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.

Some notes on ELF Earth-ionosphere waveguide daytime propagation parameters

Abstract: The transverse-electromagnetic (TEM) propagation constants for extremely low frequency (ELF) daytime propagation in the earth-ionosphere waveguide have been calculated for frequencies of 5-2000 Hz. The recently developed theory of Greifinger and Greifinger and the Wait very low frequency (VLF) exponential ionospheric-conductivity profile have been used in the analysis. It is shown that the resulting values of ELF attenuation rate, phase velocity, and ionospheric-reflection height are in excellent agreement with the measured data.

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.

A study of VLF antennas immersed in sea water: theoretical, numerical, and experimental results

Abstract: The problem of receiving VLF electromagnetic fields with an underwater electric antenna is of great interest, especially for military-application purposes. In this study, a simple and closed-form expression for the E field in sea water is derived from two physical models, which agree but are different: the surface-wave theory and the Fresnel formulas. Three configurations of immersed receiving antennas are investigated by means of computer simulations. The scattered electric field and expected measured voltage are computed, using an EFIE [electric-field integral equation], and the method of moments (MoM). Then, an equivalent-circuit model of the underwater antenna is investigated, with a focus on two dominant antenna parameters: the impedance and the voltage measured between the wire terminals. Calculations and computed results are compared with off-shore measurements.

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.

Very-low-frequency radiation spectra of lightning discharges

Abstract: Spectral a nalyses are given of t he groundwa ve por t ion of 33 sferi c waveform s recorded from cloud-t o-ground ligh t ning discharges which occu rred at distances ra nging between about 150 and 600 kilom eters from Boulder, Colo. Freq uencies of peak energy lie betwee n 5 and 20 kilocycles p er second , which ag ree fa vorably wi t h other pub lished results. The average value of e nergy calculated from the groundwave pulses was found to be 26,600 joules, whi ch is lower than va lu es derived from ot he r experim ents. Va rious pa ra m eters, such as the peak a mpli tud e and du ratio n of t he first ha lf-cy cle, a re related to the radiated energy of the stroke.