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.

Illumination of the plasmasphere by terrestrial very low frequency transmitters: Model validation

Abstract: [1] A composite model of wave propagation from terrestrial very low frequency (VLF) transmitters has been constructed to estimate the wave normal angles and fields of whistler mode waves in the plasmasphere. The model combines a simulation of the fields in the Earth-ionosphere waveguide, ionospheric absorption estimates, and geomagnetic field and plasma density models with fully three-dimensional ray tracing that includes refraction, focusing, and resonant damping. The outputs of this model are consistent with those of several previous, simpler simulations, some of which have underlying component models in common. A comparison of the model outputs to wavefield data from five satellites shows that away from the magnetic equator, all of the models systematically overestimate the median field strength in the plasmasphere owing to terrestrial VLF transmitters by about 20 dB at night and at least 10 dB during the day. In addition, wavefield estimates at L < 1.5 in the equatorial region appear to be about 15 dB too low, although measured fields there are extremely variable. Consideration of the models' similarities and differences indicates that this discrepancy originates in or below the ionosphere, where important physics (as yet not conclusively identified) is not being modeled. Adjustment of the low-altitude field estimates downward by constant factors brings the model outputs into closer agreement with satellite observations. It is concluded that past and future use of these widely employed trans-ionospheric VLF propagation models should be reevaluated.

A Portable Very Low Frequency (VLF) Communication System Based on Acoustically Actuated Magnetoelectric Antennas

Abstract: A novel very low frequency (VLF) communication system using one pair of magnetoelectric (ME) antennas has been proposed. The ME antennas are strain-mediated acoustic resonators operating at their electromechanical resonance in the VLF band. The measured near-field radiation pattern reveals ME antennas are equivalent to dipole antennas. The magnetic field radiated by the ME transmitter has been predicted along with distance ranging from 1 mm to 100 km. The measured magnetic field distribution coincided well with the prediction, and the maximum communication distance of 120 m has been achieved. With 80 V driving voltage, the power consumption of the ME transmitter has been measured as 400 mW. Furthermore, the direct antenna modulation (DAM) has also been successfully demonstrated on the ME antennas.

VLF wave stimulation experiments in the magnetosphere from Siple Station, Antarctica

Abstract: The Earth's magnetosphere is host to remarkable very low frequency (VLF) electromagnetic signals of natural origin. One of these, called a whistler, originates in lightning. Others, such as hiss and chorus, originate within the plasma itself. They are important for at least three reasons. First, they reveal the properties of the plasma through which they travel and thus can be used as remote sensing tools. Second, their high intensity and narrow bandwidths indicate the presence of a previously unknown kind of wave particle interaction that converts the kinetic energy of charged particles to coherent electromagnetic radiation. This process is called the coherent wave instability (CWI). Third, energetic charged particles are precipitated into the ionosphere through resonant scattering by these same waves, causing enhanced thermal ionization, X rays, light, and heat. To better understand and use the CWI, controlled VLF signals have been injected into the magnetosphere from Siple Station, Antarctica and received on satellites and near the conjugate point in Quebec, Canada. In addition to reproducing many puzzling natural phenomena, these experiments have provided critical new data on the CWI, laying a foundation for various theories and computer simulations. Key findings are as follows: 1) Coherent VLF signals often exhibit exponential temporal growth (∼30 dB) and saturation at levels estimated to be of order 5 pT. 2) Temporal growth requires that the input signal exceed a threshold that varies widely with time. The probable cause of the growth threshold is in situ background noise that reduces the efficiency of phase bunching by a coherent input signal whose intensity is comparable to the noise level within the frequency band of the interaction (∼100 hz). 3) Narrowband triggered emissions can be entrained by Siple frequency ramps of different slope but of much lower (−20 dB) amplitude. The mechanism of entrainment is not yet understood. 4) For two equal amplitude input waves spaced 20 Hz apart, the temporal growth of each component is almost totally suppressed. For larger spacings, 40–100 Hz, the lower frequency is more suppressed than the upper. For 10 < Δƒ < 100 Hz, unsymmetrical sidebands at integer multiples (up to seventh order) of Δƒ are created, along with subharmonics. The integer sidebands are attributed to emission growth triggered by one beat and suppressed by the next. Taken together, the spectrum of the stimulated sidebands and sub-harmonics is thus more noise-like than the transmitted spectrum. 5) Simulated hiss shows coalescence of selected noise wavelets into longer and stronger chorus-like emissions, suggesting that chorus and hiss originate in the same mechanism. Future objectives of a VLF wave injection facility include (1) new experiments on the physics of wave growth and wave-induced particle scattering and precipitation, (2) testing of the predictions of theories of VLF wave-particle interaction, (3) development of new techniques for remote sensing and control of space plasmas using VLF techniques, and (4) improvements in the design and operation of VLF communication and navigation systems.

VLF and LF signatures of mesospheric/lower ionospheric response to lightning discharges

Abstract: New evidence is presented of disturbances of the electrical conductivity of the nighttime mesosphere and the lower ionosphere in association with lightning discharges. In addition to extensive documentation of the characteristics of a class of events heretofore referred to as early/fast VLF events [Inan et al., 1993], our data reveal a new feature of these events, consisting of a postonset peak that typically lasts for 1–2 s. We also report the observation of short-duration VLF or LF perturbations, in which the amplitude of the subionospheric signal exhibits a sudden change within 20 ms of the causative lightning discharge, and recovers back to its original level in < 3 s. These short-duration events have characteristics similar to the previously observed rapid onset, rapid decay VLF signatures [Dowden et al.., 1994]. Both the typical and rapidly recovering events are observed primarily when the causative lightning discharge is within ±50 km of the VLF or LF great circle propagation path, indicating that the scattering from the localized disturbance is highly collimated in the forward direction. The latter in turn implies that for the parameters in hand, the transverse extent of the disturbance must be at least ∼ 100–150 km. The measured VLF signatures are compared with the predictions of a three-dimensional model of subionospheric VLF propagation and scattering in the presence of localized ionospheric disturbances produced by electromagnetic impulses and quasi-electrostatic (QE) fields produced by lightning discharges. The rapidly recovering or short-duration events are consistent with the heating of the ambient electrons by quasi-static electric fields, in cases when heating is not intense enough to exceed the attachment or ionization thresholds. When no significant electron density changes occur, the conductivity changes due to heating alone last only as long as the QE fields, typically less than a few seconds. When heating is intense enough so that attachment or ionization thresholds are exceeded, reductions or enhancements in electron density can respectively occur, in which case the medium would relax back to the ambient conditions with the time scales of the local D region chemistry, typically 10–100 s.

Morphology of VLF emissions observed with the Injun 3 satellite

Abstract: Results of a study of very low frequency (VLF) emissions observed with the Injun 3 satellite are presented. Approximately 1200 hours of VLF magnetic field strength data and approximately 6000 frequency spectra samples are investigated in this study. These data cover invariant latitudes up to 82°, all local times, and altitudes from 237 to 2785 km. Contour plots as a function of invariant latitude and magnetic local time giving the frequency of occurrence of VLF emissions above a given intensity are presented. The most intense VLF emissions observed by Injun 3 are found to occur between about 55 and 75° invariant latitude and during the local day, with the maximum intensity occurring at about 65° invariant latitude and about 8 to 10 hours magnetic local time. The region of most intense VLF emissions was found to move to lower latitudes during geomagnetically active periods. The principal types of VLF emissions occurring in this region are ELF hiss and chorus, with the ELF hiss usually being the most intense.