Showing papers on "Very low frequency published in 2000"

Reviving Fossil Radio Plasma in Clusters of Galaxies by Adiabatic Compression in Environmental Shock Waves

Abstract: We give for a plasma with a history of several expansion and contraction phases an analytical model of the evolution of a contained relativistic electron population under synchrotron, inverse Compton and adiabatic energy losses or gains. This is applied to different scenarios for evolution of radio plasma inside the cocoons of radio galaxies, after the activity of the central engine has ceased. It is demonstrated that fossil radio plasma with an age of even up to 2 Gyr can be revived by compression in a shock wave of large-scale structure formation, caused during the merging events of galaxy clusters, or by the accretion onto galaxy clusters. We argue, that this is a highly plausible explanation for the observed cluster radio relics, which are the regions of diffuse radio emission found in clusters of galaxies, without any likely parent radio galaxy seen nearby. An implication of this model is the existence of a population of diffuse, ultra-steep spectrum, very low frequency radio sources located inside and possibly outside of clusters of galaxies, tracing the revival of aged fossil radio plasma by the shock waves associated with large-scale structure formation.

Modeling electromagnetic propagation in the Earth-ionosphere waveguide

Abstract: The ionosphere plays a role in radio propagation that varies strongly with frequency. At extremely low frequency (ELF: 3-3000 Hz) and very low frequency (VLF: 3-30 kHz), the ground and the ionosphere are good electrical conductors and form a spherical Earth-ionosphere waveguide. Many giants of the electromagnetics (EMs) community studied ELF-VLF propagation in the Earth-ionosphere waveguide, a topic which was critically important for long-range communication and navigation systems. James R. Wait was undoubtedly the most prolific publisher in this field, starting in the 1950s and continuing well into the 1990s. Although it is an old problem, there are new scientific and practical applications that rely on accurate modeling of ELF-VLF propagation, including ionospheric remote sensing, lightning remote sensing, global climate monitoring, and even earthquake precursor detection. The theory of ELF-VLP propagation in the Earth-ionosphere waveguide is mature, but there remain many ways of actually performing propagation calculations. Most techniques are based on waveguide mode theory with either numerical or approximate analytical formulations, but direct finite-difference time-domain (FDTD) modeling is now also feasible. Furthermore, in either mode theory or FDTD, the ionospheric upper boundary can be treated with varying degrees of approximation. While these approximations are understood in a qualitative sense, it is difficult to assess in advance their applicability to a given propagation problem. With a series of mode theory and FDTD simulations of propagation from lightning radiation in the Earth-ionosphere waveguide, we investigate the accuracy of these approximations. We also show that fields from post-discharge ionospheric currents and from evanescent modes become important at lower ELF (/spl lsim/500 Hz) over short distances (/spl lsim/500 km). These fields are not easily modeled with mode theory, but are inherent in the FDTD formulation of the problem. In this way, the FDTD solution bridges the gap between analytical solutions for fields close to and far from the source.

FORTE radio-frequency observations of lightning strokes detected by the National Lightning Detection Network

Abstract: This work compares simultaneous observations of lightning from two complementary systems. FORTE is a low-Earth-orbit satellite carrying radiowave and optical instruments for the study of lightning. The radio receivers aboard FORTE observe very high frequency (VHF) emissions from the air-breakdown process preceding (and sometimes accompanying) a lightning current. The National Lightning Detection Network (NLDN) is a ground-based array of sensors in the contiguous United States observing the low-frequency (LF) and very low frequency (VLF) radiation from vertical currents. Prior to the launch of FORTE in 1997, essentially no work had been done on the statistical correlations between (1) ground-based LF/VLF and (2) spaced-based VHF remote sensing of lightning. During a 6-month campaign in April-September 1998, FORTE took most of its triggered VHF data over and near the contiguous United States, and NLDN data were specially postprocessed in a loosened-criterion mode providing enhanced detection range beyond the coastline and borders of the array itself. The time history of reported events from the two systems was compared, and event pairs (each pair containing one event from FORTE, the other from NLDN) which were candidate correlations (closer than 200 ms from each other) were scrutinized to determine whether the members of a pair actually came from the same discharge process. We have found that there is a statistically significant correlation, for a subset of FORTE events. This correlation is most likely to occur for intracloud and less likely to occur for cloud-to-ground discharges. The correlated VHF and NLDN events tend to occur within ±30 μs of each other, after correction for the propagation of the VHF signal to FORTE from the NLDN-geolocated stroke location. Most correlations outside of ±30 μs turn out to be merely a statistical accident. The NLDN-furnished geolocation allows the correlated FORTE-detected VHF pulses to be better interpreted. In particular, we can deduce, from the lag of the VHF ground-reflection echo, the height of the VHF emission region in the storm.

Temporal and spatial forecasting of the foF2 values up to twenty four hours in advance

Abstract: Radio waves of a wide range of frequencies from very low frequency (VLF) to high frequency (HF), (broadly 3 to 30 MHz) can be propagated to great distances via the ionosphere. Since the largest variability occurs in the F-region the objective of this paper is to demonstrate a neural network model with the backpropagation algorithm which is designed to forecast the foF2 values of the highly nonlinear ionosphere up to 24 hours in advance. In other words, the model forecasts all values from 1 to 24 hours ahead. By using foF2 data for three European Ionospheric stations this neural network based model can forecast foF2 values both in time and in space for those three stations. The model seems promising for practical work since the root mean square errors involved are within reasonable limits.

Matched filtering for the measurement of conjugately ducted VLF transmissions

Abstract: The application of matched filtering to the problem of very low frequency (VLF) radio signals ducted between magnetically conjugate points on the Earth by ionospheric plasma structures has been examined theoretically and implemented in recent field experiments. With accurate knowledge of the source VLF transmission the subionospheric component of the received waveform can be effectively removed, and the characteristics of the remaining ducted signal can be accurately estimated. Although the technique has been shown not to produce strictly consistent estimates of the subionospheric and ducted signal amplitudes, it is able to efficiently recover the transit delays and amplitudes of ducted man-made whistler mode emissions for reasonable values of the relevant signal-to-noise ratios. This is substantiated by the use of matched filtering to analyze data collected by radio receivers placed at the conjugate ends of a magnetic field line, thereby recovering the amplitudes and transit delays of ducted signals with time resolution as small as 30 s. An arbitrary degree of accuracy can be obtained in these measurements through the use of bit stream extraction and comparison. Further correlation analysis can be performed to extract Doppler shifts due to drifting ducts, and the small time resolution makes possible studies of ducting under rapidly changing conditions.

A nonintrusive power system arcing fault location system utilising the VLF radiated electromagnetic energy

Abstract: This paper describes research being conducted by the Power and Energy Systems Group at the University of Bath into a novel technique for monitoring the condition of transmission and sub-transmission circuitry using the electromagnetic radiated energy emitted from a power system arcing fault, together with the propagation effects of very low frequency (VLF) radio waves. The emission of an atmospheric radio wave, collectively known as a sferic, from a power system arcing fault will be characterised and is similar to the way a lightning strike induces a sferic. The paper describes the hardware infrastructure necessary to be able to monitor the condition of transmission and sub-transmission circuitry, together with the real-time intelligent algorithm which enables discrimination between power system arcing faults and other sources of impulsive radio noise in the VLF spectrum The technique described in this paper is not limited to monitoring a specific line or circuit as no physical connection is required to the system, instead the described system monitors a geographic area and therefore it can be economically used on sub-transmission circuitry.

A Novel Algorithm for the Numerical Simulation of Collision-Free Plasma-Vlasov Hybrid Simulation

Abstract: Numerical simulation of collision-free plasma is of great importance in the fields of space physics, solar and radio physics, and in confined plasmas used in nuclear fusion. This work describes a novel completely general and highly efficient algorithm for the numerical simulation of collision-free plasma. The algorithm is termed Vlasov Hybrid Simulation (VHS) and uses simulation particles to construct particle distribution function in the region of phase (r, v) space of interest. The algorithm is extremely efficient and far superior to the classic particle in cell method. A fully vectorised and parallelised VHS code has been developed, and has been successfully applied to the problem of the generation of VLF triggered emissions and VLF 'dawn chorus', due to the nonlinear interaction of cyclotron resonant electrons with narrow band VLF band waves (~kHz) in the earth's magnetosphere.

Errors in direction finding of VLF waves caused by a sloping ionosphere

Abstract: Lightning discharges emit very low frequency (VLF) radio waves commonly called “atmospherics.” In the past, lightning source locations were determined by the triangulation method from recordings at three widely separated stations in Japan. The three direction-of-arrival (DOA) lines, however, sometimes formed large triangles resulting in large uncertainties in source location. To approach this problem, the variational treatment is used here in which the sloping ionosphere plays an important role. By obtaining the time required to propagate from the source to the receiver via the Earth-ionosphere waveguide, it is found that the direction of propagation of a VLF wave can be bent along a parabolic curve. Using this concept of parabolic propagation, the locations of lightning discharges are compared with those determined by the triangular method.

On the sensitivity of the lower ionosphere to a strong atmospheric energy action

Abstract: Detailed investigations of the ionospheric and magnetospheric perturbations induced by earthquakes, volcanic eruptions and strong man-made actions such as explosions and special supersonic flights indicated that those phenomena can not be accounted for in the traditional framework of the acoustic-gravity mechanisms. A clue can be found in the particular sensitivity of the electrical conductivity of the D- layer to weak stochastic perturbations of electron concentration. The calculations show that the electron Pedersen conductivity, which is usually small in the homogeneous ionosphere, increases drastically because of ionospheric irregularities. Some refined effects in the Extremely Low Frequency (ELF) and Very Low Frequency (VLF) ranges can be predicted.