Propagation Of VLF Radio Waves Research Articles

Modal treatment in two dimensions theoretical foundations of VLF-radio wave propagation using the normalized airy functions

The goal is to take advantage of the Earth-ionosphere wave guide’s fundamentally two-dimensional wave propagation, utilizing the normalized Airy functions (NAFs) in a complex domain. It is demonstrated that the typical working formula of VLF radio-mode theory may be obtained simply from orthogonality reflections, devoid of the requirement of sophisticated argumentation in the open unit disk. The combination of the expressions is given by considering the symmetry-convex illustration of the NAFs.

VLF radio wave anomalies associated with the 2010 Ms 7.1 Yushu earthquake

The VLF radio signals recorded both from the ground based VLF radio wave monitoring network and the DEMETER satellite are investigated during the 2010Ms 7.1 Yushu earthquake. The ground-based observations show that the disturbance intensity of VLF wave’s amplitude relative to the background gets an enhancement over 22% at 11.9kHz, 27% at 12.6kHz and 62% at 14.9kHz VLF radio wave along the path from Novosibirsk - TH one day before the main shock, as compared to the maximum 20% observed during non-earthquake time. The space based observations indicate that there is a decrease of the signal to noise ratio (SNR) for the power spectral density data of 14.9kHz VLF radio signal at electric field four days before the main shock, with disturbance intensity exceeding the background by over 5% as compared to the maximum 3% observed during non-earthquake time. The geoelectric field observations in the epicenter region also show that a sharp enhancement from ∼340 to 430mV/km simultaneously appeared at two monitors 14days before main shock. The comparative analysis from the ground and space based observations during the earthquake and non-earthquake time provides us convincible evidence that there exits seismic anomalies from the VLF radio wave propagation before the 2010 Ms 7.1 Yushu earthquake. The possible mechanism for VLF radio signal propagation anomaly during 2010 Yushu earthquake maybe related to the change of the geoelectric field nearby the earthquake zone.

Propagation Effectiveness Prediction and Simulation of VLF Communication

The vlf radio wave propagation is usually treated as a waveguide problem. It should be noted that many of the parameters that describe the boundaries of the earth-ionosphere waveguide are not known with great accuracy over all regions and times. In order to know the effect of underwater platform receiving in corresponding region and time, this paper makes use of the waveguide mode theory to predict the propagation effectiveness of vlf radio, offers the service of propagation environment for the system to adjust the variable parameters or abstain from propagation attenuation, makes the system performance matching the channel character and advancing the effects of vlf communication.

3D modelling of VLF radio wave propagation in terrestrial waveguide allowing for localized large-scale ionosphere perturbation

The problem of radio wave propagation allowing for 3D localized lower ionosphere irregularity appears in accordance with the necessity of the theoretical interpretation of VLF remote sensing data. The various processes in the Earth's crust and in space (earthquakes, magnetic storms, sporadic E-layers, lightning induced electron precipitations, rocket launches, artificial ionosphere heating, nuclear explosions, etc.) may cause different power and size ionospheric disturbances. This paper presents a further development of the numerical–analytical method for 3D problem solving. We consider a vector problem of VLF vertical electric dipole field in a plane Earth-ionosphere waveguide with a localized anisotropic ionosphere irregularity. The possibility of lowering (elevating) of the local region of the upper waveguide wall is taken into account. The field components on the boundary surfaces obey the Leontovich impedance conditions. The problem is reduced to a system of 2D integral equations taking into account the depolarization of the field scattered by the irregularity. Using asymptotic ( kr⪢1) integration along the direction perpendicular to the propagation path, we transform this system to a system of 1D integral equations. The system is solved in the diagonal approximation, combining direct inversion of the Volterra integral operator and the subsequent iterations. The proposed method is useful for study of both small-scale and large-scale irregularities. We obtained estimates of the TE field components that originate entirely from field scattering by a 3D irregularity.

Three-dimensional subionospheric VLF field diffraction by a truncated highly conducting cylinder and its application to the Trimpi effect problem

[1] By means of a three-dimensional full-wave analytical-numerical technique we consider the diffraction of a VLF point source field by a localized perturbation in the lower ionosphere, which is taken as a truncated highly conducting cylinder. The surface impedance concept is additionally used to model the Earth-ionosphere waveguide. Our approach is based on the preliminary asymptotic integration of the rigorous two-dimensional integral equation in order to facilitate its further numerical handling, and an original computational algorithm is proposed to obtain a solution of the approximate equation. By reducing CPU time, the procedure developed enables us to study both small and comparatively large irregularities. The theoretical computations have yielded the following findings: (1) the transverse dimension of the reflecting area, which can explain the experimental result, may be reduced as compared with that suggested before, (2) scattering may be significant not only in the forward direction but also in the backward direction and out-of-the-way directions, and (3) the same ionospheric perturbation produces a noticeably smaller effect on subionospheric VLF radio wave propagation for daytime conditions than for nighttime ones.

ELF and VLF radio waves

This review covers developments in ELF and VLF radio-wave propagation research over the last 50 years of the Journal of Atmospheric and Solar-Terrestrial Physics. A review of such a large field, over such a long period, cannot be fully comprehensive and the authors have therefore covered important areas which have they themselves have found interesting. The survey begins with a review of work on natural and man made sources of ELF and VLF radiation. This is followed by sections on experimental and theoretical studies of unperturbed (ambient) ELF and VLF radio propagation. Schumann resonance research, which is currently undergoing a renaissance, is then reviewed. A review of research into transient perturbations of ELF and VLF propagation follows, extending from the early work on nuclear explosions up to the current work on sprites. The review concludes with a brief summary of the VLF navigation systems of the USSR and USA, (Alpha and Omega) whose development and life-span covered most of the last 50 years.

An asymptotic three‐dimensional technique to study radio wave propagation in the presence of a localized perturbation of environment

This paper presents an analytical‐numerical approach to model problems that are ubiquitous in the theory of radio wave propagation. The method is particularly efficient for considering three‐dimensional diffraction by a localized perturbation of the environment. Here we are concerned with VLF and LF (less than 100 kHz) radio wave propagation in the presence of an irregularity of finite lateral extent (ionospheric disturbance or Earth's surface inhomogeneity). The surface impedance concept is used to model the Earth‐ionosphere waveguide. Our approach is based on preliminary asymptotic integration of the rigorous two‐dimensional integral equation in order to facilitate its further numerical handling. An original computational algorithm is proposed to invert an approximate equation. By reducing CPU time, the developed procedure enables study of both small and comparatively large irregularities. In spite of accepted asymptotic compromises, most of the three‐dimensional effects that are intrinsic to similar problems are obtained during numerical simulations.

The propagation of mixed polarization VLF (f ≤5 kHz) radio waves in the Antarctic Earth‐ionosphere waveguide

During 1986, a series of special VLF transmissions at ∼3 kHz and ∼5 kHz were made from the crossed dipole antenna at Siple Station, Antarctica, which simulated transmissions from a single horizontal dipole at a number of different orientations. The subionospheric signals thus excited were recorded at four Antarctic stations: Faraday, Halley, South Pole and Arrival Heights (McMurdo Sound). The signals excited broadside to the dipole were seen to exhibit characteristics notably different from those of signals excited along the axis of the antenna, showing a minimum in received power (the depth of the minimum decreasing for the more highly attenuating paths) and increases in apparent arrival azimuth error and elevation angle. A simple computer model for mode propagation close to 5 kHz in the Antarctic Earth‐ionosphere waveguide showed that these variations were the consequence of two effects: the preferential excitation of quasi‐transverse magnetic (QTM) modes along the axis of the antenna and the lower attenuation of quasi‐transverse electric (QTE) modes (with respect to QTM modes) over the Antarctic ice sheet. These results have important implications for studies involving the subionospheric propagation of signals with frequencies at the lower end of the VLF band and over highly attenuating surfaces, showing that QTE modes play a significant and sometimes dominant role. The propagation characteristics derived from the model may be applied to studies of the effect of burst energetic electron precipitation on subionospheric VLF signals (the Trimpi effect). Furthermore, the mode structure of subionospheric VLF signals radiated from the Siple transmitter, or a similar facility in Antarctica, may be controlled through the appropriate choice of signal frequency and antenna arrangement.

Some new results in improving position accuracy with OMEGA

Abstract Some of the factors affecting the accuracy of positioning with the OMEGA radionavigation system are discussed. A simplified algorithm of computing the diurnal corrections for OMEGA is developed. It is based on the calculation of the mean velocity of propagation of VLF radiowaves as a function of the effective height of a spherical waveguide. The formulae to compute the latter are given. A composite signal method proposed by Pierce and its modification by Papousek and Reder are discussed; it is shown that the accuracy of the modified version is not better than that of the original. Both in Pierce's method and its modifications the waveguide is treated as a plane. Its sphericity is taken into account in this article, which results in better compensation of the phase variations of OMEGA signals. It is shown that fractionally rational transformations of the phase count may result in an additional error. To reduce that error, a high reliability of the ambiguity resolution is required at all used frequ...

The role of atmospheric planetary‐scale waves in the D region winter anomaly

Characteristics of the D region winter anomaly are examined, by using ionospheric data along a meridian chain of stations near Japan. The data were obtained by means of long‐distance VLF radio wave propagation and HF radio wave vertical sounding. Interesting features are the equatorward extension of the anomaly and the variability of its duration. These features are further discussed in comparison with radiance data measured by pressure modulator radiometer channel 3000 (maximum weight near 80‐km altitude in the mesosphere) on board Nimbus 6 during two winters, 1975/1976 and 1976/1977. Meteorological conditions during these winters were quite different, a major stratospheric sudden warming taking place during the latter winter but not during the former winter. The key result of this study is that the occurrence of the winter anomaly is basically associated with the amplification of a planetary‐scale wave with zonal wave number 1 in the mesosphere. At the same time we find that the local appearance of the anomaly near Japan is controlled by the movement and latitudinal structure of the excited wave 1. It is further indicated that the increase in mesospheric local temperature is not always associated with the occurrence of the anomaly, suggesting that another dynamical effect of planetary‐scale waves is important for the occurrence of the anomaly.

An application of invariant imbedding to VLF radio wave propagation

The problem of VLF wave propagation in the earth‐ionosphere waveguide is formulated in terms of a two‐point boundary value problem which is subsequently replaced by an initial value problem. As a result we easily derive the ionospheric reflection coefficient of an arbitrarily polarized plane wave reflected off an anisotropic medium.

The D-region winter anomaly and dynamical effects of atmospheric planetary-scale waves.

A study is presented of the D-region winter anomaly, observed by ground based measurements of long-distance VLF radio wave propagation and HF radio wave vertical sounding at several stations located in and near Japan over 3 winters from 1974 to 1977. These winters have different meteorological conditions (i. e., a mayor stratospheric sudden warming in the winter 1976/77, a minor warming in 1974/75 and no warmings in 1975/76). The activity of the anomaly is compared with that of planetary-scale waves using atmospheric data in the lower stratosphere and radiance data in the upper stratosphere and mesosphere obtained by the PMR (Pressure Modulator Radiometer) on board Nimbus 6 satellite. The latitudinal extent and duration of the anomaly are shown to be primarily controlled by the amplification of planetary-scale wave with zonal wavenumber 1. When a major warming took place as a result of an unusual amplification of wave 1, an intense event of the anomaly was observed for a long time. However, the low-latitude boundary was high (≥40°N). The situation was similar with the case of minor warming except that the low-latitude boundary was normal (-35°N). When the wave 1 was amplified but no warmings were observed, relatively weak events of the anomaly occurred. The duration was dependent on the wave 1 activity but generally short. The low-latitude boundary became low in latitude (-30°N) with the wave 1 activity. It is further demonstrated that a localized temperature increase excited by the vertical propagation of wave 1 into the mesosphere has a close relation to the enhancement of electron density in the D-region during winter anomalous days. This suggests that the decrease of effective electron recombination coefficient due to the temperature increase is a primary cause of the winter anomaly.

Effect of a heated patch of auroral ionosphere on VLF-radio wave propagation

In the early 1960s, during the period of atmospheric nuclear tests, much theoretical interest developed in the effects of localized ionospheric depressions on the propagation of very low frequency (VLF) radio waves1–4. Similar VLF-propagation effects are also produced by the localized dumping of electrons from the radiation belts after wave–particle interactions5,6. Both nuclear explosions and particle precipitation events are of a transient nature, however, and no experimental study has yet been made to confirm these early theoretical predictions. With the development of a unique high frequency (HF) heating facility near Tromso, Norway, the generation of movable controlled anomalies in the D-region has become possible. We describe here some initial observations, made in Norway, of the effect of such a movable D-region anomaly on the VLF signals received from the 12.1-kHz Omega transmitter at Aldra. The observations confirm the validity of earlier theoretical predictions.

The influence of localized precipitation‐induced D‐region ionization enhancements on subionospheric VLF propagation

A two‐dimensional mode propagation model has been used to investigate the effect of localized ionospheric perturbations on the propagation of VLF radio waves in the earth‐ionosphere waveguide. Computations have been performed for the NSS transmissions at 22.3 kHz from Annapolis, Maryland to Eights Station, Antarctica, where anomalous short‐duration signal amplitude changes, in coincidence with whistlers, were first noted. The calculations suggest that two ionization regions, one very near the transmitter and another near the receiver, may need to be present at the same time in order to obtain the large amplitude increases (>3 dB) that are observed to occur on occasion. It is suggested that the former is due to transmitter‐induced electron precipitation, while the latter is due to precipitation associated with the whistler. We also find that small amplitude changes can be obtained under less restrictive ionospheric perturbation conditions.

Lower ionospheric disturbances observed in long-distance VLF transmission at middle latitude

An attempt is made to investigate disturbances in the relative phase of long-distance, cesium-controlled, VLF radio wave propagation from NLK (Jim Creek, U.S.A.) to HCM (Nishinomiya, Japan). The propagation path covers a longitudinal range of about 100° and ranges from L~ 1.2 to L ~ 3.2 in invariant latitude. VLF phase disturbances following major geomagnetic storms indicate that the altitudinal and latitudinal extents of ionization enhancement in the D-region depend on the strength of the storm. The after-effect observed in the night-time VLF phase becomes pronounced especially in winter condition of the D-region. There is another type of VLF phase disturbance in the night-time which is not related to storm but associated with the winter anomaly observed in the daytime at middle latitudes (40°–45°N) but below L = 2. This result suggests that the enhanced ionization occurs in the D-region above about 80 km not only in the daytime but also in the night-time for a group of winter anomalous days. It is likely that the main cause of the enhanced ionization is related to the vertical propagation of planetary waves into the mesosphere, resulting in a sudden warming.

Stellar X-ray effects on VLF radio wave propagation

Full wave mode theory combined with electron profiles given by ionospheric theory yields results that support the reports of X-ray star effects on VLF radio signals.

The Omega navigation system--An overview

The Omega worldwide VLF radio navigation system is discussed. Following a brief historical perspective, the current and projected system configurations are described with emphasis on the operational characteristics of transmitting station equipment. The Omega positionfixing process is examined and the available user equipment is reviewed. The characteristics of VLF radio wave propagation, which have a significant impact on Omega navigation accuracy, are discussed. Finally, the various elements of the Omega user community are described along with the likely nature of future increased applications of the Omega navigation system.

Influence of ‘two-hop’ skywave on VLF propagation over short distances in the Earth-ionosphere waveguide

The article discusses VLF radio wave propagation in the range from 200 to 1000 km from the transmitter. The ground and ionospheric reflection coefficients are described and propagation in a one- and two-hop model is investigated. It is found that during the day a two-hop wave is relatively unimportant, whereas during nighttime conditions it may become the dominant skywave component. A comparison between results from full wave analysis based upon ray theory and waveguide mode theory predictions is presented.

The accuracy and resolution of model ionospheres derived from VLF propagation parameters

An inversion technique is outlined for calculating the resolution and gross error associated with D-region electron density profiles obtained by modelling from a given set of VLF radiowave propagation parameters. Two profiles, representing quiet and disturbed ionospheric conditions, are examined in detail and the height resolution and accuracy obtainable from the amount of observational data used in constructing these profiles is determined.

VLF propagation at auroral latitudes

VLF radio wave propagation at auroral latitudes (60°–75°) is frequently disturbed by precipitated electrons. Ionospheric activity as revealed by optical aurora, magnetic disturbances, or riometer absorption usually shows rather good correlation with VLF anomalies, especially during night. Large VLF disturbances which occur during both day and night, and predominantly on signals arriving at Kiruna from the south, often show poor correlation with other indications of ionospheric disturbances. They are believed to be caused by high energy electrons (>200 keV) precipitated mainly in a narrow zone below L = 6. These events are probably closely related to Bailey's relativistic electron precipitations although VLF signals provide a more sensitive indicator than forward scatter transmissions. Sudden commencements (SC's) of geomagnetic storms are connected with high latitude electron precipitation. The related VLF anomalies are small and coincide with the SC or are delayed up to a few minutes.