Showing papers on "Very low frequency published in 1971"
215 citations
TL;DR: The VLF-E.M. (Very Low Frequency-Electro Magnetics) method was first used for geophysical surveying in 1964 as mentioned in this paper, and many hundreds of surveys have been carried out; several variations on the original design have been introduced; and the method has become airborne.
121 citations
TL;DR: In this paper, a theory is presented to explain the origin of triggered discrete VLF emissions that is more complete than earlier theories, in that it is not restricted to the discussion of kinematical relations but evaluates the dynamics of the problem.
Abstract: A theory is presented to explain the origin of triggered discrete VLF emissions that is more complete than earlier theories, in that it is not restricted to the discussion of kinematical relations but evaluates the dynamics of the problem. Resonant electrons are phase correlated with the wave magnetic field by a finite length whistler train moving in the opposite direction. The time for phase correlation is of the order of the period of oscillation of a particle in the effective ‘potential well’ of the wave. It is recognized that the wave acceleration due to the inhomogeneous magnetic field of the earth must be small enough for the particle to stay trapped in the potential well. The phase-correlated electrons are subject to an instability in the form of an emitted whistler with a growth rate γ/ω ∼ (n/no)2/5(υ⊥/c)2/5 (ωp/Ω)2/5, where (n/no) is the fractional density of the resonant particles, υ⊥ is their mean transverse velocity, and ωp and Ω are local cold plasma and gyrofrequencies. The emitted frequency varies according to ω = k(ω)υ∥ − Ω, where υ∥ is the zero order longitudinal velocity of the resonant electron, and the wave vector k is a function of frequency ω through the whistler dispersion relation. The theory is in good agreement with observation.
120 citations
TL;DR: In this paper, the diurnal and latitudinal occurrence and intensity patterns of ELF, VLF, and LF whistler-mode noise emissions were observed with the Alouette 2 satellite.
Abstract: Statistical studies have been made of the diurnal and latitudinal occurrence and intensity patterns of ELF, VLF, and LF whistler-mode noise emissions observed with the Alouette 2 satellite. The ELF emissions occur characteristically below 1 kHz, are peculiar to the daylight hours, and show a peak in average intensity in the invariant latitude range of 50°–70°. In the VLF range, broadband emissions extending upward from the LHR frequency show a maximum average intensity near local noon and a Λ of about 77°. Along the auroral oval toward earlier and later hours, they show a decreasing intensity. The apparent high-frequency end of this emission band extends upward from 100 kHz to the local fN or fH frequency in the LF range. The LF emissions were studied at a fixed frequency of 200 kHz, and maximum intensities were found along the day and evening portions of the auroral oval. This type of noise also shows a latitude dependence on Kp similar to that of the auroral phenomena. A second region of peak intensity, at about the position of the nighttime plasmapause, appears in the LF statistical results, but a complete study has not been made of that region.
70 citations
TL;DR: In this article, a self-consistent treatment of this process enables us to compute the frequency spectrum of these waves, assuming that the energy diffusion of electrons occurs in a characteristic time much larger than the time required for pitch-angle diffusion.
45 citations
TL;DR: In this paper, the frequency of the key-down signals from NAA (14.7 and 17.8 kHz) was recorded at Eights station, Antarctica, during unmodulated (?key-down') transmissions, each pulsation lasting up to 2 min.
Abstract: Whistler-mode signals from station NAA (14.7 and 17.8 kHz), Cutler, Maine, show periodic fluctuations (?pulsations') in amplitude and bandwidth. The data were recorded at Eights station, Antarctica, during unmodulated (?key-down') transmissions from NAA lasting up to 2 min. In three of four instances, the pulsations consist of a series of moderate enhancements of the amplitude and bandwidth of the signal, each pulsation lasting about 50 msec. The fourth instance, however, was unusual in that the key-down signal exhibited remarkably regular and intense amplitude variations. In all four occurrences, the period of the pulsation was in the range from 0.3 to 0.6 sec. In three occurrences, this period was roughly the same as the one-hop whistler-mode delay along the field-line path; however, no demonstrable mechanism to explain this association could be found. An explanation of pulsations in terms of multipath fading effects could not be supported by the data. More likely explanations include intrinsic oscillation in the emission generation mechanism, natural oscillation in the energetic-particle population, or modulation of the VLF growth rate by Pc 1 micropulsations in the region of wave growth.
37 citations
TL;DR: Injun 5 data from the Injun 5 satellite were used in this article to analyze the color frequency-time spectrograms of VLF electric and magnetic field electric fields.
Abstract: Color frequency-time spectrograms of VLF electric and magnetic field Poynting flux data from Injun 5 satellite
36 citations
TL;DR: In this paper, a theoretical study of cyclotron instability both in linear and nonlinear regimes in conjunction with the generation of VLF emissions in the magnetosphere is made, and it is shown that the change of frequency with time of whistler triggered emissions as well as characteristics of ASE are well explained by resonant nonlinear behaviour of cycloton instability considered in the present paper.
34 citations
TL;DR: VLF Poynting flux measurement technique on Injun 5 satellite using one electric and one magnetic antenna, discussing magnetic orientation errors effects as discussed by the authors, discussed magnetic orientation error effects.
Abstract: VLF Poynting flux measurement technique on Injun 5 satellite using one electric and one magnetic antenna, discussing magnetic orientation errors effects
26 citations
TL;DR: In this article, the attenuation and phase velocity of ELF and VLF radio waves propagating under a realistic anisotropic ionosphere were computed and compared with the existing experimental data except for the heavily attenuated region.
26 citations
TL;DR: In this paper, the null effect was shown to occur when the enhancement of the intensity of the solar soft X-ray flux is small and moreover when the representative solar zenith angle on the VLF propagation circuit is large.
Abstract: IT sometimes happens that even when the flux from a soft X-ray flare on the Sun is sufficient to ionize the neutral constituents of the Earth's atmosphere at the level where very low frequency (VLF) radio waves are reflected, no sudden phase anomaly is observed on VLF propagation circuits that are in the sunlit hemisphere of the Earth, or are in a daytime condition even though part of the circuit is in darkness1,2. This is referred to as a “no SPA effect” or “null SPA effect”. I have explained1,2 the null effect in terms of waveguide theory using the exponential ionospheric model3 and I concluded that the null effect would occur when the enhancement of the intensity of the solar soft X-ray flux is small and moreover when the representative solar zenith angle on the VLF propagation circuit is large. At that time I predicted the existence of “negative SPAs” when the conditions that give rise to the null effect are strongly developed2.
TL;DR: In this article, the frequency spacing between the interference bands is given by the inverse of the time interval between the lightning strokes, which accounts for a number of peculiar characteristics which could not be accounted for with the gyrofrequency harmonic interaction hypothesis.
Abstract: : Whistlers received with the Injun 5 satellite are frequently observed to have bands with decreased signal amplitude at equally spaced frequency intervals. The frequency spacing between the bands is typically about 10 to 30 Hz. As many as 30 such bands have been observed on a single whistler. Subsequent investigations have shown that the bands are instead due to double or multiple strokes in the initial lightning discharge which result in destructive interference of the whistler signal at equally spaced frequency intervals. The frequency spacing between the interference bands is given by the inverse of the time interval between the lightning strokes. This simple explanation for these bands accounts for a number of peculiar characteristics which could not be accounted for with the gyrofrequency harmonic interaction hypothesis.
TL;DR: In this paper, the phase recovery rates differ for the long and short propagation paths monitored and the phase anomaly on the long path persists even after the X-ray flux has returned to its pre-flare value, suggesting a relatively slow recombination rate for the flare enhanced ionization in the lowest region of the ionosphere from which these waves are reflected.
TL;DR: The Pioneer 8 observations of magnetosheath and interplanetary VLF electric fields, consisting of hourly ranges of the potential amplitude in a broadband channel (0.1 to 100 kHz) and in a 15% bandpass channel centered on 400 Hz, were presented in this article.
Abstract: We present Pioneer 8 observations of magnetosheath and interplanetary VLF electric fields, consisting of hourly ranges of the potential amplitude in a broadband channel (0.1 to 100 kHz) and in a 15% bandpass channel centered on 400 Hz. Significant signals are correlated with position with respect to earth, and with solar wind plasma parameters obtained from the lunar orbiting Explorer 35 spacecraft. We detect two principal features: noise bursts or spikes with duration less than approximately 10 sec, and persistent signals with durations typically 1 day or more. The noise bursts coincide with plasma and magnetic field discontinuities where these data are available for comparison. The persistent signals correlate loosely with solar wind density, and this correlation holds whether the density increases are due to interplanetary shocks, the ‘snow plow’ effect, or other processes. Although the experiment is too limited to provide unambiguous determination of the wave modes, at present it appears most likely that ion acoustic waves have been detected.
TL;DR: In this paper, it is suggested that the integrated effect of all the X-ray sources illuminating the southern hemisphere is to enhance the ionization in the Southern Hemisphere nighttime D-region, thus, producing the smaller diurnal phase change observed at Tucuman compared with that observed on a Boulder recording of the NBA signal over a propagation path of the same length in the northern hemisphere.
Abstract: In recent years, measurements by rocket, balloon, and satellite of cosmic X rays have suggested the possibility of observing ionospheric effects from celestial X-ray sources such as Scorpius XR-1, using low and very low frequency radio transmissions. The behavior of the nighttime D region under ionization by the X-ray spectrum of the Scorpius source is estimated for several stellar zenith angles. Although Scorpius XR-1 is the strongest source in the southern hemisphere, the integrated flux of all the other X-ray sources is comparable to that of Scorpius XR-1 and appears to modify the effect of Scorpius XR-1 alone on VLF data recorded in the southern hemisphere. Observations of the VLF transmission NBA (18 kHz, Balboa, Panama) made at Tucuman, Argentina, during 1963–1964 show a sidereal time variation in the diurnal amplitude change that follows the variation in the per cent illumination of the propagation path by Scorpius XR-1 during the nighttime hours. Even though the diurnal amplitude change can be shown to have a sidereal time variation, the diurnal phase change at Tucuman, Argentina, does not vary much throughout the year and yet remains consistently lower than that observed over a path of the same length from NBA to Boulder, Colorado, in the northern hemisphere. Therefore, it is suggested that the integrated effect of all the X-ray sources illuminating the southern hemisphere is to enhance the ionization in the southern hemisphere nighttime D-region, thus, producing the smaller diurnal phase change observed at Tucuman compared with that observed on a Boulder recording of the NBA signal over a propagation path of the same length in the northern hemisphere.
TL;DR: In this article, the effects of auroral activity on the VLF signal NPM 23.4 kHz received in Kiruna are discussed, and the observations are interpreted in terms of the isotropic propagation model of Wait and Spies.
TL;DR: In this paper, the first time riser whistlers were observed in a ground station was during the routine recording of whistlers at a low latitude station, the High Altitude Research Laboratory, Gulmarg (geomagnetic latitude 24° 10′ N, geomagnetic longitude 147° 24′ E).
Abstract: THIS communication reports riser whistlers observed for the first time in a ground station. They were noticed during the routine recording of whistlers at our low latitude station, the High Altitude Research Laboratory, Gulmarg (geomagnetic latitude 24° 10′ N, geomagnetic longitude 147° 24′ E). Because these whistlers appear as rising tones in the sonagrams, they resemble the “riser whistlers” reported by Pfeiffer et al.1, from the Injun III satellite observations.
TL;DR: In this paper, it was shown that a variety of ion waves in the ELF and VLF bands can become unstable in the polar wind when the relative flow velocity between the hydrogen ions and oxygen ions enters the range 5-10 km/sec.
Abstract: Considerations based on plasma kinetic theory suggest that a variety of ion waves in the ELF and VLF bands can become unstable in the polar wind when the relative flow velocity between the hydrogen ions and oxygen ions enters the range 5–10 km/sec. Peak growth rates of ∼100 sec−1 seem achievable for wavelengths typically 10 times the Debye length (wavelengths of ∼2 meters in the polar wind). The threshold flow velocity for instability generally decreases as altitude increases within the altitude range 2000–5000 km, where the waves are expected to appear. These numerical estimates are based on a heuristic calculation for a specific instability (ion-acoustic). Other ionic two-stream instabilities (e.g., ion-cyclotron) belonging to the more general class may be equally important in the earth's polar wind. Electrostatic waves in the appropriate frequency range have been observed at altitudes 2500–4500 km over the polar regions by a VLF receiver on the OV3-3 satellite. These waves may be important in impeding the escape of polar-wind hydrogen and helium ions.
TL;DR: In this paper, a VLF receiver was used to detect broadband electrostatic emissions from the satellite OV3-3 (1966-70A) at frequencies between 400 Hz and 14.5 kHz at altitudes between 2500 and 4500 km.
Abstract: : Broadband electrostatic emissions are detected by a VLF receiver aboard the satellite OV3-3 (1966-70A). Enhancements of the electric-field intensity in ten bandpass channels at frequencies between 400 Hz and 14.5 kHz are observed when the spacecraft passes over the south polar region at altitudes between 2500 and 4500 km (satellite apogee). The power spectral density of these emissions decreases approximately as the inverse fourth power of the observed frequency. The most interesting feature of these emissions is the range of invariant-magnetic-latitudes (INVL) over which they are observed. The low-latitude limit at which they are detected is typically 72 to 76 degrees INVL in local morning and 68 degrees INVL near midnight. This dependence suggests that these emissions occur on magnetic-field lines along which the polar wind flows. During geomagnetically active periods, electrostatic waves with a high-frequency cutoff between 1.3 and 3.9 kHz are also observed in this altitude range. These emissions occur at latitudes below the low-latitude limit of the polar-electrostatic waves and above the latitude of the plasmapause. (Author)
TL;DR: In this paper, the power spectral density of continuous-phase minimum-bandwidth waveforms composed from a set of N mutuallyorthogonal chips (pulsed sinusoids of equal amplitude and duration and constant frequency) is derived.
Abstract: The power spectral density of continuous-phase minimum-bandwidth waveforms composed from a set of N mutuallyorthogonal chips (pulsed sinusoids of equal amplitude and duration and constant frequency) is derived. The spectrum and its derivation for this class of signals are of particular interest in highpower extremely low frequency/very low frequency (ELF/VLF) communications where noise immunity is a prime consideration, but the signal bandwidth and transients at the transmitting antenna must be minimized. Exact formulas are also given without derivation for wider bandwidth signals in the same class but of less practical importance. Although the power spectra of more general binary signals are already known, the special case presented here permits a much simpler derivation than that given in an earlier paper [1]. In the N -ary case a previously derived formula for the spectra of continuous-phase multilevel FM signals [2] is augmented and evaluated to explicitly yield new spectral formulas useful in describing the frequency-hopping mode.
01 Jan 1971
TL;DR: In this paper, the results of analyses of the regular features in these records, such as the diurnal and seasonal variations, the sunrise and the sunset effects, the afternoon hump5 and the midnight fall6,7, have been reported previously.
Abstract: INTEGRATED intensities of VLF atmospherics in the 27 kHz band have been recorded at this station since 1959 and in the 10 and 1 kHz bands since 1962. The results of analyses of the regular features in these records, such as the diurnal and seasonal variations1, the sunrise and the sunset effects2–4, the afternoon hump5 and the midnight fall6,7, have been reported previously. A new phenomenon occurring at random was observed in these records from the very beginning but its analysis was postponed until there were sufficient data for its complete understanding. In this communication we report the chief results of this analysis.
TL;DR: In this paper, it is shown that the VLF waves propagating at non-zero angles with the particle beam are amplified by the magnetosphere's magnetic field, and that a charged particle beam is assumed to move through magnetosphere.
TL;DR: In this article, a full-wave method was used to examine the internal reflection of very low frequency radio waves incident from above on a model of the nighttime lower ionosphere, and particular attention was paid to the changes occurring as the wave normal direction moves outside the acceptance cone for penetration to the Earth's surface.
TL;DR: In this article, the authors return to the basic nonlinear equations of motion where some near-instability related effect is undoubtedly present, however, they do not seem to produce the kind of oscillation necessary for a modulation effect.
TL;DR: In this paper, the electric field strength of a very low frequency radio signal near selected terrain features of fractional wavelength dimensions, using a specially equipped helicopter, was measured using dipole generated mathematical equipotential surfaces which locally resemble the Earth's surface.
TL;DR: Coupled vacuum mode equations modified for cylindrical stratification for calculation of propagation characteristics of earth-ionosphere waveguide were used in this paper to calculate the propagation characteristics.
Abstract: Coupled vacuum mode equations modified for cylindrical stratification for calculation of propagation characteristics of earth-ionosphere waveguide