Showing papers on "Very low frequency published in 1972"

Vlf hiss and related plasma observations in the polar magnetosphere.

Abstract: This paper presents a study of auroral-zone vlf hiss and low-energy charged-particle observations with the Injun 5 satellite. The results of this study provide a direct verification of the association between auroral-zone vlf hiss and intense fluxes of low-energy electrons with energies on the order of 100 eV to several keV. On the dayside of the magnetosphere, these low-energy electrons are identified with the dayside polar-cusp region observed at higher latitudes with the Imp 5 satellite. At other local times, through the dawn and dusk regions and into the nightside of the magnetosphere, the vlf hiss and low-energy electron precipitation regions are believed to correspond to the extension of the dayside polar cusp into the distant plasma sheet and downstream magnetosheath on the nightside of the magnetosphere. Intense fluxes of upgoing electrons are often observed in a narrow latitudinal band near the low-energy electron precipitation bands. These upgoing electrons are believed to be associated with another type of vlf emission called a saucer, which is frequently observed with Injun 5.

Comparison of very-low-frequency auroral hiss with precipitating low-energy electrons by the use of simultaneous data from two Ogo 4 experiments

Abstract: Determination of the origin of auroral hiss by comparing the records of a vlf experiment (0.3 to 18 kHz) with simultaneous data obtained by an auroral-particle experiment having detectors for precipitating electrons at 0.7, 2.3, and 7.3 keV. It is found that, on the dayside of the earth, the occurrence of vlf hiss correlates well with precipitation events at 0.7 keV, but in general very poorly with activity in the higher-energy channels. Exact correlation between variations in vlf hiss intensity and in electron fluxes is rare even at 0.7 keV. In addition, vlf hiss tends to be observed over a somewhat larger spatial region than precipitating 0.7-keV electrons. It is concluded that, on the dayside, auroral hiss is generated by soft (E less than 1 keV) 'cusp region' electrons and that the lack of detailed correlation between the two phenomena is caused by propagation effects as the hiss travels downward and spreads from the generation region.

The structure of the magnetosphere as deduced from magnetospherically reflected whistlers

Abstract: Very low frequency (VLF) electromagnetic wave phenomenon called the magnetospherically reflected (MR) whistler was investigated. VLF (0.3 to 12.5 kHz) data obtained from the Orbiting Geophysical Observatories 1 and 3 from October 1964 to December 1966 were used. MR whistlers are produced by the dispersive propagation of energy from atmospheric lightning through the magnetosphere to the satellite along ray paths which undergo one or more reflections due to the presence of ions. The gross features of MR whistler frequency-time spectrograms are explained in terms of propagation through a magnetosphere composed of thermal ions and electrons and having small density gradients across L-shells. Irregularities observed in MR spectra were interpreted in terms of propagation through field-aligned density structures. Trough and enhancement density structures were found to produce unique and easily recognizable signatures in MR spectra. Sharp cross-field density dropoff produces extra traces in MR spectrograms.

Observations of D-region modifications at low and very low frequencies.

Abstract: WE describe here an experiment in which the modulation radiated by a high frequency radio transmitter is transferred by an ionospheric process to waves of low and very low frequency (LF and VLF). This is the first observation of so called “cross modulation” for waves of frequencies below 167 kHz and this result has important implications regarding the electron energy and collision frequency in the lowest regions of the ionosphere.

Dispersive Auroral Hiss

Abstract: AT the frequencies of auroral hiss (up to 500 kHz), the propagation of radio energy through the magnetosphere is in the whistler mode, which is dispersive: the velocity of propagation is dependent on frequency. Here I report bursts of auroral hiss so brief as to have distinctly exhibited whistler-mode dispersion. The duration of each of these bursts was of the order of 0.1 s. I have chosen the name “fast hisslers” to distinguish them from the more slowly varying, periodic hissler emissions reported by Carpenter et al.1 and to indicate their kinship with auroral hiss. These hisslers were observed at a ground station and therefore differ from the 0.05 s duration “auroral beam modes” observed by Smith2 with the very low frequency receiver on the Alouette II satellite.

VLF timing: Conventional and modern techniques including omega

Abstract: VLF signals from 10 to 30 kHz play an important role in long-range time dissemination, communications, and navigation. A brief history of VLF transmissions is given with particular emphasis on the time dissemination capabilities of the Omega navigational system. Traditional methods are compared with modern capabilities. Traditional lead-edge time dissemination by VLF normally can provide an accuracy of a few milliseconds, although stabilized transmitters can permit an accuracy of better than 100 µs. Frequency comparisons to an accuracy of a few parts in 1011per day typically are made with commercially available equipment. The present four-station Omega system will be expanded to an eight-station worldwide network in the near future. With implementation of this global network, carrier ambiguities can be resolved at a receiving station with a cesium frequency standard over intercontinental ranges; phase can be maintained to better than 3 µs and frequency to about 1 part in 1012. Continuous maintenance of phase to better than 1 µs is indicated when using specialized techniques. Propagation aspects of VLF transmissions limit received accuracy; however, this is tempered by low attenuation of stable signals to long ranges.

Transpolar propagation of long radio waves

Abstract: This report presents the results of a theoretical analysis and a laboratory simulation of certain transpolar VLF/ELF propagation phenomena. The calculations are based on daytime ionospheric models representative of ambient conditions and of conditions that prevail during polar-cap absorption (PCA) events. The laboratory simulation utilized a wave guide that models VLF propagation in the earth-ionosphere cavity. The influence of the Greenland icecap is included in both the theoretical and experimental approaches. The calculations predict, in agreement with actual transpolar propagation data, that much larger signal losses will be suffered on paths that cross Greenland than on paths that do not. Furthermore, the calculations correctly predict that a given PCA will typically produce much larger amplitude degradations and phase advances on signals that cross Greenland than on ones that propagate only over sea water and/or relatively highly conducting ground. The data from the laboratory model is in good general agreement with actual transpolar propagation measurements and our theoretical results. In addition to causing sizable ground losses, the presence of thick ice at the lower boundary of the earth-ionosphere wave guide distorts the structure of the modes greatly from that which prevails for propagation over highly conducting ground. This distortion manifests itself in ambiguities in mode numbering and in an increase in the losses due to ionospheric heating. Ions contribute significantly to the propagation phenomena during moderate and intense PCA events.

Observation of very-low-frequency whistler-mode waves in the region of the radiation-belt slot

Abstract: Following a sudden-commencement magnetic storm, band-limited VLF emissions between 3.9 and 10.4 kHz were observed at high altitudes within the plasmasphere (L ∼ 2.5 to 3.6) by a receiver aboard spacecraft OV1-14 (1968–26B). The maximum spectral intensities of 1.1 mγ/Hz1/2 and 2.8 μv/Hz1/2 on the magnetic and electric antennas, respectively, occur in the 5.6-kHz channel. The waves can resonantly interact with energetic electrons above ∼50 kev at L = 2.5, and the measured wave intensities are sufficiently high to account for the short lifetimes of medium-energy electrons in the slot region between the inner and outer Van Allen radiation zones. A comparison with ground-based VLF data, suggests that the waves are generated by band-limited amplification of incoherent noise.

Amplitude Variations in Artificially Stimulated v.l.f. Emissions

Abstract: AN understanding of the basic mechanisms involved in wave-particle interactions in a magneto-active plasma is currently of interest in contexts ranging from planetary and pulsar magnetospheres to galactic jets. In the Earth's magnetosphere, which is an excellent laboratory for the study of an infinite collisionless plasma with an inhomogeneous magnetic field, ground and satellite observations of discrete very low frequency (v.l.f.) emissions are providing information on the interaction of waves in the whistler mode with asymmetric velocity distributions of energetic electrons in the radiation belts. The emissions observed consist of rising, falling, hook-shaped, or approximately steady tones, triggered in the equatorial region of the magnetosphere by natural whistlers, by unknown natural sources, by short (150 ms) pulses from high power (106 W) ground transmitters1, or by long (∼ 1 s) pulses from low power (∼ 100 W) ground transmitters of the Omega network2. The basic properties3 of the emissions to be accounted for on a microscopic theory are the narrow bandwidth, the frequency variations, the amplitudes observed (sometimes an order of magnitude greater than the amplitude of the triggering pulse), the triggering delay of order 100 ms and the fact that for ground observations triggering is generally seen at frequencies ω such that ω≈0.5 Ω0, where Ω0 is the equatorial value of the electron gyrofrequency on the appropriate field line. I shall report here the observation of an additional property: large regular and irregular classes of amplitude fluctuation within artificially stimulated v.l.f. emissions triggered by low power Omega pulses of frequency 10.2 kHz received on the ground in the magnetically conjugate zone of the transmitter after following a magnetospheric trajectory.

VLF-LF Wide Shift FSK Antenna Feed Networks

Abstract: : The problem of broadbanding VLF and LF antennas for wide shift FSK modulation has been solved by recourse to static feed networks, which exhibit either two separate passbands centered at the mark and at the space frequency respectively, or a single wide passband extending over the frequency shift. Several configurations of one-port and of two-port networks have been DEVELOPED: these exhibit minimum number of elements, with values that are economically realizable. The new antenna feed networks could be applied to existing antenna installation without excessive modifications.

Theoretical and Experimental Studies of VLF and LF Waves.

Abstract: : The results of the research on the production, transmission and reception of VLF and LF waves is represented in 9 compact chapters. A new improved equivalent circuit diagram for the helix is calculated giving a good explanation of the measured effects over a large frequency range. The receiving characteristic of a ferrite rod antenna in the elliptically polarized field is investigated specifying a method that allows all ellipse parameters to be measured simultaneously. The phase measurement at VLF-wave propagation and the device built for this purpose are described and an improved universally usable digital phase measuring equipment for a large frequency range is particularized. The various pehnomena, e.g. reflection, refraction, and wave guiding, that are occurring at VLF and LF wave propagation are described and explained. A detailed description of a long-time registration of commercial VLF transmitters is given.

Wave Propagation between Earth and Ionosphere Taking into Account Different Regions of Ground Conductivity and a Variable Height of the Ionosphere.

Abstract: : It has been reported that VLF signals are sometimes damped severely when crossing the ice of Greenland under day light conditions. This anomalous attenuation occurs simultaneously with a strong proton flux coming from the sun which is known to increase the ionization of the D-layer. In order to find a theoretical explanation of this effect, in the First Year Technical Report the propagation of the field of an electric dipole between earth and ionosphere is investigated. The model proposed consists of a spherical wave guide; the surface conductivity of the inner sphere corresponds to that of sea water except for a trapezoidal domain covered by ice, while the outer surface corresponds to the boundary of the ionosphere. (Author)

Collisional losses in a very-low-frequency duct associated with the lower-hybrid-resonance frequency

Abstract: Calculations of loss in a multi-ion horizontal duct that may trap and guide VLF waves indicate that losses are small for some configurations and not for others. Under appropriate conditions signals can be guided horizontally for hundreds of kilometers without excessive attenuation.

An experimental investigation of the power spectrum of phase modulation induced on a satellite radio signal by the ionosphere

Abstract: The power spectrum of phase modulation imposed upon satellite radio signals by the inhomogeneous F-region of the ionosphere (100 - 500 km) was studied. Tapes of the S-66 Beacon B Satellite recorded during the period 1964 - 1966 were processed to yield or record the frequency of modulation induced on the signals by ionospheric dispersion. This modulation is produced from the sweeping across the receiving station as the satellite transits of the two dimensional spatial phase pattern are produced on the ground. From this a power spectrum of structure sizes comprising the diffracting mechanism was determined using digital techniques. Fresnel oscillations were observed and analyzed along with some comments on the statistical stationarity of the shape of the power spectrum observed.

Interpretation of VLF phase data

Abstract: The specific applications of very low frequency phase tracking are described. The requirements for correct interpretation of very low frequency phase data are defined. The effects of the lower ionosphere and the ground along the path of signal propagation are analyzed. The following subjects are discussed: (1) interpretation equipment, (2) representation of very low frequency waves, (3) diurnal effects and mode interference phenomena, (4) antipodal interference, and (5) overall effects resulting from solar flares, galactic X-rays, and geomagnetic parameters.

Effect of a Large Amplitude Wave Packet and Second Order Resonance on the Stimulation of VLF Emissions

Abstract: This is an extension of a model of VLF emissions triggered by whistlers or man-made signals discussed by Das (1968), and a short introduction will be needed. Following the work of Kennel and Petschek (1966) a background noise level was assumed in the model;then the effect of a whistler mode wave packet on the particles was studied, which can be compared to pitch angle diffusion. The concept is equivalent to that of the quasilinear theory by Vedenov et al (1961, 1962), Engel (1965), Andronov and Trakhtengerts (1964), and Lutomirskiand Sudan (1966), but the problem was treated in a slightly different way which is applicable to the physical situation.

Long range navigation utilizing VLF transmissions

Abstract: The highly stable propagation characteristics of VLF transmissions make them an ideal source of world-wide navigation data The arrival phase of a VLF signal is continuously compared with a local standard The change in the phase difference between the received signal and the local standard is converted to relative motion with respect to the VLF transmitter Very low frequency transmissions form the basis of a very accurate long range navigation system The characteristic of these transmissions that make them ideal for such a use is that their arrival phase at a point distant from the transmitter is both stable and predictable