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.

Use of Random Excitation and Spectral Analysis in the Study of Frequency-Dependent Parameters of the Cardiovascular System

Abstract: If a randomly varying signal or noise is used as input, it is possible to study the input-output relations of a system, over a wide band of frequencies, by the use of power spectrum analysis. This technique has been applied to the cardiovascular system, by deliberately introducing irregularity in the pressure and flow pattern, by random electrical pacing of the heart. Examples are given of the determination, by spectral analysis in the range of 0.25 to 25 cycles/sec, of aortic and femoral arterial impedance in the dog, and also of the transmission ratio for pressure oscillations along the aorta. The results agreed satisfactorily with those obtained by the Fourier analysis of single pulses. When the time scale of the irregularities was made sufficiently great, it was possible to examine aortic impedance in the very low frequency range 0.00125 to 0.125 cycle/sec. In this range the operation of the baroreceptor reflexes became apparent.

Subionospheric VLF signatures of nighttime D region perturbations in the vicinity of lightning discharges

Abstract: A 12-hour sequence of perturbations of subionospheric VLF signals observed in association with lightning provided preliminary evidence that the ionospheric regions perturbed in these events may be confined to within ∼150 km of the lightning discharges, and that intracloud flashes as well as cloud-to-ground lightning may be important in producing the perturbations. High-resolution analysis of event signatures indicated the presence of two different classes of events. For one set of events, observed during the most active central 6 hours of the observation period, a ∼0.6-s delay between the causative lightning and VLF event onset and a ∼1-s onset duration was observed, consistent with previously suggested models of the gyroresonant whistler-particle interaction that leads to particle precipitation and perturbation of the Earth-ionosphere waveguide. However, another set of events, observed during the first 2 hours of the observation period, exhibited a very different temporal signature, characterized by a much smaller (<50 ms) delay and sometimes also very short (<50 ms) rise times. Such events are possibly related to previously reported cases of similarly early/fast events and may involve a more direct coupling between the lightning discharge and the lower ionosphere.

Quasi-trapped vlf propagation in the outer magnetosphere.

Abstract: Quasi-trapped whistler mode propagation and generation by trapped electrons in magnetosphere, noting refraction index and wave reflection

Theory of triggered VLF emissions

Abstract: A theory is presented to explain the origin of triggered discrete VLF emissions that is more complete than earlier theories, in that it is not restricted to the discussion of kinematical relations but evaluates the dynamics of the problem. Resonant electrons are phase correlated with the wave magnetic field by a finite length whistler train moving in the opposite direction. The time for phase correlation is of the order of the period of oscillation of a particle in the effective ‘potential well’ of the wave. It is recognized that the wave acceleration due to the inhomogeneous magnetic field of the earth must be small enough for the particle to stay trapped in the potential well. The phase-correlated electrons are subject to an instability in the form of an emitted whistler with a growth rate γ/ω ∼ (n/no)2/5(υ⊥/c)2/5 (ωp/Ω)2/5, where (n/no) is the fractional density of the resonant particles, υ⊥ is their mean transverse velocity, and ωp and Ω are local cold plasma and gyrofrequencies. The emitted frequency varies according to ω = k(ω)υ∥ − Ω, where υ∥ is the zero order longitudinal velocity of the resonant electron, and the wave vector k is a function of frequency ω through the whistler dispersion relation. The theory is in good agreement with observation.