Showing papers on "Very low frequency published in 1970"
TL;DR: The first direct evidence of magnetospheric ducts was obtained from the Stanford University broadband VLF experiments on Ogo 1 and Ogo 3 as mentioned in this paper, where five discrete ground whistler ducts were encountered by Ogo3 on the inbound pass of June 15, 1966, between L = 4.7 and 4.1.
Abstract: While the great majority of ground whistlers are interpreted as indirect evidence of magnetospheric ducts, the first direct evidence of ducts was obtained from the Stanford University broadband VLF experiments on Ogo 1 and Ogo 3. Five discrete whistler ducts were encountered by Ogo 3 on the inbound pass of June 15, 1966, between L = 4.7 and 4.1. Each duct was characterized by reception at the satellite of ducted whistlers with a distinct spectral shape, and of the high-frequency portions of whistlers (leakages) that propagated inward from outer ducts. The data were interpreted in detail by ray tracing in a model magnetosphere that includes ducts of enhanced ionization. The following conclusions resulted: (1) the L shell thicknesses of the observed ducts ranged between 0.035 and 0.070 earth radii, and the interduct separations ranged between 0.017 and 0.18 earth radii; (2) the dimension of the ducts in longitude was estimated to be of the order of 4°, or 0.3 earth radii at the equator, which is a factor of ∼4–8 greater than the L shell dimension; (3) the whistler ducts are much more likely to be enhancements than troughs; (4) the minimum enhancement factors needed to trap frequencies up to half the equatorial electron gyrofrequency are on the order of 8%, with smaller values producing upper cutoffs at lower frequencies; (5) the limited spreading of the calculated leaked rays is in general agreement with the corresponding regions of observation and relative signal amplitudes; (6) the low cutoffs of leaked signals are probably due to accessibility; (7) cyclotron and Landau interactions are likely to play a role in upper cutoffs of leaked signals; (8) the upper cutoff of ground whistlers near fH0/2 is a trapping (rather than absorption) effect; (9) the hydrostatic type of distribution of ionization along the field lines is applicable in the plasmasphere; and (10) the travel times (and frequency of minimum delay) of ducted whistlers can be calculated with good accuracy by assuming purely longitudinal propagation.
210 citations
01 Jan 1970
TL;DR: In this article, a phenomenological theory of discrete emissions has been advanced, based on cyclotron resonance between energetic electrons and narrow band whistler mode waves traveling along the static magnetic field.
Abstract: Very low frequency whistler mode noise from the magnetosphere frequently appears in the form of narrow band tones of variable frequency called discrete VLF emissions. These tones may appear spontaneously or they may be triggered by transmissions from ground based VLF stations (Helliwell, 1965). They can be observed from within the plasmasphere out to the magnetopause and at frequencies from 300 to 30000 Hz. A phenomenological theory of discrete emissions has been advanced, based on cyclotron resonance between energetic electrons and narrow band whistler mode waves traveling along the static magnetic field (Helliwell, 1967). An important feature of this theory is the maintenance of oscillations through feedback between the waves and the electrons over a path that is long compared with the wavelength.
34 citations
TL;DR: The mean particle distribution functions observed between 100 and 400 km at L∼37 are in agreement with other measurements made in the same energy range and at the same L value; their measurements extend the results toward lower energies and lower altitudes as mentioned in this paper.
Abstract: The mean particle distribution functions observed between 100 and 400 km at L∼37 are in agreement with other measurements made in the same energy range and at the same L value; our measurements extend the results toward lower energies and lower altitudes The proton energy distribution is fitted by two experimental curves with e-folding energies E0∼50 kev up to energies of the order of 400 kev and E0∼1 Mev above The same is generally true for the electron distribution, although the break in the spectrum is less marked, the e-folding energies being E0∼20 kev up to 100 kev and E0∼100 kev above The ratio J⊥/J∥ is usually smaller for protons than for electrons, especially for one flight during which it was smaller than 1 Some characteristics of the particle fluxes have been observed in relation to VLF emissions: a flux increase of the newly injected electrons, both trapped and precipitated, is observed during dawn chorus events A highly characteristic fluctuation (T∼80 sec) of the trapped electron intensities has been detected in coincidence with a VLF periodic hiss emission and a hydromagnetic wave of the same periodicity This phenomenon is interpreted in the frame of reference of the quasi-linear theory as a pitch-angle scattering associated with VLF emission
21 citations
TL;DR: A PRELIMINARY study of knee-whistlers observed in the late morning of a quiet day, July 26, 1967, by means of a wide-band VLF goniometer receiver at Halley Bay has revealed the existence of marked longitudinal variation just outside the plasmapause.
Abstract: A PRELIMINARY study of knee-whistlers observed in the late morning of a quiet day, July 26, 1967, 1206–1236 UT (Kp = 2–), by means of a wide-band (0.5 to 11 kHz) VLF goniometer receiver at Halley Bay (333.4° E of N, 75.52° S, generalized invariant latitude at 100 km: Λ′ = 61.02°) has revealed the existence of marked longitudinal variation (∼3: 1) in electron tube content on an L-shell (L ≃4.7–4.8) just outside the plasmapause.
19 citations
TL;DR: In this paper, the authors used ionization models for phase disturbances, HF absorption and solar protons in 1967 PCA events, using ionization model, and showed that VLF phase disturbances and HF absorption are correlated.
Abstract: VLF phase disturbances, HF absorption and solar protons in 1967 PCA events, using ionization model
16 citations
TL;DR: In this article, the attenuation suffered by ELF and VLF radio waves propagating under ionospheres having realistic variations of electron density and collisional frequency with height was investigated.
13 citations
12 citations
TL;DR: The results of full-wave calculations indicate that transpolar VLF waves suffer considerably more attenuation during a moderately strong polarcap absorption (PCA) event than during the undisturbed daytime.
Abstract: The propagation of electromagnetic waves in the VLF range in the earth-ionosphere waveguide has been theoretically analyzed. The results of full-wave calculations indicate that transpolar VLF waves suffer considerably more attenuation during a moderately strong polarcap absorption (PCA) event than during the undisturbed daytime. Computed height profiles of dissipation and field strengths show clearly that this increased attenuation is due mainly to ohmic losses associated with ion heating. Calculations based on a model of a relatively weak PCA event indicate no pronounced increase in attenuation at most VLF frequencies. For this model, ions are found not to be an important factor. The calculations do not include the possible effects of the Greenland ice cap. During a moderately strong PCA event, the height range over which long-path VLF waves interact strongly with the ionosphere is substantially lower than in the undisturbed daytime. For all ionospheric models considered, the width of the height range of substantial wave-ionosphere interaction at a given frequency was less than about 20 km. VLF probing experiments must therefore use several frequencies and incidence angles if a broader ionospheric height range is to be sampled effectively.
11 citations
TL;DR: Night-time propagation of very low frequency waves in the terrestrial ionosphere shows considerable variability, both in phase and in amplitude, in comparison with the relatively stable daytime conditions.
Abstract: NIGHT-TIME propagation of very low frequency (VLF) waves in the terrestrial ionosphere shows considerable variability, both in phase and in amplitude, in comparison with the relatively stable daytime conditions. The night-time anomalies are not yet well understood, a number of them being associated with the presence of energetic particles originating from solar proton events1. The explanation of others remains obscure.
11 citations
TL;DR: In this article, three rockets were launched along the magnetic field lines, up to an altitude of ∼400 km, to study the wave-particle interactions that take place during natural VLF emissions.
Abstract: Three rockets were launched along the magnetic field lines, up to an altitude of ∼400 km, to study the wave-particle interactions that take place during natural VLF emissions. The group delay time was measured for the structured elements of dawn chorus, using a correlation technique. This time is shown to be in good agreement in the ascending part of the flight with the theoretical one deduced from the Appleton-Hartree formula with the use of simultaneous ionospheric soundings giving the electron density above the station. Changes at the end of the flight show discontinuities in the electron density profiles and set an upper limit to the area on which VLF signals detected on the ground are coherent. From the intensity measurements, comparison is made with the theory of attenuation and reflection of VLF waves (f<2 kHz) when passing through the ionosphere. A general agreement was found between the theoretical and experimental results, as far as the shape of the amplitude variation as a function of height is considered. Large discrepancies in the absolute values are observed which are attributed to oblique incidence of the waves. Polarization measurements permit us to ascribe a circular polarization to these waves even at VLF, except in the lower ionosphere (h<120 km) where it shows a tendency to become a elliptical.
30 Jun 1970
TL;DR: In this article, the authors developed a digital computer model for predicting the characteristics of VLF atmospheric noise, which was used to compute electromagnetic power radiated from every region of the earth.
Abstract: : This report describes the development of a digital computer model for predicting the characteristics of VLF atmospheric noise. Five algorithms which form the basis for the model are: (1) An algorithm for computing electromagnetic power radiated from every region of the earth; (2) An algorithm for computing the standard deviation of power radiated for every region of the earth; (3) An algorithm for the propagation of energy within the earth-ionosphere waveguide; (4) An algorithm which uses the results of 1, 2, and 3 to compute noise field intensities (E sub v), standard deviations (sigma N), V sub d's and direction of arrival values for any location on the earth's surface, for any hour, any month and at any frequency from 10 to 30 kHz. (5) An algorithm to take the output of 4 and prepare contour plots, polar plots, diurnal plots, frequency interpolation plots and data inputs to computer programs for communications system analysis. The prediction model was used to compute E sub v, sigma N and V sub d at 20 kHz, in a 1 kHz bandwidth, for every 10 degrees longitude and every 10 degrees latitude (from 80 degrees S to 80 degrees N). These were computed for each hour and month and were used in the preparation of contours for noise prediction maps. The calculations, contour plots, etc. were used in the preparation of a VLF atmospheric noise prediction manual to be published by the United States Government. Calculated values from the prediction model were compared with measured values of E sub v and generally agreed with + or - 3 dB; sigma N within + or - 1 dB; and V sub d within + or - 2 dB. (Author)
TL;DR: In this paper, the effects of various choices of ion collision frequencies and molecular weights on the wave propagation parameters of wave propagation have been investigated and compared with atmospheric conductivity measurements, and it has been shown that ion collision frequency that exceeds the collision frequency between neutral particles and of molecular weights that are larger than the molecular weights for the usually assumed atmospheric constituents.
Abstract: Calculated parameters of wave propagation illustrate the effects of various choices of ion collision frequencies and molecular weights. Comparisons with atmospheric conductivity measurements tend to justify the use of ion collision frequencies that exceed the collision frequency between neutral particles and of molecular weights that are larger than the molecular weights for the usually assumed atmospheric constituents.
TL;DR: In this article, the generalised thin-film optical method is used to analyze propagation phenomena in a typical night-time ionosphere at frequencies in the range 10-300 kHz, the critical frequency being 24 kHz.
04 Aug 1970
TL;DR: In this article, the authors compared the computed electric field strengths obtained for the two models when using the same input parameters, and found that the models as they now exist do not produce exactly the same computational results and the degree of difference between the two computations is dependent upon propagation frequency and the electron density profile used for the ionosphere.
Abstract: : Several mathematical models for describing VLF radio wave propagation in the earth-ionosphere waveguide have been presented in the literature. The Wave Hop model and the Waveguide mode model are investigated. The computerized versions of these propagation models are examined by comparing the computed electric field strengths obtained for each model when using the same input parameters. It is found that the two models as they now exist do not produce exactly the same computational results and the degree of difference between the two computations is dependent upon propagation frequency and the electron density profile used for the ionosphere. (Author)
TL;DR: In this paper, the authors present the results of a theoretical analysis of the propagation of electromagnetic waves in the very-low-frequency (VLF) and extremely-lowfrequency (ELF) ranges in the earth-ionosphere cavity during solar flares.
Abstract: This study presents the results of a theoretical analysis of the propagation of electromagnetic waves in the very-low-frequency (VLF) and extremely-low-frequency (ELF) ranges in the earth-ionosphere cavity during solar flares. The calculations, which are based on nominal models of ambient conditions and solar flare X-ray fluxes and spectra, result in mode-attenuation coefficients that are in good general agreement with observations. The propagation is enhanced relative to ambient conditions at ELF and upper VLF frequencies but is degraded in the lower VLF range. This general behavior is quite insensitive to the details of the assumed X-ray spectrum; its main requirement is a sufficient concentration of X-ray energy in the 1- to 8-A wavelength band. This class of spectrum tends to cause a moderate lowering of the D region, accompanied by an increase in the height gradient of conductivity in the reflection region; this in turn causes the observed phenomena at ELF and VLF during sudden ionospheric disturbances.
01 Apr 1970
TL;DR: In this article, mode-scattering coefficients arising from ionospheric perturbations at sunrise and sunset lines are reported, based on an analysis that exploits the concept that radio waves at low frequencies propagate in a nonuniform, spherical, ''Earth-ionosphere waveguide? with finite, surface-impedance boundaries''.
Abstract: Computations of mode-scattering coefficients arising from ionospheric perturbations at sunrise and sunset lines are reported in the paper. The computations are based on an analysis that exploits the concept that radio waves at low frequencies propagate in a nonuniform, spherical, `Earth-ionosphere waveguide? with finite, surface-impedance boundaries. The computed data are compared with available experimental data at 18kHz for both day-night and night-day propagation paths. The computed data for v.l.f. radio frequencies indicate that, for an ionospheric effective-height change of 20km, the magnitudes of the first-second and second-first mode-conversion coefficients pass through minima about 16kHz provided that the length of the transition region is less than 800km.
TL;DR: In this article, the theoretical and experimental effects of mutual resistance on array efficiency gain over one dipole are presented and the antenna radiation pattern needed to get omnidirectionl coverage at VLF is presented.
Abstract: Horizontal VLF transmitting dipole arrays are much easier and less expensive to construct than are the vertical VLF transmitting antennas in use at the present time. The horizontal dipoles have much greater power-radiating capability and bandwidth. The theoretical and experimental effects of mutual resistance on array efficiency gain over one dipole are presented. The radiation efficiency is greatly increased by increasing the wave velocity along the resonant dipole. Radiation characteristics of a theoretical 18-dipole array on Hawaiian lava are shown. The antenna radiation pattern needed to get omnidirectionl coverage at VLF is presented. It is shown that directivity is needed because of the nonreciprocal east-west propagation attenuation.
01 Jan 1970
TL;DR: The final author version and the galley proof are versions of the publication after peer review that features the final layout of the paper including the volume, issue and page numbers.
Abstract: • A submitted manuscript is the version of the article upon submission and before peer-review. There can be important differences between the submitted version and the official published version of record. People interested in the research are advised to contact the author for the final version of the publication, or visit the DOI to the publisher's website. • The final author version and the galley proof are versions of the publication after peer review. • The final published version features the final layout of the paper including the volume, issue and page numbers.
TL;DR: In this paper, a narrow band VLF receiver is used to collect electron density measurements in lower ionosphere by narrow-band VF receiver flown in Tomahawk rocket during quiet daytime.
01 Jan 1970
TL;DR: In this article, a very low frequency phenomenon associated with the propagation of proton whistlers in the upper ionosphere has been found in the VLF recordings from the Injun 3 and Alouette satellites.
Abstract: A NEW very low frequency phenomenon associated with the propagation of whistlers in the upper ionosphere has recently been found in the VLF recordings from the Injun 3 and Alouette satellites1. This phenomenon showed up on a frequency–time spectrogram as a rising tone which started immediately after the reception of a short fractional-hop whistler. The spectrogram showed a rapid rise in frequency with an asymptotic approach to the cyclotron frequency for the protons in the plasma surrounding the satellite. These signals have been called proton whistlers. Analysis has shown that they are excited because of mode coupling or conversion from the electron whistler mode with right-hand polarization to the ion cyclotron wave with left-hand polarization2. The frequency at which polarization reversal or coupling occurs has been called the crossover frequency, which is below the ion cyclotron frequency. More recently it was reported that the ion wave at a helium cyclotron frequency has been observed in addition to the proton whistler in one of the Alouette II records3.
TL;DR: In this paper, the distortion of short, quasi-monochromatic VLF pulses by dispersion and multimode propagation in the earth-ionosphere waveguide was studied, and the delays between received pulses on three waveguide modes were calculated with a β = 0.5 ionospheric electron density profile having an ionosphere height of 70 km (day) and 90 km (night).
Abstract: The distortion of short, quasi-monochromatic VLF pulses by dispersion and multimode propagation in the earth-ionosphere waveguide is studied. The delays between received pulses on three waveguide modes are calculated with a β = 0.5 ionospheric electron-density profile having an ionospheric height of 70 km (day) and 90 km (night). The fractional spreading of short pulses caused by dispersion on individual modes is also calculated. Delays of up to 0.7 msec between pulses arriving on different waveguide modes were obtained. The dispersive spreading on the important modes is only a few percent for a 2-msec pulse.
TL;DR: In this paper, the role of multiple reflections is demonstrated by determining the error introduced by a first order calculation of reflection coefficients and by the W.K.B. calculation of transmission coefficients.
30 Jun 1970
TL;DR: In this paper, the VLF stmospheric noise prediction model described by Maxwell, et al. was used to compute the expected direction of arrival of atmospheric noise energy and the energy arriving in each 10 degree azimuthal sector was determined and was related to effective field intensity in dB relative to one microvolt per meter in a one kilohertz bandwidth.
Abstract: : The VLF stmospheric noise prediction model described by Maxwell, et al was used to compute the expected direction of arrival of atmospheric noise energy. The energy arriving in each 10 degree azimuthal sector was determined and was related to effective field intensity in dB relative to one microvolt per meter in a one kilohertz bandwidth. In other words, if the receiving antenna has unity gain over a 10 degree sector and zero gain in all other directions, the field intensity indicated on these plots would be observed.
01 Jan 1970
01 Jan 1970
TL;DR: Polar ionosphere auroral oval position detection by satellite observations of naturally occurring VLF and man-made HF plasma waves is described in this paper, where satellite observations are used to detect the position of the polar ionosphere oval.
Abstract: Polar ionosphere auroral oval position detection by satellite observations of naturally occurring VLF and man-made HF plasma waves
01 Dec 1970
TL;DR: Measurements of urban, suburban, and rural radio frequency noise interfering with aircraft communication have been carried out in this article, showing that urban and suburban radio frequency noises interfere with aircraft communications.
Abstract: Measurements of urban, suburban, and rural radio frequency noise interfering with aircraft communication
TL;DR: In this article, the reflection properties of typical D - and E -region models for radiowave frequencies varying from 16 kHz (VLF) to 3.0 MHz (HF) have been calculated by full wave, ray tracing and phase integral methods.