Showing papers on "Very low frequency published in 1977"

VLF Emissions Associated with Enhanced Magnetospheric Electrons.

Abstract: VLF emissions and enhancements of low-energy electrons have been simultaneously observed during periods of geomagnetic disturbances by the equatorial orbiting Explorer 45 satellite. These events are characterized by the following: (1) the VLF emissions occur outside the plasmasphere in the nightside of the magnetosphere, (2) the emissions consist of two frequency regimes, one below the local electron gyrofrequency, and the other above it, (3) the emissions below the gyrofrequency are relatively broadband whistler waves characteristic of chorus, (4) the emissions above the gyrofrequency are electrostatic and typically have components near 3/2 the gyrofrequency, (5) the onset of emissions coincides with abrupt increases outside the plasmasphere in 1-10 keV electrons to intensities of the order of 10 to the 8th el/sq cm/s/sr/keV, and (6) the cessation of the emissions coincides with a drop in the electron intensities to their preenhancement levels, which are of the order of 10 to the 6th el/sq cm/s/sr/keV or less.

Equion, an Equatorial Ionospheric Irregularity Experiment

Abstract: : This paper describes a coordinated rocket and ground based study to investigate Equatorial Spread-F irregularities. Measurements by the Jicamarca backscatter radar, ionosondes, airglow instrumentation and of radio scintillations were correlated with high spatial resolution measurements of electron densities, particle fluxes and VLF-ELF fields from a rocket probe. Experimentally the authors find: (1) Very high positive and negative electron density gradients are responsible for the enhanced radar echoes; (2) Energetic particles are not part of the Spread-F phenomena; (3) wave measurements are strongly correlated with the Doppler character of the radar signals; and (4) That there is a one to one correspondence of the large scale irregularities responsible for scintillations of satellite radio transmissions, and ionosonde spread echoes with the small scale irregularities responsible for the backscatter of VHF radar signals. Implications of the data are discussed.

Longitudinal variations of very‐low‐frequency chorus activity in the magnetosphere: Evidence of excitation by electrical power transmission lines

Abstract: The distribution of VLF chorus in the high-altitude magnetosphere as detected by the Ogo 3 satellite was investigated. Chorus occurrence frequency over 4,668 samples of 5 min of broadband data was computed for Northern Hemisphere bins of 10 deg by 10 deg in dipole invariant latitude and longitude. Peaks in activity were noted over Alaska, the eastern U.S. and Canada, western Europe, and western Siberia, and were shown not to be the result of biased sampling. It is suggested that power line radiation of only about 1 W or less at a given harmonic frequency may be sufficient to stimulate the observed chorus activity and that such power levels can be expected in industrial areas or from large distribution networks. Preliminary results show that chorus starting frequencies are highly correlated with 60 Hz harmonics in the American sector and with 50 Hz harmonics in the European sector.

VLF transmission induced slot electron precipitation

Abstract: : Analysis of electron observations in the drift and bounce loss cones of the slot region of the magnetosphere indicates that the predominant mode in which electrons in the 100 to 400 keV energy range arrive in the drift loss cone is via discrete events. These events are usually traceable back to the vicinity of a high-power-level VLF transmitter. No gradual buildup of electron flux, proceeding eastward from the South Atlantic Anomaly, is observed. Estimates of the loss rate due to the discrete events show that this process can result in a depletion of 50% per day of the electron flux in the slot region. The wave-particle interaction must occur relatively low on the field line (lambda approximately 30 deg to 50 deg) because of the relationship of particle energies and wave frequencies involved. Additional scattering of the particles near the equator, either by power-line harmonic emissions or naturally occurring ELF hiss, is required to transport the particles to the lower interaction region. (Author)

Apparatus including bifrequency electromagnetic wave generation means for sympathetic excitation of detached conductive structures

Abstract: A method and means is described for a concealed structure locating and surveying exciter apparatus, and more particularly, to a portable device which couples, either by direct or indirect connection, high frequency alternating current energy into a more conductive structure which is usually obscured in a less conductive covert medium. The preferred embodiment provides two different frequency high stability sources of first oscillations the outputs of which are alternately switched or gated by a very low frequency source of second oscillations. The alternating first oscillations are combined in a summing amplifier and caused to drive an output coupling means. The amplified outout is further modulated, usually at an audio frequency rate, by a low frequency source of third oscillations. The result is a composite electromagnetic waveform output which switches alternatively between two distinctly spaced radio frequency signals at a slow, very low frequency rate while the two radio frequency emissions are simultaneously amplitude modulated by a steady pitch audio tone.

Influence of altitude and displacement currents on plane‐wave em fields

Abstract: Airborne electromagnetic mapping by wavetilt measurement is especially suitable for highly resistive strata like the permafrost terrains of northern Canada. A theoretical study has been made on the influence of the altitude of the aircraft and displacement currents on the measured wave tilt values over a range of frequency from VLF (very low frequency) to BCB (broadcast band). A computer modeling study has shown that the effects of displacement currents cannot be ignored over resistive ground if the frequency is greater than 100 kHz. The altitude effect is significant beyond 20 m for medium frequencies and resistive ground. The phase of the wavetilt is almost always more sensitive to these factors than the amplitude. When the top layer is thicker than three skin depths in that layer, the effects of lower layers on the wavetilt is negligible. It is also apparent that before attempting to interpret wavetilt data over a layered medium, one should carefully determine the nature of the source field. The result...

Effective electron density distributions describing VLF/LF propagation data

Abstract: : This report presents vlf/lf field strength measurements obtained between 9 and 60 kHz for several radio propagation paths. These include several radials outward from Hawaii, several transmission paths across the continental United States, and high latitude propagation over the Greenland Ice Cap. Effective exponential electron density profiles are derived yielding predicted signal levels that simulate the experimental data.

Active Very Low Frequency Experiments on the Magnetosphere from Siple Station, Antarctica

Abstract: V.l.f. coherent waves transmitted from Siple Station, Antarctica, provide a novel means for the study and control of the magnetosphere and ionosphere. Through cyclotron resonance the waves exchange energy with radiation belt electrons, causing wave growth (ca. 10 $^3$ in power), wave-wave interaction and modification of the distribution of electrons. The measured growth rates give information on the flux of energetic electrons along the path of propagation. Precipitation induced by v.l.f. waves from lightning, ground transmitters and power grids, alters the ionosphere, affecting propagation (e.l.f., v.l.f., l.f., h.f.) and providing a controlled forcing function for the study of ionospheric processes. The active experiments are supported by the L = 4 network of whistler stations in the Antarctic; they provide data on the distribution and movement of the magnetospheric plasma. Both the active and the passive v.l.f. experiments will contribute important data to the I.M.S. (International Magnetospheric Survey) and to the associated space experiments of the I.S.E.E. (International Sun-Earth Explorers) and E.E. (Electrodynamic Explorer) programmes.

Suppression effects associated with VLF transmitter signals injected into the magnetosphere

Abstract: : Magnetospheric growth of coherent VLF signals transmitted from Siple Station, Antarctica is inhibited by whistler mode echoes of earlier transmitter signals This new phenomenon, called echo-induced suppression, is observed at least a third of the time that transmissions from Siple Station are detected at the receiving station in Roberval, Quebec, Canada Suppression levels as high as 20 dB are observed Though the echo is usually much weaker than the direct signal the level of suppression is directly related to the amplitude of the echo The echoes reduce triggering of emissions as well as the growth of the signal Echo-induced suppression is not explained by linear wave-wave interference The echoes are thought to restrict growth by reducing the coherence of the total input signal According to quasi-linear theory, wave growth is regulated by changes in the particle pitch angle distribution that results from wave-particle interaction These new results suggest that coherent waves tend to limit their own growth, even when modification of the particle pitch angle distribution is unimportant Other phenomena such as whistler-induced suppression of signal growth are thought to be related to echo-induced suppression Wave-induced growth suppression provides an indirect verification of the existence of discrete ducts for the propagation of VLF signals in the magnetosphere

Vlf pulse timing: limitations and potential as a companion to omega

Abstract: The potential accuracy of VLF pulse (envelope) timing is severely limited by vagaries of the propagation medium and atmospheric noise due to spherics. These limitations are discussed in depth. The conclusion is that VLF pulse-timing accuracy corresponding to about 5 n. mi. should be possible over very long ranges up to 5000 n. mi. A hypothetical system employing pulse transmitters co-located at the Omega sites is explored. It is proposed that such a pulse system and the present phase-only Omega system could serve to complement each other as a coarse-fine system.

Induced precipitation of inner zone electrons. Volume 1: Observations

Abstract: : Narrow precipitation spikes of energetic electrons observed in the inner zone drift loss-cone during the 1968-1970 period by instrumentation on the OV1-14 and OV1-19 satellites are shown to have characteristics which are consistent with pitch-angle scattering produced through a resonant interaction with ground-based VLF transmissions. Analysis of the pitch-angle distributions indicates that for virtually all of the events, the electrons last interacted with the atmosphere in the vicinity of 55 deg to 62 deg East Longitude. The L-dependency of the spikes as a function of energy is consistent with scattering by a monochromatic wave. The presumption is that UMS, located at 44 deg E. and operating at a frequency of 16.2 kHz during this period, was responsible for these precipitation events.

INVERT: A computer program for obtaining D-region electron density profiles from VLF reflection coefficients

Abstract: : INVERT is a FORTRAN computer program designed to study the feasibility of obtaining electron density distributions of the D-region of the ionosphere. These electron density profiles would be derived from measurements of ionospheric reflection coefficients. The radio propagation frequencies would be limited to the VLF band, particularly 3-20 kHz. This report contains a discussion of the analytical approach taken in INVERT, a FORTRAN listing of the program, instructions for using the program and some sample calculations using simulated data. The program has not been used with real data. (Author)

Effect of geophysical disturbances on ionospheric radio wave propagation and emergency communication

Abstract: Some results of a study of the effect of geophysical disturbance due to solar flares, thunderstorms and meteor showers on the ionospheric radio wave propagation characteristics in the entire frequency range of radio communication ranging from VLF to HF, are presented. The results reveal that a fadeout in one band may be accompanied by an enhancement or no change of signal level in other band, and thus indicate the possibility of evolving an interband frequency diversity system involving VLF, LF, MF and HF bands to cope with the fadeouts associated with different geophysical disturbances.

Stimulation of ULF (Ultra-Low-Frequencies) Geomagnetic Pulsations by Controlled VLF (Very-Low-Frequencies) Transmissions into the Magnetosphere

Abstract: : This report presents the results of an investigation of a proposed method for the controlled artificial generation of ultra-low-frequency (ULF) hydromagnetic waves of class Pc 1 (0.2 to 5 Hz) in the ionosphere and magnetosphere. In this method, which is called the VLF method, a large ground- based very-low-frequency (VLF) transmitter is used to stimulate the ULF waves by injecting pulses of VLF waves into the magnetosphere. A second possible method of ULF wave generation, the peninsula method, is discussed in a companion report. Combining the theoretical and experimental results obtained during this research, it is suggested that naturally-occurring repetitive VLF activity can stimulate Pc 1 pulsation events, and it is further suggested that such VLF activity may be a major source of stimulation for Pc 1 pulsations. Thus, future experiments on ULF wave generation with ground-based VLF transmitters would probably benefit greatly if they were combined with a program of simultaneous observations of naturally-occurring VLF and ULF activity.

VLF/LF Reflectivity of the Polar Ionosphere 1 January-3 July 1976

Abstract: : This report provides a summary of high latitude ionospheric reflectivity as observed by the USAF high resolution VLF/LF ionosounder operating in northern Greenland. Ionospheric reflectivity parameters, including reflection heights and coefficients, are presented as a function of time of day. VLF long path propagation measurements, along with magnetometer and riometer data, are presented as supplementary information.

Low-Cost High-Performance VLF Receiver Front-End

Abstract: A VLF receiver front end has been designed using standard linear IC's. The basic methods have been evaluated extensively on the omega 10.2-kHz channel but are readily adaptable to any other VLF frequency in the 10.2-20 kHz region. Applications for the modules exist in position location, time-frequency measurements, and signal propagation. The set provides control gates, zero-crossing signals, and analog outputs to interface with any type of digital logic, microprocessor, or analog signal processor.