Ground-based VLF Transmitters Research Articles

The influence of the ionospheric disturbance on the ground based VLF transmitter signal recorded by LEO satellite – Insight from full wave simulation

The earthquakes could induce ionospheric electron density disturbance at D/E, F layer and even topside of ionosphere. Subsequently the ionospheric disturbance would change the electromagnetic field from the ground based VLF transmitter. Nowadays, the electromagnetic field recorded by Low earth orbit (LEO) satellite have been used to study the possible seismic anomalies frequently. It is essential to study systematically how the ionospheric disturbance influences the electromagnetic field at the altitude of LEO satellite theoretically. In this paper, we construct a full wave model to simulate the influence of the amplitude, altitude, and scale of the ionospheric disturbance on the electromagnetic field in the ionosphere from the ground based VLF transmitter. The result shows the electromagnetic field at the altitude of satellite changes obviously when the wave encounters an ionospheric disturbance in the D/E layer. The variation of the electromagnetic field increases with the amplitude and scale of the ionospheric disturbance. However, the influence of the ionospheric disturbance in the F layer just has a local effect on the electromagnetic field. The influence of ionospheric disturbance in the nighttime on the variation of electromagnetic field is smaller compared with it in the daytime. These conclusions can improve our understanding of using electromagnetic field observed by satellite to study seismic anomalies.

DEMETER observations of manmade waves that propagate in the ionosphere

Abstract This paper is a review of manmade waves observed by the ionospheric satellite DEMETER. It concerns waves emitted by the ground-based VLF and ELF transmitters, by broadcasting stations, by the power line harmonic radiation, by industrial noise, and by active experiments. Examples are shown including, for the first time, the record of a wave coming from an ELF transmitter. These waves propagate upwards in the magnetosphere and they can be observed in the magnetically conjugated region of emission. Depending on their frequencies, they perturb the ionosphere and the particles in the radiation belts, and additional emissions are triggered.

Terrestrial VLF transmitter injection into the magnetosphere

Very Low Frequency (VLF, 3–30 kHz) radio waves emitted from ground sources (transmitters and lightning) strongly impact the radiation belts, driving electron precipitation via whistler‐electron gyroresonance, and contributing to the formation of the slot region. However, calculations of the global impacts of VLF waves are based on models of trans‐ionospheric propagation to calculate the VLF energy reaching the magnetosphere. Limited comparisons of these models to individual satellite passes have found that the models may significantly (by >20 dB) overestimate amplitudes of ground based VLF transmitters in the magnetosphere. To form a much more complete empirical picture of VLF transmitter energy reaching the magnetosphere, we present observations of the radiation pattern from a number of ground‐based VLF transmitters by averaging six years of data from the DEMETER satellite. We divide the slice at ∼700 km altitude above a transmitter into pixels and calculate the average field for all satellite passes through each pixel. There are enough data to see 25 km features in the radiation pattern, including the modal interference of the subionospheric signal mapped upwards. Using these data, we deduce the first empirical measure of the radiated power into the magnetosphere from these transmitters, for both daytime and nighttime, and at both the overhead and geomagnetically conjugate region. We find no detectable variation of signal intensity with geomagnetic conditions at low and mid latitudes (L < 2.6). We also present evidence of ionospheric heating by one VLF transmitter which modifies the trans‐ionospheric absorption of signals from other transmitters passing through the heated region.

Transmitter-induced modulation of subionospheric VLF signals: Ionospheric heating rather than electron precipitation

[1] The controlled keying of ground-based VLF transmitters with periodic on/off sequences allows the detection of weak but measurable cross-modulation effects on other subionospheric VLF probe signals used for VLF remote sensing. In this paper, we reexamine previously published and additional cases of such events and determine that the initial interpretations of such cross modulation as being due to electron precipitation is likely incorrect. Rather, such events appear to be fully consistent with ionospheric heating caused by the keyed signal, even when the probe VLF signal path lies thousands of kilometers from the heating VLF transmitter. The 21.4 kHz transmitter NPM located in Lualualei, Hawaii, is keyed on/off in periodic sequences, and that same periodicity is observed on the subionospherically propagating probe signal generated by the 24.8 kHz transmitter NLK of Jim Creek, Washington. Previous initial conclusions published for these experiments do not hold under detailed review due to the lack of discernible onset delay and lag time in the observed perturbations, which eliminates transmitter-induced precipitation of electron radiation as a possible cause. Detailed testing of the receiver shows instrumental cross-modulation to not be a concern in these observations. It is thus concluded that the observed perturbations, despite occurring on a probe signal pathway that is 1750 km away from NPM at its point of closest approach, are due to direct ionospheric heating by the keyed VLF transmitter NPM. Results indicate that the VLF transmitter may affect the overlying ionosphere over much larger lateral regions than previously believed.

Correction to “Optical signatures of radiation belt electron precipitation induced by ground‐based VLF transmitters”

In the paper “Optical signatures of radiation belt electron precipitation induced by ground-based VLF transmitters” by R. A. Marshall et al. (Journal of Geophysical Research, 115, A08206, doi:10.1029/2010JA015394, 2010), calculations in section 4, “Implications for Experimental Detection,” were not correctly updated from an earlier draft during the production process. The correct calculations and statements are given here. In section 4.1, the computations of the SNR contained numerous errors. Paragraph 32 should be replaced with the following. “The best narrowband filters on the market have bandwidths of ∼12 A and a peak transmission of ∼0.5. Integrating in our two emissions, this yields an airglow signal of 12 R in the 4278 A channel and (250 + 12× 5) = 310 R in the 5577 A channel. We can compute a system sensitivity as follows: 1 R (10 photons cm−2 s−1 (4π sr)−1) into an optical system with 10 cm diameter and 10◦ × 10◦ field of view (0.03 sr), through the filter with transmission of 0.5, yields 9.4 × 10 photons s−1 hitting the detector. As an example detector, the Hamamatsu R5900U series of photomultiplier tubes has a quantum efficiency (QE) of ∼0.17 at 5577 A, a dark current at the anode of ∼1 nA, and a gain

Investigation of VLF and HF waves showing seismo-ionospheric anomalies induced by the 29 September 2009 Samoa earthquake (<i>M</i><sub>w</sub>=8.1)

Abstract. In Samoa Islands, a powerful earthquake took place at 17:48:10.99 UTC (06:48:10.99 LT) on 29 September 2009 with a magnitude Mw=8.1. Using ICE (Instrument Champ Electrique) and IMSC (Instrument Magnetic Search Coil) experiments onboard the DEMETER (Detection of Electromagnetic Emissions Transmitted from Earthquake Regions) satellite we have surveyed possible variations in electromagnetic signals transmitted by the ground-based VLF transmitter NPM in Hawaii and in HF plasma waves close to the Samoa earthquake during the seismic activity. The indices Dst and Kp were used to distinguish pre-earthquake anomalies from the other anomalies related to the geomagnetic activities. In a previous study we have shown that anomalies in IAP (plasma analyzer) and ISL (Langmuir probe) experiments onboard the DEMETER and also TEC (Total Electron Content) data appear 1 to 5 days before the Samoa earthquake. In this paper we show that the anomalies in the VLF transmitter signal and in the HF range appear with the same time scale. The lack of significant geomagnetic activities indicates that these anomalous behaviors could be regarded as seismo-ionospheric precursors. It is also shown that comparative analysis is more effective in seismo-ionospheric studies.

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).

Formation of electron beams by the interaction of a whistler wave packet with radiation belt electrons

The purpose of this paper is to discuss the properties of electron beams formed by cyclotron interactions between radiation belt electrons and a quasi-monochromatic whistler wave packet from a ground-based VLF transmitter. The beams are formed due to trapping of the electrons at the forward edge of the wave packet, their acceleration inside the wave packet, the escape of the accelerated electrons from the moving backward edge of the wave packet, and their following free motion in an inhomogeneous magnetic field. A combination of these processes provides the main features of the spatial-temporal evolution of the beams which are investigated both analytically and numerically. It is shown that one or two beams can appear at one point at the same time, and that the density of the beams increases during their expansion. Motion of the pumping wave packet in the inhomogeneous magnetic field provides the variations of the initial velocity and position of the beam injection which change the spatial and temporal gradients of the parallel velocity of the beam, in contrast with the case of the pure adiabatic motion of an individual electron. Such a behaviour can be significant for the generation of secondary emissions. Numerical calculations demonstrate a wide variety of the spatio–temporal patterns of the beam parallel velocity depending on the plasma and wave packet parameters. It is shown that the most significant parameters which determine the beam characteristics are the wave packet length about the equator, its group velocity, and the initial energy and pitch angle of the electrons.

Simultaneous triggered VLF emissions and energetic electron distributions observed on POLAR with PWI and HYDRA

We report simultaneous observations of energetic 1–20 keV electrons and VLF emissions triggered within the plasmasphere by pulses from ground based VLF transmitters, using the PWI and HYDRA instruments on the POLAR spacecraft. The 1–20 keV electrons have the correct energy to interact with the input pulses through gyoresonance. Emissions are generated by the pulses only when the particle flux is enhanced well above background and the particle pitch angle distribution is very highly anisotropic, with the average equatorial pitch angle exceeding ∼75°. Because of these high pitch angles, the particles are trapped typically within 7° of the magnetic equator. Only pulses which propagate within whistler mode ducts are observed to trigger emissions. The observed pitch angle anisoptropies are much larger than those assumed in present models of the VLF emission triggering phenomenon, and thus our observations provide a new starting point for understanding the emission process.

Self-consistent theory of triggered VLF emissions: An analytical approach

The self-consistent analytical approach to the problem of triggered ELF/VLF emissions is developed. The self-consistency includes calculation of energetic electron beams produced by a quasimonochromatic whistler wave packet from a ground based VLF transmitter and analysis of secondary whistler wave generation by this beam, taking into account the second order cyclotron resonance effects. The theory permits estimation of the maximum amplification of the secondary waves as well as dynamical frequency spectrum of triggered emission. A simple computer program has been developed to find the dynamic spectrogram of triggered VLF signals with spatial and temporal dependences of electron beam velocity and geomagnetic field.

Formation of electron beams under the interaction of a whistler wave packet with the radiation belt electrons

The purpose of this paper is to discuss the properties of electron beams formed by cyclotron interaction between radiation belt electrons and a quasi-monochromatic whistler wave packet from a ground-based VLF transmitter. Those electron beams are thought to be the source of triggered ELF/VLF emissions as secondary whistler waves. The beams are formed due to trapping of electrons at the forward front of the wave packet, their acceleration inside the packet, escape of the accelerated electrons from the moving backward front of the packet, and their following free motion in an inhomogeneous magnetic field. Combination of these processes provides the main features of the spatial-temporal evolution of the beams which are investigated analytically and numerically. Movement of the pumping wave packet in the inhomogeneous magnetic field provides the variations of the initial velocity and position of the beam injection which change the spatial and temporal gradients of the parallel velocity of the beam opposite direction to a pure adiabatic motion. Such a behavior can be significant for the generation of secondary emissions.

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.

Effects of artificially injected electron beams on the characteristics of ground VLF transmitter signals in the ionosphere

Spectral broadening of signals from a ground-based VLF transmitter was observed onboard the Magion-3 sub-satellite during electron beam injection from the lntercosmos-25 (APEX) satellite. Broadening of the order of 300–500 Hz was apparently correlated with the 2 s-periodic sequence of the electron gun pulses which were modulated in amplitude with frequencies 30.5 < f m < 31,250 Hz. No effects were found for those electron gun pulses with f m = 62.5, 125 and 250 kHz. The observations were made in the middle latitude ionosphere at altitudes between 1175 and 1580 km; the distance between satellite and sub-satellite was about 250 km. The electron gun current and the energy of electrons were 100 mA and 10 keV, respectively, and the duration of the elementary pulse of current was 2 μs. The data are interpreted in terms of the scattering of whistler mode waves into quasi-electrostatic waves by periodic small-scale plasma inhomogeneities or ELF plasma turbulence created by the pulsemodulated electron beam in the ionosphere.

Interpretation of multiple peaks observed in the quiet-time energy spectra of inner belt electrons

Multiple peaks observed in the energy spectra of electrons at the lower edge of the inner radiation belt ( L ⩽ 1.3) in the South Atlantic Anomaly region during magnetically quiet periods are interpreted in terms of cyclotron resonance involving whistler-mode waves radiated from multiple ground-based VLF transmitters. Our results show that out of six peaks observed at L = 1.22 and L = 1.31, three peaks corresponding to higher energies are formed as a result of pitch angle diffusion into the loss cone driven by first-order cyclotron resonance interaction involving the waves radiated from transmitters NSS (21.4 kHz), OMEGA (13.6 kHz) and OMEGA (10.2 kHz). We suggest that the remaining peaks at lower energies in the spectra are formed as a result of cyclotron resonance involving third harmonics of the waves radiated from some of the above and other transmitters. Other possibilities suggested for the formation of multiple peaks are also discussed.

DE‐1 observations of lower hybrid waves excited by VLF whistler mode waves

Recent satellite data show high amplitude lower hybrid (LH) waves excited by electromagnetic (EM) whistler mode waves throughout magnetospheric regions where small scale magnetic‐field‐aligned plasma density irregularities exist. One important consequence in the auroral acceleration region is the heating of suprathermal ions by the excited LH waves. To evaluate such heating it is necessary to know the wavelength (λ) range of the waves, information not previously available since most past observations were made with long (l ≥ 75; l=antenna length) electric dipole antennas which have very poor response for λ &lt; l. New observations using the short 9 m electric dipole antenna on the DE‐1 spacecraft show that LH waves with λ ≤ 10 m are excited by EM input waves from ground‐based VLF transmitters. Sequential observations on the short (9 m) and long (200 m) dipole antennas show that the long antenna generally has no measurable response to LH waves with λ ≤ 26 m. Since most previous correlative observations of LH waves and ion conics in the low altitude auroral acceleration region have involved long electric antennas, it is suggested that the intensity of the LH waves in this region may have been seriously underestimated.

Storm-time precipitation of resonant electrons at the lower edge of the inner radiation belt

Pitch angle scattering and diffusion of energetic electrons at the lower edge of the inner radiation belt ( L = 1.2) in disturbed magnetic conditions are studied in terms of cyclotron resonance involving naturally occurring ELF/VLF waves and signals radiated from ground-based VLF transmitters. Our results show that the diffusion coefficients are very sensitive to wave magnetic fields. The average lifetimes of energetic electrons at resonant frequencies of 550 Hz and 3.2 kHz ( E R > 1 MeV) are found to be about 2 h and 4 days, respectively, which indicate significant precipitation caused by such waves, whereas the same at resonant frequencies of 9.6 and 16 kHz are found to be about 5 and 28 years indicating that the corresponding resonant electrons are stably trapped. However, the lifetimes of such electrons reduce to hours also if the wave magnetic fields are increased. This may explain L-dependent peaks occurring at higher L values between 1.4 and 1.8. Further, as the calculated energy flux agrees well with the observed energy flux in the low-latitude zone of electron precipitation the results confirm wave-particle interaction involving cyclotron resonance as a source of these electrons.

Wave-electron interactions induced by VLF transmissions

The five papers discussed all involve interactions between waves produced by ground-based VLF transmitters and energetic electrons in the magnetosphere. Most of these studies concern the electron-cyclotron interaction or that between whistler mode waves and counterstreaming electrons. The most important consequence of this is electron pitch angle diffusion into the loss cone, and so precipitation. New features studied are finite band input waves, skew wave normal interaction, precipitation flux-time profiles and slow growth instabilities.

Results from the SEEP active space plasma experiment: Effects on the ionosphere

An active satellite‐ground coordinated space plasma experiment was conducted from May to December, 1982, in which electrons were precipitated from the radiation belts into the ionosphere by the controlled injection of VLF signals from ground‐based transmitters. The results confirm the hypothesis that electrons can be precipitated from the radiation belts by ground‐based VLF transmitters, and they provide information relating to the effects of such precipitation on the ionosphere. The ionization produced in the atmosphere of the northern hemisphere at L = 2.3 by the modulated signals from ground‐based VLF transmitters was shown to be as great as one ion pair/cm3 s at 80 km altitude. The ionization at comparable positions produced by naturally occurring electron precipitation varies greatly, and can be as low as 0.1 ion pair/cm3 s, but is also sometimes larger than 100 ion pairs/cm3 s at times of lightning flashes.

The theory of VLF Doppler signatures and their relation to magnetospheric density structure

The published spectrograms of signals from ground-based VLF transmitters observed aboard satellites show a wide variation from sample to sample. In general, the observed signals are Doppler shifted by the motion of the satellite, but in some published samples there is no observed Doppler shift. A ray tracing study of differing VLF Doppler signatures observed by the Ogo 4, FR 1, and Isis 2 VLF experiments has shown that each signature can be reproduced by a distinct class of electron density gradient models. Large positive and negative Doppler shifts (∼100 Hz) are reproduced by a strong decreasing electron density gradient which interacts with the magnetic field curvature gradient between L ∼ 2 and L ∼ 3. Presence of a strong signal with no Doppler shift is shown to be a result of trapping of upgoing rays by steep density drop-offs. Merging of the spectral components in the Doppler shifted signals is interpreted in terms of scattering by small-scale field-aligned density irregularities. These results allow VLF Doppler observations to be used as a new diagnostic tool in determining magnetospheric density structure below L ∼ 3 and in estimating the effective coverage of the lower ionosphere by the ground-based VLF transmitter in the excitation hemisphere.

Ionospheric interaction of VLF radio waves

Ground-based VLF transmitters may modify the lower ionosphere. The largest heating effects are observed under conditions when the quasi-longitudinal approximation of ionospheric fields becomes inaccurate for the idealized homogeneous anisotropic ionosphere model, which occurs north of transmitters located at moderate northern latitudes. Temperature changes of the order of 10 per cent or by 20°K can be expected for the nighttime E-layer with 1000 kW of transmitted power. This temperature increase causes an increase of electron collision frequency and also a short term increase of electron density following the hypothesis of LeLevier. The resulting change of reflection coefficients is of the order of 1 per cent and the wave fields in the ionosphere are changed by several per cent for frequencies in the VLF range; incident ELF signals are affected to a lesser degree. A neglect of electron density changes leads to smaller changes of reflection coefficients and of wave fields. The estimated changes of ionospheric parameters are comparable to those observed in current ionospheric modification experiments at higher frequencies. The calculated electron temperature changes are much smaller than in models which use an isotropic ionosphere and free space fields for estimating the amplitude of the heating signal.