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.

Very low frequency high voltage sinusoidal electrical testing method, systems and apparatus

Abstract: Very low frequency (VLF) high voltage (HV) sinusoidal electrical test waves provide for testing AC electrical power installations and equipment having large electrical capacitances. VLF HV sinusoidal electrical waves are suitable for testing characteristics and/or qualities of insulation on long, buried electric power cables. Capacitance of a load being tested by VLF HV sinusoidal waves is discharged during a second half of each positive half-wave and during a second half of each negative half-wave by using a sequence of resistive discharge paths. Each successive discharge path in a sequence has less resistance than its predecessor for creating a sequence of progressively-reduced discharge Time Constants. Solenoid-operated switch contacts bring successive discharge paths into action. Also, solenoid-operated switch contacts reverse polarity to create positive and negative half waves of the VLF HV sinusoidal electrical test waves. To avoid inadvertent switch closure, downward spring-bias requires solenoids be energized for overcoming spring force and gravity to close switches upwardly. AC voltage is amplitude-modulated sinusoidally by moving a brush along transformer taps arranged with sine-function voltage differences. Alternatively, such modulation is achieved by a revolving heart-shaped-cam cyclically operating in opposite directions a variable autotransformer brush. Stepping-up amplitude-modulated AC voltage to 40 kilovolts or more, then rectifying and polarity reversing successive half waves creates VLF HV sinusoidal test waves.

Variance of transionospheric VLF wave power absorption

Abstract: To investigate the effects of D ‐region electron density variance on wave powerabsorption, we calculate the power reduction of very low frequency (VLF) wavespropagatingthroughtheionospherewithafullwavemethodusingthestandardionosphericmodel IRI and in situ observational data. We first verify the classic absorption curves ofHelliwell’susingourfullwavecode.ThenweshowthattheIRImodelgivesoverallsmallerwave absorption compared with Helliwell’s. Using D‐region electron densities measuredbyrocketsduringthepast60years,wedemonstratethatthepowerabsorptionofVLFwavesissubjecttolargevariance,eventhoughHelliwell’sabsorptioncurvesarewithin±1standarddeviation of absorption values calculated from data. Finally, we use a subset of therocket data that are more representative of the D region of middle‐ and low‐latitude VLFwave transmitters and show that the average quiet time wave absorption is smaller thanthat of Helliwell’s by up to 100 dB at 20 kHz and 60 dB at 2 kHz, which would make themodel‐observation discrepancy shown by previous work even larger. This result suggeststhat additional processes may be needed to explain the discrepancy.

Decay of a vertical plasma column: A model to explain VLF sprites

Abstract: VLF sprites are identified by high angle scattering of VLF transmissions by the conducting plasma columns which appear as the luminous columns of “red sprites”. VLF sprites are “early/fast Trimpis” and probably vice versa. Recently discovered properties of early/fast Trimpis are the logarithmic decay of the amplitude of the scattered signal and monotonic variation of its phase. These properties are explained in terms of scattering from a vertical column or set of columns extending from 50 km (or lower) altitude to about 80 km.

Altitude profiles of localized D region density disturbances produced in lightning‐induced electron precipitation events

Abstract: A three-dimensional model of very low frequency (VLF) radio wave propagation in the Earth-ionosphere waveguide in the presence of lower ionospheric disturbances is used to quantitatively interpret VLF signatures of lightning-induced electron precipitation (LEP) events observed in two previously reported cases One case is that of a 285-kHz signal originating in Puerto Rico and propagating to a receiver in Lake Mistissini, Quebec The other case involves a 240-kHz signal originating in Cutler, Maine, and received at Stanford, California In both cases, high-resolution measurements of the VLF signals were made to accurately document characteristic signatures of LEP events (Inan et al, 1988b, 1990) The comparison of the model calculations with the data yields information about the altitude profiles of electron density of both the extra ionization produced by the LEP events and of the ambient ionospheric D region The comparisons are carried out using generally accepted values of the spatial extent of the disturbed regions and the intensity of the particle flux constituting the LEP burst

LIII. The propagation of very low frequency radio waves to great distances

Abstract: Summary In discussing the propagation of very low frequency radio waves to great distances, it is convenient to treat the space between the earth and the ionosphere as a wave-guide. This treatment is applied to some experimental measurements made by Dr. Weekes on signals of frequency 16 kc/s, over a range of distances from 340 km to 3640 km from the sender. It is shown that the measurements can be explained in terms of the four least attenuated wave-guide modes. If the earth's magnetic field is neglected, and the ionosphere is assumed to be a sharply bounded homogeneous, ionized medium, then it is shown that the height of the boundary is 69·1±0·5 km.