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.

Whistler wave propagation and whistler wave antenna radiation resistance measurements

Abstract: Whistler waves are a common feature of ionospheric and magnetospheric plasmas. While the linear behavior of these waves is generally well understood, a number of interesting observations indicate that much remains to be learned about the nonlinear characteristics of the mode. For example, in space, very low frequency (VLF) emissions triggered by whistler modes launched from ground-based transmitters have been observed. Emission is assumed to come from transverse currents formed by counterstreaming electrons that are phase bunched by the triggering signal. In the laboratory, it has been shown that with increasing amplitude of the driving signal applied to an antenna, the whistler mode radiation pattern forms a duct with diameter of the order of the parallel wavelength. The ducted waves were observed to propagate virtually undamped along the length of the plasma column. These observations have prompted an Naval Research Laboratory's (NRL) Space Physics Simulation Chamber study of whistler wave dynamics. The goals are to investigate whistler wave ducting, self-focusing, and amplification, and to study nonlinear whistler-plasma interactions.

Very broadband analysis of a swarm of very low frequency earthquakes and tremors beneath Kii Peninsula, SW Japan

Abstract: [1] We have conducted a temporal broadband seismic observation in Kii Peninsula, southwest Japan, and detected a swarm of 110 very low frequency earthquakes (VLFs) and deep low frequency tremors During three days of the swarm activity, VLFs and tremors occur concurrently in two localized regions separated by ∼10 km Stacking analyses are also employed to detect VLF signals of a period longer than 50 s, whose focal mechanisms are determined for the first time from data and shown to be consistent with the subducting plate motion Evaluation of the VLF seismic moment implies that a substantial portion of the SSE seismic moment is released as a VLF swarm

A wave‐wave interaction in whistler frequency range in space plasma

Abstract: The quantitative theory is developed for the frequency spectrum broadening and sideband structure of VLF signals in the upper ionosphere. The theory is based on a common theory of wave-wave interaction in plasma and includes the transformation effects of a quasi-monochromatic whistler wave into quasi-electrostatic whistler mode waves (l waves) near the resonance cone with their interaction with field-aligned plasma density (electric field) inhomogeneities. Such inhomogeneities can be ion-acoustic or ion cyclotron electrostatic waves. We consider both cases of regular (deterministic plases) and irregular (random phases) inhomogeneities. The spectral characteristics and interaction length of l waves and some nonstationary effects are considered, which permit us to obtain the quantitative interpretation of main experimental data. The conditions of high coherence of interacting waves are investigated, which are compared with the results of bicoherence analysis of sideband structure of VLF signals. The estimations of bicoherence value are given for such type of wave-wave interaction.

Unusual behavior of Very Low Frequency signal during the earthquake at Honshu/Japan on 11 March, 2011

Abstract: We present evidence of unusual Very Low Frequency (VLF) signal amplitude variation during the devastating earthquake of magnitude 9.0 which occurred at Honshu, in Japan on 11 March, 2011. We use the SoftPAL very low frequency receiver placed at Ionospheric and Earthquake Research Centre of Indian Centre for Space Physics, located at Sitapur (Lat. 22°30′N, Long. 87°47′E). We observe significant changes in signal amplitude from JJI (Lat. 32°05′N, Long. 131°51′E) station transmitting at a frequency of 22.2 kHz prior to the earthquake. We analyze signal amplitude for almost 2 weeks to establish a possible seismo–ionospheric correlation. We observe significant shift of the sunrise terminator time up to 2 days before the earthquake and the shift is found to be maximum on the day of the earthquake. In addition, we observe unusual increase of the D-layer disappearance time during the earthquake and the value becomes maximum on the day of the earthquake. These findings generally agree with our previous findings reported elsewhere.

The effects of a small local change in phase velocity on the propagation of a VLF radio signal

Abstract: Cha nges in th e propagation of a VLF sign a l due to t he cha ngrs in phasr \" ('loc it,\\' causeci by a sm all localized depression in the h eight of the ionosphe re a long or near t il(' lin e jo ining th e t ra nsmitter a nd r eceiver arc in vestigated usin g r lcmr ntary dirrl'l1ct ion theo ry. It is shown t hat both the ph ase a nd amplitude of th e reer iH'd s ig na l ma.v br changed if t he horizo ntal extent of t he depression is great enough a nd if it is su ffi cicnt ly close to the p ath. If th e dep ress ion is at great er distances from th e p ath, thr amp li t ud e a nd phase of t he recei\\'ed sig na l Ill errly osc illates abou t t he undisturbed values. An accoun t is given of th e use of Co rnu 's spiral in est im a ting t he effects of a pa rt icu lar disturbance.