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.

The Modulated Precipitation of Radiation Belt Electrons by Controlled Signals from VLF Transmitters

Abstract: The first direct observations of the precipitation of radiation belt electrons by the controlled injection of VLF signals from a ground based transmitter were recently reported from data acquired in the SEEP (Stimulated Emission of Energetic Particles) experiment. That outstanding example of time-correlated wave and electron data has now been enhanced by the finding of four additional modulated events out of 65 satellite passes when one of the U. S. Navy VLF transmitters at Cutler, Maine (NAA) or at Annapolis, Maryland (NSS) was being modulated in a 3s ON/2s OFF format. During each of these events the fluxes of precipitating electrons were observed repeatedly to display a characteristic time behavior with respect to the transmitter modulation: a relatively slow rate of increase after start of the ON period leading to a maximum about 2 seconds later. Details of this consistent pattern and the statistics of occurrence of modulation events are presented along with comparisons of the absolute fluxes of precipitating electrons observed during normal transmitter operation with those recorded when one of the transmitters was modulated.

On phase changes in very‐low‐frequency propagation induced by an ionospheric depression of finite extent

Abstract: An approximate analysis is given for VLF propagation in the earth-ionosphere waveguide of variable height. The height changes are not restricted to points along the great circle path connecting the transmitter and receiver. By choosing a simple example, we show that ionospheric perturbations lying within the first Fresnel zone are most effective in modifying the phase of the received signal. This paper is a sequel to an earlier one published in the Journal in 1961 about ionospheric depressions that were infinite in their transverse dimension.

Impact of modulation frequency on cross-phase modulation effect in intensity modulation-direct detection WDM systems

Abstract: The impact of modulation frequency on the crossphase modulation (XPM) effect in intensity modulation (IM)-direct detection wavelength-division multiplexing systems is investigated theoretically and numerically. A simple expression for IM is derived, verified by simulation and its validity is presented. The dependence of XPM-induced IM on the fiber length, fiber dispersion, channel separation and pump modulation frequency is assessed. It is shown that at very low frequency the walkoff effect has almost no influence on the XPM-induced IM efficiency which increases with the square of the frequency; at higher frequency the IM efficiency can be reduced significantly by the walkoff and scales linearly with modulation frequency.

VLF line radiation observed by satellite

Abstract: VLF line radiation received by the ISIS 1 and 2 satellites over New Zealand is found to fall into two distinct classes. The first of these consists of magnetospheric lines (MLs) which are characterized by a broadband appearance and by frequency drifts of a few tens of hertz per minute, similar to those reported elsewhere. Both their initial frequencies and their frequency spacings were, however, found to be essentially random rather than multiples of 50 or 60 Hz. The hourly variation in ML occurrence showed no correlation with electrical load in possible electrical mains systems sources. No clear decrease in occurrence on weekends was evident. This first satellite survey with significant numbers of MLs found no evidence of a relationship with power line harmonics. The second class of satellite-received VLF lines consists of “tram lines” (TLs) which are characterized by their very narrow bandwidth and zero frequency drift. TLs appear to lie close to harmonics of 50 or 60 Hz. An example of this class is presented.

Models of ionospheric VLF absorption of powerful ground based transmitters

Abstract: [1] Ground based Very Low Frequency (VLF, 3–30 kHz) radio transmitters play a role in precipitation of energetic Van Allen electrons. Initial analyses of the contribution of VLF transmitters to radiation belt losses were based on early models of trans-ionospheric propagation known as the Helliwell absorption curves, but some recent studies have found that the model overestimates (by 20–100 dB) the VLF energy reaching the magnetosphere. It was subsequently suggested that conversion of wave energy into electrostatic modes may be responsible for the error. We utilize a newly available extensive record of VLF transmitter energy reaching the magnetosphere, taken from the DEMETER satellite, and perform a direct comparison with a sophisticated full wave model of trans-ionospheric propagation. Although the model does not include the effect of ionospheric irregularities, it correctly predicts the average total power injected into the magnetosphere within several dB. The results, particularly at nighttime, appear to be robust against the variability of the ionospheric electron density. We conclude that the global effect of irregularity scattering on whistler mode conversion to quasi-electrostatic may be no larger than 6 dB. Citation: Cohen, M. B., N. G. Lehtinen, and U. S. Inan (2012), Models of ionospheric VLF absorption of powerful ground based transmitters, Geophys. Res. Lett., 39, L24101, doi:10.1029/2012GL054437.