Showing papers on "Very low frequency published in 1987"
TL;DR: In this paper, the spectral, temporal, and polarization characteristics of radio emissions from Uranus were analyzed and two smooth radio components, one occurring in the lowest frequency range (SLF) from 20 kHz to 347 kHz, and the other one reaching the highest observed frequencies, i.e., 865 kHz (SHF).
Abstract: Radio emission from Uranus was discovered a few days before the closest approach of Voyager 2 with Uranus on January 24, 1986. The planetary radio astronomy experiment recorded several types of emissions in the frequency range 1–900 kHz; they differ by their spectral, temporal, and polarization characteristics. The analysis of the observations led us to distinguish two smooth radio components, one occurring in the lowest frequency range (SLF) from 20 kHz to 347 kHz, and the other one reaching the highest observed frequencies, i.e., 865 kHz (SHF). The SLF component is left-hand polarized when observed in the northern magnetic hemisphere and right-handed in the southern magnetic hemisphere, without polarization reversal from dayside to nightside. The SHF component is only observed in the nightside of Uranus, and it is left-hand polarized. Several bursty emissions were observed: some of them are broadband bursts and occur in the 78- to 750-kHz range (b-bursty), others are narrow-band bursts and occur in the 40- to 270-kHz range (n-bursty). The broadband bursts were observed at low magnetic latitudes of the spacecraft and mainly when it was in the southern magnetic hemisphere; their polarization is left-handed. The narrow-band bursts were observed in the northern magnetic hemisphere and are not correlated with the magnetic latitude of the spacecraft; their polarization is right-handed. Sporadic bursts of short duration were observed mainly in the northern magnetic hemisphere with right-hand polarization. Two periodic events were observed, ahead of the inbound bow shock and inside the magnetosphere of Uranus. Very low frequency emissions at 1.2 and 20.4 kHz are also discussed; some of them are correlated with in situ phenomena. From the polarization study we deduce the possible source locations of the main components in the frame of our hypothesis. The source regions are characterized by the latitude and longitude of the footprints of the magnetic field lines through the source expressed in the Uranographic longitude system (ULS). The SLF source would be fixed in magnetic longitude at the northern magnetic pole in the range 13°< latitudeULS<38°, 26°
39 citations
TL;DR: In this paper, a simple but quite accurate analytical expression suitable for ray-tracing was derived for the night electron density in the height range 170-1400 km, based on non-isothermal diffusive equilibrium and O + /O friction.
36 citations
01 Apr 1987
TL;DR: In this article, a computer which obtains waveguide mode solutions for very low frequencies and low frequencies (VLF/LF) is described, which allows for multiple homogeneous segments to be specified, allowing for consideration of variations in the earth-ionosphere waveguide.
Abstract: : A computer which obtains waveguide mode solutions for very low frequencies and low frequencies (VLF/LF) is described. The program allows for multiple homogeneous segments to be specified, allowing for consideration of variations in the earth-ionosphere waveguide. Path geometry and geophysical parameters can be computed by the program. Ionospheric disturbances due to man-made or naturally events can also be modeled using the program. Keywords include: Longwave propagation, Wavelength mode, and Propagation path.
26 citations
Journal Article•
TL;DR: In this paper, les proprietes de l'amplification d'ondes coherentes et des interactions non lineaires dans la magnetosphere demontre les relations liant ces proprietes aux caracteristiques du bruit naturel, including les rayonnements de raies magnetospheriques.
Abstract: On presente les proprietes de l'amplification d'ondes coherentes et des interactions non lineaires dans la magnetosphere et on demontre les relations liant ces proprietes aux caracteristiques du bruit naturel, incluant les rayonnements de raies magnetospheriques
19 citations
TL;DR: In this paper, the amplitude and phase of VLF radio signals from the Omega transmitters on La Reunion Island and in Argentina have been made on routine Antarctic re-supply nights from Christchurch, New Zealand.
15 citations
TL;DR: In this paper, a model was proposed to predict the amplitudes produced by lower power HF radars (partial reflection experiments) in auroral regions, which are typically an order of magnitude below those obtained for the Tromso heater but may nevertheless provide useful ionospheric information as a byproduct of the radar's normal operation.
Abstract: Pulses of radio waves in the ELF and VLF range can be generated in the auroral ionosphere by the transient conductivity perturbation (in the presence of an ambient electric field) caused by artificial heating using pulsed high-power HF radio waves. Examples of such waves generated by the Max-Planck heating facility near Tromso are briefly reviewed. The waves generated can be used to provide information on the horizontal ionospheric electric field. A model which has successfully explained these results quantitatively is used to predict the amplitudes produced by lower power HF radars (partial reflection experiments) in auroral regions. The model results show ELF/VLF amplitudes which are typically an order of magnitude below those obtained for the Tromso heater but may nevertheless provide useful ionospheric information as a byproduct of the radar's normal operation.
13 citations
TL;DR: In this article, the ratio of the frequency shift to the rate of change of group delay was found to be the same as the ratio which would be expected from signals which have travelled through the equatorial ionosphere.
12 citations
TL;DR: In this paper, the ionospheric array factors for a linear and a planar array of Hertzian dipole sources were developed and their properties were examined and the principle of pattern multiplication was applied to include the effect of the ionosphere array element.
Abstract: A very low frequency (VLF) or extremely low frequency (ELF) dipole source has been created within the lower ionosphere by modulating the atmospheric dynamo currents with a ground-based high power HF source from the Arecibo Observatory. The authors and their colleagues have demonstrated that ELF or VLF generated in this way and injected into the earth-ionosphere waveguide could be received a few thousand kilometers away. The injection properties due to an array of ionospheric dipoles as a function of array geometry and element currents that will allow steerable ELF/VLF radiation within the earth-ionosphere waveguide are investigated theoretically. The ionospheric array factors for a linear and a planar array of Hertzian dipole sources are developed and their properties examined. The principle of pattern multiplication is then applied to include the effect of the ionospheric array element. This provides a means for predicting the field strengths at a remote receiving site due to a steerable linear or planar array of ionospheric sources generated by high power HF periodic plasma heating.
12 citations
TL;DR: In this article, the authors measured the frequency of 14.881 kHz whistler-mode signals at the conjugate point (L = 1.93) in South Australia of the VLF transmitter in Eastern U.S.R.
12 citations
TL;DR: In this paper, the anisotropy of the electron distribution function is determined by the injection processes and by further modifications, e.g. convection and drift, which is large enough to allow the growth of VLF waves responsible for scattering of electrons into the loss cone.
10 citations
Journal Article•
TL;DR: In this article, a VLF-sferics analyzer was used at the German Antarctic von Neumayer Station from January to June 1983 to record sferics from distant lightning events in the frequency range between 5 and 9 kHz.
23 Jun 1987
TL;DR: In this article, the effects of propagation conditions in the ELF/VLF/LF frequency band on the design and performance of digital communications systems are described, and the communications channel from a ground-based or airborne transmitting antenna through the earth ionosphere waveguide to the receiver is discussed.
Abstract: : The effects of propagation conditions in the ELF/VLF/LF frequency band on the design and performance of digital communications systems are described in this report. Also, discussed in this report is the communications channel from a ground-based or airborne transmitting antenna through the earth- ionosphere waveguide to the receiver. Questions about coverage, noise, dispersion, time variance, and propagation disturbances are also summarized here. Keywords include: VLF Propagation, LF Minimum shift keying, ELF TACAMO, Dispersion, and Earth-ionosphere waveguide.
TL;DR: In this paper, the wave normal directions of deterministic VLF signals that can be approximated by plane waves are determined by a matched filtering and subsequent parameter estimation technique, based on a priori information about the frequency-time behaviour of the signal.
TL;DR: In this article, two years of phase and amplitude records of the 12.9 kHz signals emitted from Omega-Argentina (43.20°S; 294.60°E) and received at Tucuman (26.90°S, 294.70°E), were used to study the behavior of the electron concentration at the reflection level of very low frequency (VLF) waves.
TL;DR: Anomalous advances of OMEGA VLF navigation system (10.2-kHz) signals during three extended periods (totaling 60 days) of solar cosmic ray event activity during 1982 are examined and compared with energetic proton, alpha particle, and electron precipitation into the polar cap ionosphere as mentioned in this paper.
Abstract: Anomalous advances of OMEGA VLF navigation system (10.2-kHz) signals during three extended periods (totaling 60 days) of solar cosmic ray event activity during 1982 are examined and compared with energetic proton, alpha particle, and electron precipitation into the polar cap ionosphere. Time profiles of the OMEGA 10.2-kHz Hawaii-to-Norway path phase advance are found to be almost congruent with the time profiles of the logarithms of the energetic proton fluxes precipitating into the polar cap as determined from instruments aboard the polar orbiting NOAA-6 satellite and the GOES-2 geostationary satellite. Although both energetic alpha particles and electrons contribute to the anomalous phase advance experienced by the 10.2-kHz Hawaii-to-Norway OMEGA signal, a simple, empirical algorithm reasonably relates the observed phase advance to the observed differential fluxes of 6-MeV solar cosmic ray protons.
Journal Article•
30 Nov 1987
TL;DR: In this article, an active satellite-ground coordinated space plasma experiment was conducted in which electrons were precipitated from the radiation belts into the ionosphere by the controlled injection of VLF signals from ground-based transmitters.
Abstract: : An active satellite-ground coordinated space plasma experiment was conducted in which electrons were precipitated from the radiation belts into the ionosphere by the controlled injection of VLF signals from ground-based transmitters. In this experiment Stimulated Emission of Energetic Particles (SEEP), four operational VLF transmitters were operated in special controlled formats at times of overpasses of the low-altitude polar- orbiting satellite S81-1. 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. In addition, the precipitation of electrons by VLF waves associated with lightning was measured for the first time from a satellite. Many other important findings were made with the SEEP data. These include the following: 1) An assessment was made of the relative contributions of hiss, lightning and VLF transmitter waves to electron precipitation from the slot region, 2) discovery of the frequent occurrence of multiple peaks in the energy spectra of electrons trapped at low altitudes, 3) observation of electron pulsations near the trapping boundary and 4) discovery of bremsstrahlung x-ray images of small isolated patches of energetic electron precipitation at high latitude.
01 Apr 1987
TL;DR: In this article, self-contained, freely-drifting swallow floats capable of recording very low frequency (VLF) ambient ocean noise are under development by the contractor and are ballasted to neutral buoyancy at midwater depth where they record the components of particle velocity from which sound pressure levels may be derived.
Abstract: : Self-contained, freely-drifting Swallow floats capable of recording very low frequency (VLF) ambient ocean noise are under development by the contractor. The floats are ballasted to neutral buoyancy at midwater depth where they record the components of particle velocity from which sound pressure levels may be derived. A high frequency acoustic mutual interrogation system may be used to determine relative float positions. During an experiment conducted between 16 and 18 September 1986 approximately 50 miles west of San Diego, twelve Swallow float buoys were deployed to depths of 1000 to 2000 meters over a 24 hour period. This memorandum reports the preliminary analysis of data acquired from that experiment. Keywords: Swallow floats; Ambient ocean noise; Untethered floats, Particle velocity.
Patent•
03 Dec 1987
TL;DR: In this paper, the authors proposed a method for transmitting data in drill pipes which data is obtained from a sensor at drill bit crowns, which transmits the detected measurement data via an inductive directional antenna in the longitudinal direction of the drill pipe as electromagnetic waves with a very low frequency.
Abstract: The invention relates to a method for transmitting data in drill pipes which data is obtained from a sensor at drill bit crowns. The sensor transmits the detected measurement data via an inductive directional antenna in the longitudinal direction of the drill pipe as electromagnetic waves with a very low frequency. This data is received by VLF transponders distributed at a distance from one another over the length of the drill pipe, is amplified and retransmitted in each case at separate frequencies in the VLF range. At the end of the drill pipe, the electromagnetic waves of the last VLF transponder are received by a receiving probe and passed on to an evaluation computer.
31 Aug 1987
TL;DR: In this article, the authors derived and tested tractable formulas for coupling among VLF/LF waveguide modes that occurs at boundaries separating regions of different ground conductivities.
Abstract: : Tractable formulas are derived and tested for coupling among VLF/LF waveguide modes that occurs at boundaries separating regions of different ground conductivity. Although algebraically complicated, the formulas are easily programmed and require less computer running time than numerical mode-coupling algorithms used in exact computer codes. The formulas have two desirable features; (1) computational simplicity and (2) dependence on ground conductivity on either side of a transition (while depending only slightly on the conductivity variation within the transition itself). The formulas are subjected to three approximations, valid under most circumstances: (1) substitution of an equivalent parallel-plate waveguide for the actual waveguide in the short spatial interval that contains the conductivity boundary, (2) application of the WKB approximation, requiring that all conductivity changes in the transition zone be gradual (occurring over at least 1/6 wavelength), and (3) neglection of phase (only the magnitudes of the modes are used when performing certain numerical operations). Although derived for inclusion in future computer models of VLF/LF worldwide atmospheric noise, the mode-coupling formulas can be used in any application involving such a large number of propagation paths that the length of computer running time becomes a problem. Keywords: VLF/LF Propagation; Ground conductivity; Long-Wave communication; Strategic communication; Waveguide.