Powerful VLF Research Articles

Plasma perturbations in the coexisting environment of VLF transmitter emission, lightning strokes and seismic activity

We analyzed plasma perturbations occurring in the coexisting environment of powerful VLF transmitter emission, intense lightning strokes and strong seismic activity during pregnant period. The results suggest that anomalous electron bursts with energy dispersion in the range of ~100–350 keV, forming the “wisp” signature, are due to cyclotron resonance of electrons with monochromatic waves from the powerful NWC VLF transmitters during nighttime. The intense broad band VLF emissions (up-going 0+ whistlers) are observed while the DEMETER satellite goes through the region of intense thunderstorm activities at mid-latitudes. However, the effects of intense lightning activity and pregnant earthquake have little impact on this kind of stable energy-dispersed electron structures, despite the fact that they are presumably two primary reasons for the particle precipitation in the ionosphere. The case studied here provides us a valuable opportunity to address the various sources triggering the anomalous plasma perturbations in the ionosphere.

First optical observations of energetic electron precipitation at 4278 Å caused by a powerful VLF transmitter

A summary is presented of experimental optical observations at 4278 Å from close to a powerful (~150 kW) VLF transmitter (call sign JXN) with a transmission frequency of 16.4 kHz. Approximately 2.5 s after transmitter turn-on, a sudden increase in optical emissions at 4278 Å was detected using a dedicated camera/charge-coupled device (CCD) monitoring system recording at a frequency of 10 Hz. The optical signal is interpreted as a burst of electron precipitation lasting ~0.5 s, due to gyro-resonant wave-particle interactions between the transmitted wave and the magnetospheric electron population. The precipitation was centered on the zenith and had no detectable spatial structure. The timing of this sequence of events is in line with theoretical predictions and previous indirect observations of precipitation. This first direct measurement of VLF-induced precipitation at 4278 Å reveals the spatial and temporal extent of the resulting optical signal close to the transmitter.

Perturbations of ionosphere-magnetosphere coupling by powerful VLF emissions from ground-based transmitters

The characteristics of the plasma-wave disturbances stimulated in the near-Earth plasma by powerful VLF radiation from ground-based transmitters are investigated. Radio communication VLF transmitters of about 1 MW in power are shown to produce artificial plasma-wave channels (density ducts) in the near-Earth space that originate in the lower ionosphere above the disturbing emission source and extend through the entire ionosphere and magnetosphere of the Earth along the magnetic field lines. Measurements with the onboard equipment of the DEMETER satellite have revealed that under the action of emission from the NWC transmitter, which is one of the most powerful VLF radio transmitters, the generation of quasi-electrostatic (plasma) waves is observed on most of the satellite trajectory along the disturbed magnetic flux tube. This may probably be indicative of stimulated emission of a magnetospheric maser.

DEMETER observations of ionospheric heating by powerful VLF transmitters

[1] We report DEMETER spacecraft observations of ionospheric heating produced above powerful VLF transmitters by their intense radiated electromagnetic (EM) signals. We compare the heating effects of signals from the 1 MW NWC transmitter in Australia with those produced by signals from the 885 kW NAA transmitter in Maine. Significant observable effects include perturbations in plasma density and thermal electron temperature, and the production of quasi-electrostatic (QE) VLF plasma wave bands, both over the transmitters, and, in the case of NWC, also in the magnetically conjugate region. In the regions in which the QE wave bands were observed, they were invariably accompanied by a band of ELF turbulence with maximum intensity below 300 Hz. Such turbulence has in the past been associated with the presence of small scale plasma density irregularities. This association suggests that heating effects due to NWC are far-reaching and extend along Bo into the conjugate hemisphere where they are expressed in part as small scale plasma density fluctuations.

Contrasting the efficiency of radiation belt losses caused by ducted and nonducted whistler‐mode waves from ground‐based transmitters

It has long been recognized that whistler‐mode waves can be trapped in plasmaspheric whistler ducts which guide the waves. For nonguided cases these waves are said to be “nonducted”, which is dominant for L < 1.6. Wave‐particle interactions are affected by the wave being ducted or nonducted. In the field‐aligned ducted case, first‐order cyclotron resonance is dominant, whereas nonducted interactions open up a much wider range of energies through equatorial and off‐equatorial resonance. There is conflicting information as to whether the most significant particle loss processes are driven by ducted or nonducted waves. In this study we use loss cone observations from the DEMETER and POES low‐altitude satellites to focus on electron losses driven by powerful VLF communications transmitters. Both satellites confirm that there are well‐defined enhancements in the flux of electrons in the drift loss cone due to ducted transmissions from the powerful transmitter with call sign NWC. Typically, ∼80% of DEMETER nighttime orbits to the east of NWC show electron flux enhancements in the drift loss cone, spanning a L range consistent with first‐order cyclotron theory, and inconsistent with nonducted resonances. In contrast, ∼1% or less of nonducted transmissions originate from NPM‐generated electron flux enhancements. While the waves originating from these two transmitters have been predicted to lead to similar levels of pitch angle scattering, we find that the enhancements from NPM are at least 50 times smaller than those from NWC. This suggests that lower‐latitude, nonducted VLF waves are much less effective in driving radiation belt pitch angle scattering.

Penetration of lightning MF signals to the upper ionosphere over VLF ground‐based transmitters

The MF data recorded by the low‐altitude satellite DEMETER have been used to survey the MF waves around the Earth. A global map of the MF emissions indicates that there exists a wave activity in the frequency range 2–2.5 MHz above the main powerful VLF ground‐based transmitters operating in the frequency range 18–50 kHz. It is shown that this is due to the high‐frequency part of whistlers induced by the thunderstorm activity. They can penetrate trough the ionosphere at the locations of the transmitters because these transmitters induce large ionospheric perturbations. This means that an integrated map over several months is able to show these MF emissions above all main VLF transmitters. The discrepancy between intensities of the emissions in winter and summer (in the Northern Hemisphere) is explained by considering the geographic variations of the plasma frequency below the satellite. It is shown that the MF waves spread in longitude in the hemisphere opposite to the VLF transmitters.

Radiation belt electron precipitation by man‐made VLF transmissions

Enhancements of drift‐loss cone fluxes in the inner radiation belt have been observed to coincide with the geographic location of the powerful VLF transmitter NWC. In this paper we expand upon the earlier study to examine the occurrence frequency of drift‐loss cone enhancements observed above transmitters and the intensity of the flux enhancements and to demonstrate the linkage to transmitter operation. Our study has confirmed the strong dependence that these enhancements have upon nighttime ionospheric conditions. No enhancements were observed during daytime periods, consistent with the increased ionospheric absorption. We have also confirmed the persistent occurrence of the wisp features east of the NWC transmitter. The enhancements are initially observed within a few degrees west of NWC and are present in 95% of the nighttime orbital data east of the transmitter for time periods when the transmitter is broadcasting. No enhancements are observed when NWC is not broadcasting. This provides conclusive evidence of the linkage between these drift‐loss cone electron flux enhancements and transmissions from NWC. When contrasted with periods when NWC is nonoperational, there are typically ∼430 times more 100–260 keV resonant electrons present in the drift‐loss cone across L = 1.67–1.9 owing to NWC transmissions. There are almost no wisp‐like enhancements produced by the transmitter NPM, despite its low‐latitude location and relatively high output power. The lack of any wisp enhancement for L < 1.6 suggests that nonducted propagation is an inefficient mechanism for scattering electrons, which explains the lower cutoff in L of the NWC‐generated wisps and the lack of NPM‐generated wisps.

First in‐situ observations of strong ionospheric perturbations generated by a powerful VLF ground‐based transmitter

This paper is related to observations by the satellite DEMETER of strong ionospheric perturbations close to the VLF transmitter NWC in Australia (L = 1.41). Electrostatic waves from HF to ELF ranges are generated and strong turbulence appears. Fluctuations of electron and ion densities are observed as well as increase of temperature. The perturbations are located to the geographic North of the transmitter and cover an area of ∼500,000 km2 which is centred at the altitude of the satellite (700 km) around the magnetic field line at L = 1.41. The phenomenon is due to the electron and ion heating induced by the powerful transmitter VLF wave. This perturbation is in addition to the already known precipitation of energetic particles which interact with the VLF wave through a cyclotron resonance mechanism. The particle precipitation zone is located south of the transmitter at a slightly larger L‐shell value (1.9).

Global diagnostics of the ionospheric perturbations related to the seismic activity using the VLF radio signals collected on the DEMETER satellite

Abstract. The analysis of the VLF signals radiated by ground transmitters and received on board of the French DEMETER satellite, reveals a drop of the signals (scattering spot) connected with the occurrence of large earthquakes. The extension of the "scattering spots" zone is large enough (1000–5000 km) and, probably, it increases with the magnitude of the "relative" earthquake. A possible model to explain the phenomenology, based on the acoustic gravity waves and the ionosphere turbulence, is proposed. The method of diagnostics applied to this study has the advantage to be a global one due to the world wide location of the powerful VLF transmitters and of the satellite reception. However, a specific disadvantage exists because the method requires rather a long time period of analysis due to the large longitudinal displacements among the successive satellite orbits. At the moment, at least, one month seems to be necessary.

Investigation of powerful VLF disturbances

Investigation of powerful VLF disturbances

Theoretical model of possible disturbances in the nighttime mid-latitude ionospheric D region over an area of strong-earthquake preparation

We present a theoretical model of possible electron-density disturbances in the nighttime mid-latitude ionospheric D region, preceding strong earthquakes. It is found that the electron density in the nighttime D region over an earthquake epicentral zone can considerably increase before severe earthquakes. The horizontal size of the area of disturbed electron density is about 300 km. The disturbance effect is expected to be more pronounced if a powerful VLF transmitter operates in the vicinity of an imminent earthquake epicentral zone. In this case, a very dense ionization layer of daytime D-layer type can be formed at the altitudes of the upper nighttime mesosphere and can give rise to the effect of strong absorption of HF radio waves propagating over the earthquake preparation area.

VLF monitorig of atmosphere-ionosphere boundary as a tool to study planetary waves evolution and seismic influence

It is shown that sounding of the lower ionosphere boundary by subionospheric signals from powerful VLF transmitters can be a useful tool for investigation of the Planetary Waves (PW) with periods in a range of 2–30 days. A specific spring-time transition in the PW dynamic periodograms is revealed from the analysis of several years data using Tsushima, Japan VLF transmitter (10.2 kHz) along the path with length of about 1000 km. Earthquake influence in the periodograms could be sometime recognized as an appearance of specific wavelets. We discuss a possibility of PW transportation from the bottom to the upper atmosphere as modulation of shorter-scale gravity waves (GW) inside the troposphere and subsequent demodulation of the GW at the atmosphere-ionosphere boundary for explanation of the observational results.

Sounding the magnetosphere by signals from VLF radio transmitters

Abstract A new remote sensing method for the Earth's magnetosphere is suggested. The method is based on the parametric reflection of whistler mode waves from the level in the magnetosphere where a whistler wave frequency coincides with the local lower hybrid resonance frequency. In this region a sufficiently intense whistler signal generates two electrostatic plasma waves which can produce a reflected whistler wave. The known localization of the whistler reflection leads us to suggest a new method of electron density measurements in the magnetosphere, using a grid of frequencies from a powerful ground-based VLF transmitter. Parametric reflection of whistler signals in the magnetosphere can appear from naturally occurring short-scale low frequency turbulence (ion sound or ion cyclotron waves). In this case, using this new method we can obtain important information about this turbulence. Some experimental data which demonstrate the appearance of VLF echo signals from the magnetosphere with anomalously small group time delays are discussed from this point of view.

Local time and seasonal variations in the precipitation of energetic electrons from the inner radiation belt by cyclotron resonance with waves from powerful VLF transmitters

Powerful ground‐based VLF transmitters resonantly scatter ∼100‐keV electrons from the inner radiation belt onto trajectories from which they precipitate into the atmosphere as they drift eastward. To obtain a better understanding of the underlying wave‐particle mechanisms, we have studied energetic electron data from the Burst and Transient Source Experiment (BATSE) on NASA's Compton Gamma Ray Observatory (GRO) satellite. Wave‐particle interactions are detected when the satellite traverses a cloud of energetic electrons which have been scattered into quasi‐trapped trajectories by VLF waves. The previously reported seasonal variation in the frequency of occurrence of cyclotron resonance interaction is confirmed with the continuous coverage provided by GRO. These data also provide the first detailed measurements of the frequency of occurrence of the quasi‐trapped electron fluxes as a function of local time at the transmitter. The variations are more complex than just a day‐night effect, and the precipitation is most often seen in the predawn hours. As with the seasonal changes, the local time variations are governed by the transmission of VLF waves through the ionosphere.

VLF RESISTIVITY MAPPING AND VERTICALIZATION OF THE ELECTRIC FIELD1

AbstractFor over 20 years, powerful VLF transmitters have been used as electromagnetic sources for subsurface investigations in mining exploration. Measurements initially concerned the vertical component of the magnetic field or the inclination of the field and were later extended to measurement of the horizontal electric field in the direction of the transmitter, to determine the resistivity of the terrain. Measurement of the electric field is usually performed with electric lines, grounded or not, with lengths of at least 5 m.This paper presents the concept of a VLF resistivity meter with a very short electric sensor (1 m) and the results obtained with it. This technique improves the measurement of the electric field, which is in principle a point value. It also permits a higher spatial sampling rate and, by closely linking the electric sensor with the magnetic sensor on a lightweight mount, makes it possible for the instrument to be used by a single operator.In addition, transformation of the electric field data, analogous to reduction to the pole in magnetism, is proposed to correct the horizontal deformation of the anomalies created by polarization of the primary field. Comparison with direct current electrical measurements shows highly satisfactory correlations. This transformation, validated for VLF, can be extended to any electrical or electromagnetic method using a uniform primary field, i.e. gradient array in direct current or low‐frequency magnetotellurics. We call this verticalization of the electric field.Resistivity measurements and mapping using the VLF frequency range can be applied not only to mining but also to a wide range of shallow geophysical studies (hydrology, civil engineering, etc.) and are not limited to problems concerning the location of conductive targets

VLF propagation parameters derived from sferics observations at high southern latitudes

Sferics are electromagnetic pulses generated by lightning events. Their maximum spectral energy is in the frequency range below 15 kHz. These powerful natural VLF transmitters can be used to determine the propagation characteristics of the atmospheric wave guide between earth and ionospheric D layer along virtually every propagation path. A VLF-sferics-analyzer was operating at the German Antarctic von Neumayer Station from January to June 1983. This analyzer recorded sferics from distant lightning events in the frequency range between 5 and 9 kHz. The method of measurement is described. The data are evaluated, and the propagation characteristics of the atmospheric wave guide are determined as a function of azimuth and season. The result is compared with theoretical calculations. It is shown that the difference between west-to-east and east-to-west propagation is much smaller than theory predicts, indicating that the ionospheric D layer at high southern latitudes behaves less anisotropic with respect to VLF propagation than at mid-latitudes.

Unintentional man-made modification effects in the magnetosphere

Evidence continues to accumulate that even very small input signals, such as power-line harmonic radiation (PLHR), can give rise to wave-particle interactions in the magnetosphere by stimulation of emission at VLF and contribute to the formation of the electron slot (2< L<3). Over North America and its geomagnetic conjugate, the satellite data indicate a permanent zone of VLF emission which appears to originate in the industrial areas of northeastern U.S.A. and southern Canada. The more intense stimulated emission in the summer (northern hemisphere) occurs poleward of the electron slot at 3< L<5. The resultant increased electron precipitation may explain the increase in thunderstorms at L>4 in the period 1935–1970 relative to 1900–1935. There is probably an intensity threshold at which PLHR becomes important. This is consistent with ground-based data from longitudinally separated stations near L = 4 (Siple, Halley, Yakutsk). The discovery that certain industrial plants (e.g. cement works) give rise to relatively much more intense PLHR requires further investigation. Also, geomagnetically induced currents in long power lines at high geomagnetic latitudes in Canada and Alaska can result in transformer saturation and possible damage, as well as greatly increased PLHR. Powerful VLF transmitters in the range 15–25 kHz can produce peaks in the spectra of energetic electrons observed on low altitude satellites. Morphological studies of the NAA wavefield intensity above the ionosphere show strong amplification immediately to the east of the South Atlantic Anomaly.

On some effects observed during powerful VLF wave action on the ionosphere

The results of observations of the amplitude variations of pulses transmitted by a powerful VLF transmitter (ƒ = 15 kHz) at different distances are given. The duration of the pulses varied from 5 to 120 sec. The observed variations may be explained as the self‐action of a powerful wave in the lower ionosphere.