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.

High-Speed Frequency-Shift Keying of LF and VLF Radio Circuits

Abstract: Present VLF transmitters are limited in transmission speed to approximately 20 words per minute because of the bandwidth limitations of the High-Q antenna circuit. To permit an increase in transmission speed, a unique method has been developed which makes use of the variation of reactance of the antenna circuit in synchronism with frequency shift modulation of the carrier. The method makes possible the transmission of teletype or high speed Morse signals at speeds of 100 words per minute or more. This increase by a significant factor in present traffic handling capabilities of VLF stations can be accomplished by increasing the cost of the station less than ten per cent.

VLF remote sensing of high‐energy auroral particle precipitation

Abstract: Ground-based measurements of VLF transmitter signals propagating in the Earth-ionosphere waveguide can be used to determine the location of nighttime high-energy (≥100 keV) auroral particle precipitation. When the region of auroral particle precipitation passes over a VLF propagation path, disturbances in the D region of the ionosphere created by the high-energy particles perturb the amplitude of VLF signals propagating below in a characteristic manner. Continuous nighttime observations of the amplitude of the signal from the NLK transmitter (24.8 kHz, Jim Creek, Washington) were made in Gander, Newfoundland, during November 1993 and January 1994. Simultaneous images of atmospheric X rays created by auroral particle precipitation taken by the AXIS instrument on the UARS satellite were examined for times when large-scale auroral particle precipitation extended over the NLK-Gander propagation path. Quantitative characteristics of the precipitation-associated NLK signal perturbations are established from days which clearly exhibit good correlation between the AXIS images and VLF data, and a larger data set from the months of November 1993 and January 1994 is examined statistically to determine the effectiveness of the VLF technique in capturing particle precipitation events. The number of particle precipitation onsets seen in the AXIS images that can be readily identified in the VLF amplitude data is found to be almost 94%. VLF propagation model calculations show that the observed VLF amplitude decreases are consistent with propagation under conditions of enhanced lower ionosphere electron density caused by auroral electron precipitation and suggest that electrons with energies greater than 100 keV are responsible for the VLF amplitude perturbations.

Characteristics of slow earthquakes in the very low frequency band: Application to the Cascadia subduction zone

Abstract: Deep tectonic tremors detected in many subduction zones worldwide are often accompanied by very low frequency (VLF) signals, detectable by broadband seismometers but usually hidden in large ambient noise. By stacking broadband seismograms relative to tremor hypocentral times, we can recover the VLF signals. The stacked signals are then inverted to determine a moment tensor solution, using a procedure previously applied to VLF signals in Japan, Taiwan, and Mexico. Here we apply this method to the Cascadia subduction zone, where tremors and slow slip events are clearly observed. As expected, we successfully recover VLF signals for almost the entire tremor region beneath southern Vancouver Island and northern Washington State. The moment tensors are mostly well determined as low-angle thrust type, but source depths are poorly constrained. The slip direction is slightly rotated counterclockwise with respect to the local plate motion direction, probably due to bending of the subducting plate. The seismic moment measured in VLF band is proportional to the seismic energy of tremors, with a scaled energy of about 3 × 10−9. The widespread observability of VLF signals suggests that the deformation associated with tremors and SSEs is actually a very broadband phenomenon, as suggested by stochastic models.

Very low frequency disturbances and the high-altitude nuclear explosion of July 9, 1962

Abstract: On July 9, 1962, a 1.4-megaton nuclear device was exploded at an altitude of 400 km above Johnston Island in the central Pacific. The burst perturbed the relative phase of three frequency-stabilized VLF transmissions on propagation paths shielded by the earth from the direct effects of the explosion. The temporal variations of these three disturbances differ in major respects. On the NPG, Jim Creek, Washington, to APJ/JHU path, the onset and maximum were essentially instantaneous, a characteristic which generally fits the burst- related neutron-decay model of Crain and Tamarkin in which sudden ionization in the VLF altitude region is produced by neutron-decay electrons geomagnetically guided into the lower ionosphere. In addition, the NPG-APL variation shows a secondary perturbation having a 10 second period and stemming from a hydromagnetic disturbance associated with temporarily trapped neutrondecay protons of 0.4 Mev. In contrast to the NPG-APL perturbation, and among other differences, the perturbations to the transmissions from NBA (at Balboa, Panama) and WWVL (at Boulder, Colorado) are marked by a delay in the onset and maximum. The disturbance observed for the NBA-APL path, which lies almost along a geomagnetic meridian, results from ionization of the lower ionosphere by electrons that were produced inmore » the radioactive decay of fission fragments and that formed an artificial radiation belt. Here there is good qualitative agreement between the temporal variation of the VLF perturbation and the energy contained in the stream of trapped fission-decay electrons drifting azimuthally from the burst region over Johnston Island to the NBA area. The temporal VLF phase variation for the WWVL-APL path (which lies along a line of nearly constant geomagnetic latitude) indicates that the major part of this perturbation is due to ionization resuiting from the effects of geomagnetically trapped neutron-decay electrons. A relatively early and small part of this disturbance also results from the contribution of trapped fission-decay electrons. (auth)« less

Modeling of subionospheric VLF signal perturbations associated with earthquakes

Abstract: It has been reported that propagation of very low frequency (VLF) waves in the Earthionosphere waveguide might provide an indication of imminent earthquakes [Hayakawa et al., 1996; Molchanov et al., 1998]. Narrow-band data from Inubo, Japan, suggested that transmissions from Omega Japan, 1000 km away, might be influenced by pre-earthquake processes. The terminator time (TT) was defined as the time where a minimum occurred in the received phase (or amplitude) during sunrise and sunset. A few days before an earthquake the TT was observed to deviate significantly from the monthly averages, producing a longer “VLF day.” The TT effect has been explained through some rather simple modeling by a 1–2 km drop in the VLF reflection height at the lower ionospheric boundary. In this study we apply more realistic propagation models to show that the changes in VLF reflection height associated with earthquakes would have to be considerably larger (∼4–11 km) than those suggested previously in order to produce the reported effect. If the reported TT changes were caused by alterations in the VLF reflection height associated in some manner with an imminent earthquake, these effects would be commensurate with the effects of a solar flare. However, this would lead to changes in received amplitude (or phase) that would be significant at all times, and not just during the day/night transition. Hence it is not at all clear that a simple height-lowering explanation for the TT effect is correct.