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Very low frequency

About: Very low frequency is a research topic. Over the lifetime, 1540 publications have been published within this topic receiving 24233 citations. The topic is also known as: VLF.


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Journal ArticleDOI
TL;DR: In this paper, the authors used a nonlinear decomposition technique called the empirical mode decomposition EMD method with the Hilbert transform to obtain more reliable low frequency electromagneticVLF-EM data.
Abstract: Geologic noise and background electromagnetic EM waves often degrade the quality of very low frequency electromagneticVLF-EMdata.Toretrievesignalswithsignificant geologic information, we used a new nonlinear decomposition technique called the empirical mode decomposition EMD method with the Hilbert transform. We conducted a 2Dresistivitymodelstudythatincludedinversionofthesyntheticdatatotesttheaccuracyandcapabilitiesofthismethod. Next, we applied this method to real data obtained from a fieldexperimentandageologicexample.ThefilteringprocedureforrealdatastartswithapplyingtheEMDmethodtodecompose the VLF data into a series of intrinsic mode functions that admit a well-behaved Hilbert transform. With the Hilbert transform, the intrinsic mode functions yielded a spectrogram that presents an energy-wavenumber-distance distribution of the VLF data. We then examined the decomposeddataandtheirspectrogramtodeterminethenoisecomponents, which we eliminated to obtain more reliable VLF data. The EMD-filtered data and their associated spectrograms indicated the successful application of this method. Because VLF data are recorded as a complex function of the real variable distance, the in-phase and quadrature parts are complementarycomponentsofeachotherandcouldbeaHilbert transform pair if the data are analytical and noise free. Therefore,bycomparingtheoriginaldatasetwiththeoneobtained from the Hilbert transform, we could evaluate data quality and could even replace the original with its Hilbert transform counterpart with acceptable accuracy. By application of both this technique and conventional methods to real data in this study, we have shown the superiority of this new method and have obtained a more reliable earth model by invertingtheEMD-filtereddata.

56 citations

Journal ArticleDOI
TL;DR: In this article, the seafloor-to-seafloor response to a horizontal electric dipole source is explored as a function of frequency and range, showing that, compared with the response for a half-space with the lowest conductivity in the reference model, significant enhancement of the field amplitude can occur at long ranges (>100 km) and low frequencies (<1 Hz).
Abstract: A reference model for the electrical conductivity structure beneath the deep seafloor is proposed and justified using a variety of geophysical evidence. The model consists of relatively conductive sediment and crustal layers of 6.5 km extent overlying a resistive (≈10−5 S/m) subcrustal channel of 30 km thickness and terminated in a deeper conductive layer and half-space. Its seafloor-to-seafloor response to a horizontal electric dipole source is explored as a function of frequency and range, showing that, compared with the response for a half-space with the lowest conductivity in the reference model, significant enhancement of the field amplitude can occur at long ranges (>100 km) and low frequencies (<1 Hz). At the same time, marked attenuation relative to the half-space response is seen at higher frequencies. The field enhancement is due to trapping of electromagnetic energy in a leaky subcrustal waveguide, as demonstrated by computing the complex Poynting vector. The attenuation occurs in the relatively conductive sedimentary and crustal layers overlying the lithospheric waveguide when their electrical thickness exceeds a skin depth. The results indicate that attempts either to model controlled electromagnetic sources or to interpret controlled source data using half-space models for the Earth can be badly misleading. The practicality of lithospheric communications in the real Earth is also investigated. Using measured receiver noise figures and the reference model, the receiver bandwidth necessary to achieve a given signal-to-noise ratio as a function of range and frequency is estimated for a seafloor horizontal source of strength 105 A-m. The results indicate that significant (≈100 km) ranges can be achieved only around 1 Hz with a bandwidth of ≈1 Hz at a SNR of 10, yielding a very low data rate of <3.5 bits/s. Longer ranges and higher frequencies are precluded by attenuation in the sediment and crustal layers and because the conductivity in the resistive channel is too large.

56 citations

Journal ArticleDOI
TL;DR: In this paper, the authors presented evidence to support the hypothesis that the low frequency cutoff of a VLF noise band observed by the Alouette 1 satellite is due to a plasma resonance (the lower hybrid resonance) which defines a cutoff frequency for propagation transverse to the earth's field.
Abstract: Evidence is presented to support the hypothesis that the low frequency cutoff of a VLF noise band observed by the Alouette 1 satellite is due to a plasma resonance (the lower hybrid resonance) which defines a cutoff frequency for propagation transverse to the earth's field. If this hypothesis is accepted, the observations of this cutoff frequency by the satellite's VLF receiver, and simultaneous measurements of the electron plasma frequency by the topside sounder, can be used to determine an effective mean mass for the ions in the ambient plasma. The values, together with the scale height measured from the electron density profiles, set limits on the ion composition and temperature. The main support for the hybrid resonance hypothesis lies in the plausibility of the temperature and ion mass information that is derived using this interpretation and the agreement between this information and the results of other workers. In general, is found to be larger during the day than at night and to increase with latitude. In polar regions ranges from about 7 at midnight to about 13 at noon; at midnight the composition of the polar ionosphere at 1000 km is more than 60% O+, whereas at noon it is about 95% O+. At middle latitudes, L = 2 to 3, ranges from 1.8 to as high as 7. For low latitudes, L < 2, the lowest observed values of of 1.3 require that the H+ concentration be greater than 68%. An effective ionospheric temperature of about 1200°K is determined for middle latitudes, increasing to high values for high latitudes. A marked diurnal variation of temperature is found at high latitudes.

56 citations

Journal ArticleDOI
TL;DR: In this article, the Earth-ionosphere waveguide mode interference pattern in the spectra of radio atmospherics (or sferics for short), which are the high-power, broadband, very low frequency (VLF, 3-30 kHz) signals launched by lightning discharges, was measured and applied to measure the local midlatitude daytime ionospheric D region electron density profile sharpness.
Abstract: [1] We described and applied a technique to measure the local midlatitude daytime ionospheric D region electron density profile sharpness from the Earth-ionosphere waveguide mode interference pattern in the spectra of radio atmospherics (or sferics for short), which are the high-power, broadband, very low frequency (VLF, 3–30 kHz) signals launched by lightning discharges. VLF propagation simulations are used to show that the upper VLF frequency spectral minima of sferics on several hundred kilometers long propagation paths depend critically on the effective D region sharpness while depending only weakly on the effective D region height. This enables the straightforward extraction of the sharpness parameter from measured VLF spectra, which generally exhibit well-defined minima at upper VLF frequencies. By applying this technique, we calculated the profile sharpness during morning, noontime, and afternoon periods in 3 different days using sferics from ∼660–800 km away. The measured sharpness showed a weak dependence on the solar zenith angle, with values between 0.35 and 0.45 km−1 for angles from 20° to 75°. This is different from the previous narrowband measurement since the sharpness derived from narrowband VLF signals highly depends on the solar zenith angle. To better understand this discrepancy, we also used simulations to calculate the equivalent exponential profiles for International Reference Ionosphere (IRI) profiles and the empirical FIRI model profiles. The equivalent exponential profiles can best duplicate the sferic spectral characteristics for IRI and FIRI models. We find that both the magnitudes and solar zenith angle variations of the sharpness for our broadband measurements, previous narrowband measurements, and both models are completely different. This suggests the daytime ionosphere, particularly at larger solar zenith angles, may not be well described by a simple two-parameter exponential model.

55 citations

Journal ArticleDOI
TL;DR: In this paper, the effects of broadening of frequency spectra of very low frequency (VLF) signals were observed in the polar ionosphere when VLF transmitter signals were registered on the satellite "Intercosmos-19".
Abstract: The effects of broadening of frequency spectra of quasi-monochromatic very low frequency (VLF) signals were observed in the polar ionosphere when VLF transmitter signals were registered on the satellite "Intercosmos-19". Broadening of the spectrum was mainly registered on the electric antenna to the extent of Δf ≤ ± 1 kHz. Broadening of the VLF signal and intensive extra low frequency (ELF) noise with f < 1 kHz were observed simultaneously. Such effects can be explained by a transformation of the initial whistler-wave, into whistler-mode waves with wave-normals very near the resonant cone, by small-scale ionospheric irregularities. These irregularities can be caused by ion-acoustic or ion-cyclotron waves. The effect of the broadening of the VLF signal spectrum can be used as a diagnostic of ionospheric turbulence and field-aligned electric currents in the upper ionosphere.

55 citations


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Performance
Metrics
No. of papers in the topic in previous years
YearPapers
202320
202232
202156
202048
201942
201852