VLF Transmitter Research Articles

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.

Interplanetary radio transmission through serial ionospheric and material barriers

A usual first principle in planning radio astronomy observations from the earth is that monitoring must be carried out well above the ionospheric plasma cutoff frequency (∼5MHz). Before space probes existed, radio astronomy was almost entirely done above 6MHz, and this value is considered a practical lower limit by most radio astronomers. Furthermore, daytime ionization (especially D-layer formation) places additional constraints on wave propagation, and waves of frequency below 10–20MHz suffer significant attenuation. More careful calculations of wave propagation through the Earth's ionosphere suggest that for certain conditions (primarily the presence of a magnetic field) there may be a transmission window well below this assumed limit. Indeed, for receiving extraterrestrial radiation below the ionospheric plasma cutoff frequency, a choice of VLF frequency appears optimal to minimize loss. The calculation, experimental validation, and conclusions are presented here. This work demonstrates the possibility of VLF transmission through the ionosphere and various subsequent material barriers. Implications include development of a new robust communications channel, communications with submerged or subterranean receivers/instruments on or offworld, and a new approach to SETI.

Study of the North West Cape electron belts observed by DEMETER satellite

We analyzed observation data collected by the Instrument for the Detection of Particles (IDP) on board the DEMETER satellite during a period of 17 months in 2007 and 2008. In the meantime, the VLF transmitter located at North West Cape (NWC) ground station was shut down during 7 months and working for a total of 10 months. By an (on‐off) method, our analysis for the first time revealed in detail the transient properties of the space electron precipitation belt which is induced by the man‐made VLF wave emitted from NWC. We mapped the electron flux distribution and figured out the space regions that the NWC belt covered. The NWC electron spectrograms have been investigated in a wide range of the McIlwain parameter L (1.1–3.0). Furthermore, we obtained the averaged energy spectra of the NWC electrons within the drift loss cone and compared their characteristics during daytime and nighttime. Our results confirm the previous studies of the enhancement of NWC electrons, the wisp structure, and the day/night difference of the electron flux. In addition, more detailed information is provided. We provide not only evidence of a momentary flux enhancement up to 3 orders of magnitude but also a flux reduction at higher L shells with a maximum up to 60% of the original value. For the first time, the energy spectra of NWC electrons covering the entire IDP energy band are presented for both nighttime and daytime and are quantitatively compared. At the end, our results are discussed, and their agreement with the theory of wave‐particle interactions is checked.

An improved model of the lightning electromagnetic field interaction with the D‐region ionosphere

We present an improved time‐domain model of the lightning electromagnetic pulse (EMP) interaction with the lower ionosphere. This improved model inherently accounts for the Earth's curvature, includes an arbitrary number of ion species, and uses a convolutional Perfectly Matched Layer (PML) boundary. We apply an improved model of electron heating due to the lightning EMP and electrostatic fields, and we include ionization, attachment, and detachment. In addition to modeling lightning, this model can be used for long‐distance VLF wave propagation in the Earth‐ionosphere waveguide, heating of the lower ionosphere by VLF transmitters, and heating in the F‐region ionosphere by lightning. In this paper we present three initial results of this model. First, we compare results of ionospheric heating and electron density disturbances with and without electron detachment taken into account. We find that detachment is important only for the QE effects on time scales longer than 1 ms. Second, we find a simple explanation for the recently‐reported “elve doublets”, which we find are an effect of the rise and fall times of the lightning waveform. In particular, we find that all elves are doublets, and the rise and fall times of the current pulse control the brightness and separation in time of the two successive halves of the elve. Third, we find a similar simple explanation for “ring” sprites, whole columns appear in a circle symmetric around the discharge axis. We find that ring sprites can be initiated for particular current waveforms, where the QE and EMP fields in the mesosphere produce a maximum reduced field away from the discharge axis.

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.

Magnetospherically reflected, specularly reflected, and backscattered whistler mode radio-sounder echoes observed on the IMAGE satellite: 1. Observations and interpretation

[1] A survey of echoes detected in 2004–2005 during pulse transmissions from the Radio Plasma Imager (RPI) instrument on the IMAGE satellite has revealed several new features of sounder generated whistler mode (WM) echoes and has indicated ways in which the echoes may be used for remote sensing of the Earth's plasma structure at altitudes <5000 km. In this paper we describe the frequency versus travel time (f − t) forms of the WM echoes as they appear on RPI plasmagrams and discuss qualitatively their raypaths and diagnostic potentials. Based on their reflection mechanism, the WM echoes can be classified as: magnetospherically reflected (MR), specularly reflected (SR), or backscattered (BS). The MR echoes are reflected at altitudes where the local lower hybrid frequency (flh) is equal to the transmitted pulse frequency f, a phenomenon familiar from both theory and passive recordings of WM wave activity. The SR echoes (previously reported in a higher frequency range) are reflected at the Earth-ionosphere boundary, either with wave vector at normal incidence or, more commonly (and unexpectedly, due to ray bending in the layered ionosphere), at oblique incidence. The BS echoes are the result of scattering from small scale size plasma density irregularities close to IMAGE. The echoes are described as discrete, multipath, and diffuse, depending upon the amount of travel-time spreading caused by the presence of field aligned density irregularities (FAIs) along echo raypaths. The WM echoes described in this paper have been observed at altitudes less than 5,000 km and at all latitudes and at most MLTs. The diagnostic potential of these phenomena for remotely studying the distribution of plasma density and composition along the geomagnetic field line B0, as well as the presence of FAIs of varying scale sizes, is enhanced by the tendency for SR and MR echoes to be observed simultaneously along with the upward propagating signals from a spatial distribution of communication VLF transmitters. We believe that our findings about WM propagation and echoing in an irregular medium have important implications for the connection between WM waves and the Earth's radiation belts. In a companion paper by Sonwalkar et al. (2011), we employ ray tracing and refractive index diagrams in quantitative support of this paper and also present two diagnostic case studies of plasma density, ion effective mass, and ion composition along B0.

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.

The DEMETER mission, recent investigations on ionospheric effects associated with man-made activities and seismic phenomena

Two of the main scientific objectives of the DEMETER mission concerned the search for ionospheric effects due to earthquakes and the study of disturbances associated with human activities or natural phenomena such as thunderstorms. The large data base gathered during nearly 6 years of in-orbit operation has lead to making new observations and/or to improving the understanding of several, already identified, phenomena. The main effects of human activities are observed in the field of radio-electric emissions and we present several results on the EM waves generated by power networks or associated with high power VLF transmitters. The search for ionospheric disturbances associated with earthquakes relies on thorough statistical studies to disentangle seismic effects from the variations induced by the physical processes that control the ionospheric plasma and natural emissions. Several studies are presented, and one of them has demonstrated a decrease of the ELF wave intensity in the frequency range between 1 and 2 kHz a few hours before the shock.

Remediation of radiation belt electrons caused by ground based man-made VLF wave

The physics mechanisms of radiation belt electrons loss and acceleration are important issues in space physics research. Recently, France Microsatellite DEMETER has discovered the correlation between man-made VLF signals and radiation belt electrons precipitation in the NPM(the U.S. VLF transmitter located at Lualualei) experiment. Our research focuses on the explanation of the relation among affected pitch angle distribution, kinetic energy and position of electrons. This is achieved by calculating the local diffusion coeffcient based on the theory of qusi-linear diffusion with resonant interaction. Our result has a good explanation of radiation belt electron precipitation discovered by DEMETER during NPM experiment. Furthermore, we have discussed the effciency of radiation belt remediation in an artificial way.

Multi‐instrumental observations of a positive gigantic jet produced by a winter thunderstorm in Europe

At 2336:56 UTC on 12 December 2009, a bright gigantic jet (GJ) was recorded by an observer in Italy. Forty‐nine additional sprites, elves, halos and two cases of upward lightning were observed that night. The location of the GJ corresponded to a distinct cloud top (−34°C) west of Ajaccio, Corsica. The GJ reached approximately 91 km altitude, with a “trailing jet” reaching 49–59 km, matching with earlier reported GJs. The duration was short at 120–160 ms. This is the first documented GJ which emerged from a maritime winter thunderstorm only 6.5 km tall, showing high cloud tops are not required for initiation of GJs. In the presence of strong vertical wind shear, the meteorological situation was different from typical outbreaks of fall and winter thunderstorms in the Mediterranean. During the trailing jet phase of the GJ, a sprite with halo triggered by a nearby cloud‐to‐ground lightning flash occurred at a relatively low altitude (&lt;72 km). At the same time, the trailing jet and beads were reilluminated. Electromagnetic waveforms from Hungary, Poland, and the USA revealed this GJ is the first reported to transfer negative charge (approximately 136 C) from the ionosphere to the positively charged origins in the cloud (i.e., a positive cloud‐to‐ionosphere discharge, +CI), with a large total charge moment change of 11600 C km and a maximum current of 3.3 kA. Early VLF transmitter amplitude perturbations detected concurrently with the GJ confirm the production of large conductivity changes due to electron density enhancements in the D‐region of the ionosphere.

Response of the equatorial lower ionosphere to the total solar eclipse of 22 July 2009 during sunrise transition period studied using VLF signal

Changes in equatorial D‐region electron density are studied using subionospherically propagating VLF signal at 18.2 kHz over a distance of 2200 km during the total solar eclipse of 22 July 2009. There are very few studies about the eclipse's effects on the equatorial lower ionosphere in the scientific literature. In the light of that, the objective of the present work is to study the effects of the eclipse on the dynamics of the equatorial lower ionosphere during ionospheric sunrise transition period. In the present case, great circle path between VLF transmitter and receiver falls totally in partial eclipse zone, having a maximum solar obscuration of 90% and an average obscuration of 74%. Results show an average decrease of 3.2 dB in signal strength compared to control days during peak solar obscuration over the path. A comparison with previous studies shows an increase both in lower ionosphere virtual reflection height (H′) and Wait inverse scale height parameter (β); the values estimated are 74.5 km and 0.46 km−1 compared to unperturbed ionosphere values of 71 km and 0.43 km−1, respectively. During maximum eclipse over the path, the model profile shows an average 80% drop in electron density at a height of 71 km at equatorial lower ionosphere. A nonlinear variation of lower ionosphere electron density with solar radiation is found as opposed to the model study proposed by previous workers.

Quasi‐electrostatic whistler mode wave excitation by linear scattering of EM whistler mode waves from magnetic field‐aligned density irregularities

Recent observations by Starks et al. (2008) from multiple spacecraft suggest that the actual nighttime intensity of VLF transmitter signals in the radiation belts is approximately 20 dB below the level that is assumed in the model developed by Abel and Thorne (1998) and approximately 10 dB below model values during the day. In the present work, we discuss one experimentally established mechanism which might be responsible for some of this intensity discrepancy, linear mode coupling as electromagnetic whistler mode waves propagate through regions containing small‐scale (2–100 m) magnetic field‐aligned plasma density irregularities. The scattering process excites quasi‐electrostatic whistler mode waves, which represents a power loss for the input waves. Although the distribution and amplitude of the irregularities is not well known at present, we construct plausible models in order to use numerical simulations to determine the characteristics of the mode coupling mechanism and the conditions under which the input VLF waves can lose significant power to the excited quasi‐electrostatic whistler mode waves. For short propagation paths of approximately 15 km, the full‐wave model predicts power losses ranging from −3 dB (25% probability) to −7 dB (2% probability). For longer propagation paths of approximately 150 km, the full‐wave model predicts power losses ranging from −4 dB (25% probability) to over −10 dB (2% probability). We conclude that for the irregularity models investigated, the mode coupling mechanism can result in significant power loss for VLF electromagnetic whistler mode waves.

VLF electromagnetic field structures in ionosphere disturbed by Sura RF heating facility

Observation of spatial VLF field structures in an artificially disturbed ionosphere is reported. The disturbed area with horizontal sizes ∼50 km in a quiet middle‐latitude ionosphere was produced by the powerful RF Sura heating facility (56°1′N, 46°1′E). Measurements were carried out onboard the DEMETER satellite while passing the disturbed area at height ∼700 km. Spectra broadening (Δf &lt; ±1 kHz) and considerable (up to 30 dB) increase of signal intensity of VLF transmitters' signals were observed. The VLF field and electron density irregularities have similar spatial structure. The characteristics of the VLF field in disturbed by RF heating area are analyzed.

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

Optical signatures of radiation belt electron precipitation induced by ground‐based VLF transmitters

Numerical simulations are presented of optical emissions in the ionosphere due to electron precipitation caused by ground‐based VLF transmitters. Ray tracing and precipitation calculations are made to estimate the flux precipitated from the inner magnetosphere for existing ground‐based VLF transmitters as well as hypothetical transmitters with controlled parameters. The resulting precipitated fluxes are used to estimate ionization profiles through Monte Carlo simulations, and the ionization profiles are converted to photon volume emission rates. The results are extended over a range of L shells and longitudes. Results show that the NWC transmitter at North West Cape, Australia, creates the strongest optical signature because of its high power, latitude of ∼35°, and low frequency. The resulting optical signal has a peak of &lt;0.1 R; however, we calculate that this should be detectable using sensitive photometric instruments and controlled modulation experiments.

Decrease of VLF transmitter signal and Chorus-whistler waves before l'Aquila earthquake occurrence

Abstract. We investigate the VLF emissions observed by the Instrument Champ Electrique (ICE) experiment onboard the DEMETER micro-satellite. We analyze intensity level variation 10 days before and after the occurrence of l'Aquila earthquake (EQ). We found a clear decrease of the VLF received signal related to ionospheric whistler mode (mainly Chorus emission) and to signal transmitted by the DFY VLF station in Germany, few days (more than one week) before the earthquake. The VLF power spectral density decreases of more than two orders of magnitude until the EQ, and it recovers to normal levels just after the EQ occurrence. The geomagnetic activity is principally weak four days before EQ and increases again one day before l'Aquila seismic event. Our results are discussed in the frame of short- and long-terms earthquakes prediction focusing on the crucial role of the magnetic field of the Earth.

Two‐dimensional frequency domain modeling of lightning EMP‐induced perturbations to VLF transmitter signals

The lightning electromagnetic pulse creates observable modifications to the overlying D region ionosphere, exhibited optically as elves. Recent work, both experimental and theoretical, has shown that elves are accompanied by considerable electron density disturbances of up to a factor of 2 increase in local density. We investigate the possibility that these density disturbances are observed by subionospheric VLF transmitter signals as the so‐called early VLF events. We use a finite difference frequency domain model to simulate the VLF transmitter signal propagating subionospherically to a VLF receiver, under ambient conditions and through a disturbed region. We show that modeled electron density disturbances, consistent with optical observations of elves, yield small but detectable perturbations to the VLF transmitter signal and may explain at least some observed early VLF events. We further show that sequences of in‐cloud lightning pulses, as in spider lightning, may yield considerably higher density disturbances and similarly more prominent VLF transmitter perturbations. In this way, the model herein supports previously reported correlations between sferic bursts and early VLF events.

Investigation of VLF and HF waves showing seismo-ionospheric anomalies induced by the 29 September 2009 Samoa earthquake (&amp;lt;i&amp;gt;M&amp;lt;/i&amp;gt;&amp;lt;sub&amp;gt;w&amp;lt;/sub&amp;gt;=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.

Sudden amplitude and phase changes in subionospheric VLF transmitter signals observed at Agra, India

The amplitude and phase of VLF transmitter signals NWC (19.8 kHz), NPM (21.4 kHz), and NAA (24 kHz) have been monitored at Agra (geomag. lat. 17.10°N, L=1.15), India using AbsPAL receiver for a period of three years between 01 September, 2002 and 30 August, 2005. Seven cases of abrupt amplitude and phase changes have been identified which varied between 3 and 7 dB and 40° and 80° respectively. The onset duration varied around 5 sec. The observed characteristics have been examined in the light of lightning induced electron precipitation (LEP), solar flares, and lightning, and finally attributed to early/slow perturbation caused by distant sprites and lightning along the propagation paths.

VLF amplitude anomaly during the total solar eclipse of 22 July, 2009

During the total solar eclipse in India on July 22, 2009, we measured the amplitude of the fixed frequency VLF transmitter signals (f =19.8 kHz, NWC, Australia) at Agra (Geographic Lat. 27.2°N, Long. 78°NE), using a SoftPAL (Software based phase and amplitude logger) receiver. It was the longest total solar eclipse in the 21st century seen in India ever since 18 August, 1968. We analysed the VLF data for a period of fifteen days (±7 days from the date of the event) and found that the amplitude of the signal decreased on 22 July in the time sector of 2 hrs between 0530 and 0730 hours LT with the maximum depletion during the period of total solar eclipse. The result is interpreted in terms of depletion of electron density in the D region of the ionosphere caused by the solar eclipse.