VLF Transmitter Research Articles

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.

Modeling the relaxation of early VLF perturbations associated with transient luminous events

Studies show that early VLF perturbations, characterized by abrupt signal onsets and long recoveries, occur often in relation with transient luminous events, especially sprites and sprite halos. Also, most of the early VLF events are attributed to forward scattering of subionospheric VLF transmissions incident upon horizontally elongated disturbances of elevated ionization in the upper D region. This concept is supported by the similarity of early VLF event recoveries to those of lightning‐induced electron precipitation events (LEPs), which are due to electron density enhancements in the upper D region caused by lightning and whistler‐induced precipitation of radiation belt electrons. Here, the simplified Glukhov‐Pasko‐Inan model, that has been developed for LEP investigations, is applied to simulate early VLF event recoveries observed simultaneously with sprite discharges in the D region. The present study shows that: (1) early VLF events with longer (shorter) recoveries are likely to come from higher altitudes of about 80–90 km (lower altitudes of about 70–80 km) and under conditions of lower (higher) electron density elevations relative to ambient values; (2) although negative ion and positive cluster ion production plays a role in electron density relaxation at lower heights, the electron–single ion dissociative recombination is likely the key process at upper D region heights that defines the relaxation of early VLF perturbations; and (3) tentative estimates of electron density increases responsible for early VLF events may reach values possibly higher than 104 electrons per cm3 in the upper D region ionosphere.

Potential waves for pitch‐angle scattering of near‐equatorially mirroring energetic electrons due to the violation of the second adiabatic invariant

Bounce and MLT averaged diffusion rates due to gyroresonance scattering by plasmaspheric hiss, lightning generated whistlers, and whistlers due to VLF transmitters inside the plasmasphere; and chorus waves outside the plasmasphere; are computed in high resolution in pitch‐angle. The computed rates show that another scattering mechanism is required to provide pitch‐angle scatting for nearly equatorially mirroring particles at low energies outside the plasmasphere and at a wide range of energies in the slot region. We show that magnetosonic waves and ElectroMagnetic Ion Cyclotron (EMIC) waves can be in bounce resonance with electrons for a wide range of energies and pitch‐angles, and may be responsible for the scattering of electrons with equatorial pitch‐angels close to 90°. Bounce resonance may also result in an in‐situ acceleration of equatorially mirroring particles.

Correction to “Radiation belt electron precipitation by man-made VLF transmissions”

[1] In the paper “Radiation belt electron precipitation by man-made VLF transmissions” by Rory J. Gamble et al. (Journal of Geophysical Research, 113, A10211, doi:10.1029/2008JA013369, 2008), Figures 2 and 7 were switched. The correct figures and their captions are shown here.

SNR changes of VLF radio signals detected onboard the DEMETER satellite and their possible relationship to the Wenchuan earthquake

Here we used the VLF signal data received by the DEMETER satellite, transmitted from various ground VLF transmitters which are located around China, to study the changes in the signal to noise ratio (SNR) before and after the Wenchuan earthquake, which had a magnitude of 8.0. We also found that the SNRs of different frequency signals decreased significantly over the epicenter region before the earthquake, and reverted to their original levels after the earthquake. This phenomenon may be related to the earthquake.

Oblique Whistler-Mode Waves in the Inhomogeneous Magnetospheric Plasma: Resonant Interactions with Energetic Charged Particles

A consistent account is given of the theory of resonant interactions between energetic charged particles and a whistler-mode wave propagating obliquely to the non-uniform geomagnetic field in the inhomogeneous magnetospheric plasma. The basic equations for the wave field and charged particle dynamics are presented, with the emphasis being placed on the parameters governing the problem. A Hamiltonian approach is consistently used in the analysis of the particle equations of motion which are discussed in detail and solved analytically in various cases. Two applications of the theory are considered. First, we calculate the growth (or damping) rate for a whistler-mode wave propagating obliquely to geomagnetic field in the magnetosphere. Secondly, we estimate the proton precipitation into the upper atmosphere induced by a VLF transmitter signal.

Narrow Spikes Enhanced by Electron Precipitation under the Inner Radiation Belt

The observation of energetic particles trapped in the magnetosphere has made clear that narrow spikes of these particles as observed at L∼1.6 are causally generated by the precipitation of particles trapped in the inner radiation belt. Furthermore, the occurrence frequency of these spikes is well correlated with the local time as coincident with the radio power transmitted from a VLF station located in west Australia, NWC. The results suggest that narrow spikes observed at L∼1.6 are mainly caused by the radio power from VLF transmitter, NWC located in Australia.

Similar behaviors of natural ELF/VLF ionospheric emissions and transmitter signals over seismic Adriatic regions

Abstract. We report on the analysis of ELF/VLF emissions observed by the Instrument Champ Electrique (ICE) experiment onboard the DEMETER micro-satellite. We consider principally selected seismic events reported by Molchanov et al. (2006). These authors studied the VLF signals radiated by ground transmitters and received on board the DEMETER micro-satellite. They revealed a drop of the signals (scattering spot) connected with the occurrence of large earthquakes. In our investigations, we proceed to a spectral analysis of ICE observations with the aim to find if the natural ionospheric VLF/ELF emissions show, or not, a similar ''drop'' in the intensity as it is the case of the VLF transmitter signal. We combine our results with those of Molchanov et al. (2006), and we discuss the origin of such interesting ionospheric features in the frame of the investigation of the pre-seismic electromagnetic emissions. We show that the geomagnetic activity is a key parameter which could disturb the natural VLF ionospheric emissions, and also the transmitter signal. We find that it is not possible to conclude the presence, or not, of a preseismic effect when the Kp-index is higher than one.

Early VLF perturbations caused by lightning EMP‐driven dissociative attachment

We propose a new mechanism for lightning‐induced perturbations to VLF transmitter signals known as Early VLF events. This mechanism involves electron density changes due to electromagnetic pulses (EMP) from successive in‐cloud lightning discharges associated with cloud‐to‐ground discharges (CGs), which are likely the source of continuing current and much of the charge moment change in CGs. Through time‐domain modeling of the EMP we show that a sequence of pulses can produce appreciable density changes in the lower ionosphere, and that these changes are primarily electron losses through dissociative attachment to molecular oxygen. Modeling of the propagating VLF transmitter signal through the disturbed region shows that perturbed regions created by successive horizontal EMPs create measurable amplitude changes.

Statistical correlation of spectral broadening in VLF transmitter signal and low-frequency ionospheric turbulence from observation on DEMETER satellite

Abstract. In our earlier papers we have found the effect of VLF transmitter signal depression over epicenters of the large earthquakes from observation on the French DEMETER satellite that can be considered as new method of global diagnostics of seismic influence on the ionosphere. At present paper we investigate a possibility VLF signal-ionospheric turbulence interaction using additional characteristic of VLF signal-spectrum broadening. This characteristic is important for estimation of the interaction type: linear or nonlinear scattering. Our main results are the following: – There are two zones of increased spectrum broadening, which are centered near magnetic latitudes Φ=±10° and Φ=±40°. Basing on the previous case study research and ground ionosonde registrations, probably it is evidence of nonlinear (active) scattering of VLF signal on the ionospheric turbulence. However occurrence rate of spectrum broadening in the middle-latitude area is higher than in the near-equatorial zone (~15–20% in comparison with ~100% in former area) that is probably coincides with the rate of ionospheric turbulence. – From two years statistics of observation in the selected 3 low-latitude regions and 1 middle-latitude region inside reception area of VLF signal from NWC transmitter we find a correlation of spectrum broadening neither with ion-cyclotron noise (f=150–500 Hz), which possibly means poor representation of the turbulence by the noise due to its mixture with natural ELF emission (which correlates with whistler), nor with magnetic storm activity. – We find rather evident correlation of ion-cyclotron frequency noise, VLF signal depression and weak correlation of spectrum broadening with seismicity in the middle-latitude region over Japan. But in the low-latitude regions we do not find such a correlation. Statistical decrease of VLF signal supports our previous case study results. However rather weak spectrum broadening-seismicity statistical correlation means probably that passive scattering prevails upon nonlinear (active) one.

Effects of plasma density irregularities on the pitch angle scattering of radiation belt electrons by signals from ground based VLF transmitters

Recent DEMETER spacecraft observations show that VLF signals from the NPM transmitter in Hawaii often strongly excite quasi‐electrostatic whistler mode waves as the NPM signals propagate upward through plasma density irregularities. As a result of the NPM wave energy loss to the quasi‐electrostatic waves, the transmitter signals will arrive at the radiation belts with less intensity than predicted by present models of VLF wave propagation and will produce less pitch angle scattering of energetic electrons than presently believed. This type of wave energy loss may be partially responsible for the pervasive wave intensity deficit for VLF transmitter signals in the plasmasphere recently noted by Starks et al. (2008).

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.

VLF transmitter signals as a possible tool for detection of seismic effects on the ionosphere

An overall consistent scheme is presented of using VLF transmitter signal spectral broadening observed on a satellite as a detection means of seismic activity. This includes the mechanisms for formation of small-scale plasma density irregularities, and generation of quasi-electrostatic lower hybrid resonance waves due to the scattering of transmitter signal from small-scale plasma irregularities. Both points are discussed in detail on quantitative level.

Energetic electron precipitation induced by space based VLF transmitters

[1] Numerical raytracing with Landau damping is used to calculate electron precipitation that would be induced by in situ sources located at L = 2 and L = 2.5, at the geomagnetic equator and also at a geomagnetic latitude of 20° along each field line. In accordance with the Wang and Bell (1970) model of antenna radiation in a magnetoplasma, we consider frequencies immediately below, approximately equal to, and 1 kHz above the local lower hybrid resonance frequency at each location and injected rays within 3° of the resonance cone. The magnetospherically reflecting (MR) whistler-mode waves that are injected from such in situ sources propagate with a wave normal angle very close to the resonance cone. Compared to a single-pass interaction, such MR waves precipitate up to 16 times more 100 keV to 5 MeV electrons. Waves injected at initial wave normal angles closer to the magnetic field, e.g., 45°, in fact precipitate fewer >1 MeV electrons than waves injected close to the resonance cone.

Very low-frequency radio waves drain Earth’s inner radiation belt of satellite-killing electrons

A high-altitude nuclear explosion would swell the radiation belt and imperil the global positioning system and other satellites. VLF transmissions could forestall the damage.

Precipitation signatures of ground‐based VLF transmitters

Numerical raytracing with Landau damping is used to calculate >100 keV electron precipitation signatures induced by hypothetical VLF transmitters distributed broadly in geomagnetic latitude and operating at a wide range of frequencies. A one‐half second pulse from each source is simulated and attenuation to the base of the magnetosphere for geomagnetic latitudes from 10° to 60° is calculated. Source location affects induced precipitation more strongly than operating frequency or radiated power. Sources located at 35° to 45° induce the most >100 keV precipitation for the 10 to 40 kHz waves typical of ground‐based VLF sources, while locations below λ ≃ 15° or above λ ≃ 55° are least effective at precipitating energetic electrons. In all cases, induced precipitation increases as the operating frequency decreases, with 10 kHz waves from a source at λ ≃ 35° the most effective at precipitating >100 keV electrons. Precipitation signatures produced by five existing ground‐based VLF transmitters are also simulated: the NAA, NLK, NAU, NPM, and NWC VLF transmitters. NWC induces the strongest >100 keV electron precipitation signature, followed by NPM, NAU, NAA, and NLK.

Radiation belt electron precipitation due to VLF transmitters: Satellite observations

In the Earth's inner magnetosphere, the distribution of energetic electrons is controlled by pitch‐angle scattering by waves. A category of Whistler waves originates from powerful ground‐based VLF transmitter signals in the frequency range 10–25 kHz. These transmissions are observed in space as waves of very narrow bandwidth. Here we examine the significance of the VLF transmitter NWC on the inner radiation belt using DEMETER satellite global observations at low altitudes. We find that enhancements in the ∼100–600 keV drift‐loss cone electron fluxes at L values between 1.4 and 1.7 are linked to NWC operation and to ionospheric absorption. Waves and particles interact in the vicinity of the magnetic equatorial plane. Using Demeter passes across the drifting cloud of electrons caused by the transmitter; we find that ∼300 times more 200 keV electrons are driven into the drift‐loss cone during NWC transmission periods than during non‐transmission periods. The correlation between the flux of resonant electrons and the Dst index shows that the electron source intensity is controlled by magnetic storm activity.

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).

The wave-like structures of ionospheric perturbation associated with Sumatra earthquake of 26 December 2004, as revealed from VLF observation in Japan of NWC signals

The observations of subionospheric VLF waves from the Australian VLF transmitter NWC (frequency=19.8 kHz) at the Japanese receiving stations Chofu, Chiba and Kochi have been utilized to identify a possible precursor of ionospheric perturbations to the huge Sumatra earthquake of 26 December 2004. The VLF amplitude data at Japanese stations have indicated the depression in amplitude and also the enhancement in nighttime amplitude fluctuation before the earthquake. The nighttime fluctuation is composed of wave-like structures, and the wavelet analysis and cross-correlation analyses have been performed for those fluctuations. A significant enhancement in the fluctuation spectra in the period 20–30 min to ∼100 min (the frequency range of atmospheric gravity waves) is observed only before the earthquake. Then, the wave-like structures tend to propagate from the NWC–Kochi path to NWC–Chiba path with the time delay of ∼2 h, and so the wave propagation speed is estimated as ∼20 m/s. This finding might be important when we think of lithosphere–ionosphere coupling mechanism.

Solar flare induced D region perturbation in the ionosphere, as revealed from a short‐distance VLF propagation path

Short‐distance (propagation distance less than 1,000 km) VLF subionospheric propagation is, for the first time, used to study the solar flare induced D‐region perturbations of the lower ionosphere. We have used two years data for a propagation path from the Japanese VLF transmitter (call sign JJI, frequency = 22.2 kHz, and located at Ebino, Kyushu) to a receiving station of Chofu, Tokyo. For the sake of comparison, we have also used the long distance VLF path (NWC‐Chofu). First of all, the value of perturbation in amplitude (ΔA) for the short‐distance path seems to be much more enhanced (ΔA = 8∼9 dB) for this short‐path than the long‐distance paths previously studied. The experimental value of ΔA is found to monotonically increase with the logarithm of X‐ray flux. This dependence is numerically simulated by means of the FDTD (finite difference time domain) method to deduce the D‐region perturbation, {1 + C exp {−(z − z0)2/2β2} (C: enhancement factor, z0: reference height and β: vertical extent) (this modification seems to be much more realistic than the previous two parameter modeling in an exponential profile). The numerical computations with z0 ∼60 km and β ∼ 10 km have shown that the value of ΔA increase up to ∼6 dB in a range of X‐ray flux from C = 10 to C = 103. As the conclusion, the ΔA is closely related with the logarithm of solar X‐ray flux, so that we will be able to deduce the C value from the observed ΔA value.