentum space) and result in charged particle precipitation into the ionosphere and upper atmosphere. This can in turn cause charged-particle-impact ionization and change ionospheric densities and conductivities [e.g., 1]. Here we comprehensively study charged particle precipitation from the inner magnetosphere into ionosphere and their effects. We simulate ion and electron dynamics in the inner magnetosphere during moderate and strong storms, using the CIMI/BATS-R-US model [2, 3], which solve the bounce-averaged Boltzmann’ transport and magnetohydrodynamics equations including quasi-linear wave-particle interactions. We also simulate ion and electron precipitation into the mid-latitude ionosphere and estimate their impact on the ionosphere calculating ionospheric chemistry and conductivity, which affects magnetosphere-ionsphere coupled current system and Joule heating in the ionsphere.

Observations of Electromagnetic Waves at Very Low Frequency in the Near Topside Ionosphere

Abstract: The e-POP/Swarm-E Radio Receiver Instrument (RRI) on CASSIOPE detects very-low-frequency (VLF) broadcasts from national naval sources over the whole world. The propagation in question is the whistler mode whose dispersion relation allows a number of unique phenomena to be examined. Because the whistler mode is the only ionospheric magnetoionic mode propagating at VLF, the crossed 6-m dipoles feeding the RRI permit the whistler-mode elliptical electric fields to be measured. This allows the direction of arrival (DOA) of propagation at the satellite to be determined. Examples of wave reception from NWC (Australia), NLK (USA), NAA (USA), and DHO38 (Germany) show features of whistler-mode propagation in the topside ionosphere.

Propagation and Time-of-Arrival of VLF Pulses in the Earth-Ionosphere Waveguide

Abstract: Very low frequency (VLF) propagation in the earth-ionosphere waveguide has been of interest for many decades. Most computational capabilities have been performed for a single frequency, with time-dependent results (such as those applied to lightning) obtained by inverse-Fourier-transforming these single-frequency components over a spectrum of frequencies. But if time-of-arrival (TOA) of a wave pulse is of interest, a less computationally-intensive method can be employed. Since a single spectral field component in the waveguide may be represented as a sum of modal fields, and since the group velocity is different for each mode, only the mode with the largest group velocity – the lowest order mode – need be calculated to obtain TOA information. This principle is demonstrated in detail for a simple waveguide, and applied to the earth-ionosphere waveguide.