Showing papers on "Very low frequency published in 1990"

VLF heating of the lower ionosphere

Abstract: A controlled wave-injection experiment with a 28.5 kHz transmitter having a radiated power of 100 kW has revealed evidence of ionospheric heating by the VLF waves. Calculations indicate that the observed effect can be attributed to the absorption of wave energy in the lower ionosphere, which is estimated to result in a 30% enhancement in the collision frequency at 85 km. This process also represents a new means of direct coupling of lightning energy to the lower ionosphere.

Some comments on seabed propagation of ULF/ELF electromagnetic fields

Abstract: A reference model for the electrical conductivity structure beneath the deep seafloor is proposed and justified using a variety of geophysical evidence. The model consists of relatively conductive sediment and crustal layers of 6.5 km extent overlying a resistive (≈10−5 S/m) subcrustal channel of 30 km thickness and terminated in a deeper conductive layer and half-space. Its seafloor-to-seafloor response to a horizontal electric dipole source is explored as a function of frequency and range, showing that, compared with the response for a half-space with the lowest conductivity in the reference model, significant enhancement of the field amplitude can occur at long ranges (>100 km) and low frequencies (<1 Hz). At the same time, marked attenuation relative to the half-space response is seen at higher frequencies. The field enhancement is due to trapping of electromagnetic energy in a leaky subcrustal waveguide, as demonstrated by computing the complex Poynting vector. The attenuation occurs in the relatively conductive sedimentary and crustal layers overlying the lithospheric waveguide when their electrical thickness exceeds a skin depth. The results indicate that attempts either to model controlled electromagnetic sources or to interpret controlled source data using half-space models for the Earth can be badly misleading. The practicality of lithospheric communications in the real Earth is also investigated. Using measured receiver noise figures and the reference model, the receiver bandwidth necessary to achieve a given signal-to-noise ratio as a function of range and frequency is estimated for a seafloor horizontal source of strength 105 A-m. The results indicate that significant (≈100 km) ranges can be achieved only around 1 Hz with a bandwidth of ≈1 Hz at a SNR of 10, yielding a very low data rate of <3.5 bits/s. Longer ranges and higher frequencies are precluded by attenuation in the sediment and crustal layers and because the conductivity in the resistive channel is too large.

Bias Elimination and Scatter in Lightning Location by the VLF Arrival Time Difference Technique

Abstract: In the very low frequency (VLF) band lightning flashes are detectable at ranges of several thousand kilometers. Studies of experimental data show that if systematic biases were eliminated from the U.K. Meteorological Office's VLF arrival time difference (ATD) flash locating (fixing) system, the residual ATD scatter would amount to 1.4–2 µs. For the operational outstation network this would give stroke fixing errors below 1.2 km over most of western Europe. Techniques are presented for the elimination of bias to approach this precision. At longer ranges propagation effects, including those due to terrain conductivity, must he considered.

Floating sensor to detect very low frequency pressure signals

Abstract: A floating sensor system is provided to detect very low frequency pressureignals (down to 0.01 Hz). This system detects pressure fluctuations or pressure signals of interest in the ocean or other body of water in the presence of unwanted pressure signals generated by surface wave induced motion. A drifting sensor surface float follows the surface waves and in turn moves a suspended pressure sensor vertically, such that it detects the wave motion as a change in static pressure which then constitutes a noise source. A correlation circuit and a logic circuit discriminate between a first composite signal, comprised of the pressure signals of interest and wave motion noise, and a second reference signal, comprised only of the wave motion noise to provide an output signal transmitted to a monitoring station.

Electromagnetic Disturbances Associated With Earthquakes: An Analysis of Ground-Based and Satellite Data

Abstract: Several observations were made of Very Low Frequency (VLF) emissions apparently associated with earthquakes, which were recorded in- dependently at ground-based stations and on satellites. The observations at the Kerguelen station (49"26'S, 70°25'E) were made using magnetic anten- nae, on April 24 and 25, 1980, during a period when three earthquakes with magnitude Ms > 4.7 took place near the station. Several increases of electro- magnetic waves at the time of earthquakes were recorded on the polar-orbit- ing satellite AUREOL-3. The observations on the geostationary GEOS-2 satellite were made using magnetic and electric antennae during the period 1977- 198 1. Data were analysed for those cases when both intense (M, > 5) earthquakes occurred in the region close to the satellite longitude and the satellite was operating in the VLF mode. A statistical analysis, based on the enhancement of wave intensity at the time of earthquakes and using GEOS- 2 data, seems to indicate that there is a (possibly indirect) association be- tween seismic activity and some of the VLF emissions observed at the satellite. Ionospheric measurements made from the ground also showed an increase of the critical frequencyfoE, of the sporadic layer Es when earth- quakes occurred nearby. Some aspects of the relation between the VLF emissions and the seismic activity are discussed.

Wideband VLF electromagnetic bursts on the DE 1 satellite

Abstract: Wideband VLF electromagnetic bursts are observed on the DE 1 satellite by both the electric and magnetic field sensors in the frequency range of 0.650 kHz to 16.0 kHz. The impulsive signals endure for a relatively short time ({approximately}1s or less) and exist in the frequency range from well below to well above the local gyrofrequency. They are typically found at L > 4 over a {approximately} 40{degree} range of latitudes including the geomagnetic equator and are often accompanied by discrete emissions or a band of hiss. Some observed features are consistent with previous observations of electrostatic plasma waves (Ondoh et al., 1989; Reinleitner et al., 1983); however, the magnetic measurements clearly indicate that the impulsive signals are electromagnetic in nature, a result that has not been reported before. The possibility of spacecraft discharge effects as the cause of these signals is discussed.

Interpretation of ground VLF-EM data in terms of inclined sheet-like conductor models

Abstract: The response of two-dimensional, inclined, sheet-like conductors with, low conductance values to plane wave electromagnetic fields in the very low frequency (VLF) range has been evaluated by using a numerical technique. The conductance values of the conductors considered are appropriate for those produced by water and/or clay-filled fracture and shear zones in the Precambrian crystalline rocks of the Canadian Shield. The surrounding host rock was assumed to be, resistive with resistivities in the 1–10 kΩ.m range to reflect the high resistivities over the shield areas. No overburden was assumed in this analysis.

Radio noise near the earth in the 1–30 MHz frequency range

Abstract: Terrestrial radio interference presents a serious problem for sensitive low-frequency radio observations from space near the Earth The interference is both narrow band and broad band Several satelites and planetary probes have carried radio astronomy experiments so a moderate amount of information is available concerning the noise radiation from the Earth The region of space within 100 RE of the Earth is quite a hostile environment for any radio astronomy experiment Observations up to 10 MHz employing ionospheric shielding may be possible from satellite altitudes on the sunlit side of the Earth near solar maximum Observations above 10 MHz should be made from the surface of the Earth or from the Moon

Plasma heating near a VLF antenna

Abstract: The collisionless plasma heating due to a strong parametric instability of VLF oscillations, excited in the near zone of an antenna in the ionospheric plasma, is considered. An expression for the temperature is derived.

3-D Modeling of Subionospheric VLF Propagation in the Presence of Localized D-Region Perturbations Associated with Lightning

Abstract: : A theoretical model of multiple-mode, subionospheric VLF wave propagation in the presence of localized perturbations of the nighttime D-region has been developed Such perturbations could be produced, for example, by lightning-induced electron precipitation associated with a characteristic type of phase or amplitude perturbation in VLF signals known as Trimpi events Our model assumes that the ionospheric perturbation is slowly varying in the horizontal plane and that mode-coupling within the region of the perturbation is therefore negligible However, the model does assume mode-coupling along the paths between the transmitter and receiver, the transmitter and the perturbation, and the perturbation and receiver The model accounts for (1) effects of perturbations with finite extent in the dimension transverse to the great circle (GC) path between transmitter and receiver, and (2) effects of perturbations which lie off the GC path as well as on it The formulation used for the numerical calculations depends significantly on the mode refractive index of the ambient earth-ionosphere waveguide and the mode refractive index in the region of the perturbation In the calculations, values for the mode refractive index are determined from the electron-density-versus-altitude profiles of both the ambient and perturbed ionospheres

High power very low frequency/low frequency transmitting antennas

Abstract: Very-low-frequency (VLF) and low-frequency (LF) transmitting antennas are small with respect to the wavelength and are required to radiate substantial power levels Design parameters for these antennas are discussed, including the impact of technical advances in high-voltage design and bandwidth enhancement Most of these antennas are various realizations of electrically small monopoles that can be modeled by a simple series tuned circuit with one resonance From this circuit, simple formulas are derived for the maximum radiated power (P/sub max/), and intrinsic bandwidth (100% efficiency bandwidth) B/sub wo/ It is shown that P/sub max/ is proportional to the square of the effective topload area, B/sub wo/ is proportional to the volume under the topload, and both are proportional to frequency to the fourth power A normalized area and volume are defined and used as a figure of merit for comparing antenna designs These factors are calculated for several existing antennas and for a generalized umbrella toploaded monopole (UTLM) The use of multiple elements in VLF antennas is discussed >

VLF, ELF and ULF research using the Tromsø heating facility

Abstract: : A review is presented of results from the Tromso Heating facility in the areas of wave generation from ULF (mHz) to VLF (kHz) frequencies, as well as VLF wave propagation under heated ionospheres. Results from similar facilities in the USSR and USA will not be included. High power high frequency (HF) radio waves heat the electrons in the lower ionosphere through non-deviative absorption on the time-scale of microseconds resulting in the enhancement of the electron-neutral collision frequency. The heated region, typically 25 km diameter at 80 km altitude, can act as a perturbation on the upper wall of the Earth-ionosphere waveguide, thereby affecting the propagation of VLF waves. On the other hand, by amplitude modulating the heating wave the heated region can act as an oscillating current source, if there is an external driving electric field, which can itself radiate waves at the modulation frequency into the waveguide or into the magnetosphere. The auroral zone is ideally situated for experiments of the second type because of the frequent presence of large (tens of mV/m) electric fields. For experiments of the first type, the auroral zone is not quite so ideal since large natural ionospheric variations can mask the artificially produced ones. We shall review results from both types of experiment performed using the heating facility of the Max-Planck-Institut fur Aeronomie, a description of which can be found in Stubbe and Kopka (1979) and Stubbe et al. (1982a, 1985). This facility will be operated by the EISCAT scientific association from 1993 onwards.

Application of High Stability Oscillators to Radio Science Experiments Using Deep Space Probes

Abstract: The microwave telecommunication links between the earth and deep space probes have long been used to conduct radio science experiments which take advantage of the phase coherency and stability of these links. These experiments measure changes in the phase delay of the signals to infer electrical, magnetic and gravitational properties of the solar system environment and beyond through which the spacecraft and radio signals pass. The precision oscillators, from which the phase of the microwave signals are derived, play a key role in the stability of these links and therefore the sensitivity of these measurements. These experiments have become a driving force behind recent and future improvements in the Deep Space Network and spacecraft oscillators and frequency and time distribution systems. Three such experiments which are key to these improvements are briefly discussed and relationship between their sensitivity and the signal phase stability is described. The first is the remote sensing of planetary atmospheres by occultation in which the radio signal passes through the atmosphere and is refracted causing the signal pathlength to change from which the pressure and the temperature of the atmosphere can be derived. The second experiment is determination of the opacity of planetary rings by passage of the radio signals through the rings. The third experiment is the research for very low frequency gravitational radiation. The fractional frequency variation of the signal is comparable to the spatial strain amplitude the system is capable of detecting. A summary of past results and future possibilities for these experiments are presented.