Showing papers on "Very low frequency published in 1982"

Theory and computer simulations of magnetospheric very low frequency emissions

Abstract: A theory of magnetospheric VLF emission must account for the following features: (a) the triggering of monochromatic emissions by signals of sufficient strength and duration, while the background noise and weak short signals are not amplified, and (b) the occurrence of frequency changes after the emissions have reached a sufficiently large amplitude. A nonlinear mechanism exhibiting these features, with fixed and varying frequencies, is examined analytically and by computer simulation techniques. This mechanism depends on a simultaneous propagation and amplification of wave packets along geomagnetic lines to maintain the nonuniformity ratio RproportionaldelB/sub 0//B/sub w/ in the regime Vertical BarRVertical Barroughly-equal0.5, corresponding to maximum amplitication. (B/sub 0/ is the geomagnetic field and B/sub w/ is the wave magnetic field.) For a constant frequency, this condition yields triggering thresholds which are related to the properties of the magnetosphere. For a varying frequency ..omega..(t), it yields the condition partial..omega../partialtproportional..omega../sup 2//sub t/ for the large-amplitude portion of the risers, where ..omega../sub t/proportionalB/sup 1/2//sub w/ denotes the trapping frequency of the wave.

The influence of localized precipitation‐induced D‐region ionization enhancements on subionospheric VLF propagation

Abstract: A two-dimensional mode propagation model has been used to investigate the effect of localized ionospheric perturbations on the propagation of VLF radio waves in the earth-ionosphere waveguide. Computations have been performed for the NSS transmissions at 22.3 kHz from Annapolis, Maryland to Eights Station, Antarctica, where anomalous short-duration signal amplitude changes, in coincidence with whistlers, were first noted. The calculations suggest that two ionization regions, one very near the transmitter and another near the receiver, may need to be present at the same time in order to obtain the large amplitude increases (>3 dB) that are observed to occur on occasion. It is suggested that the former is due to transmitter-induced electron precipitation, while the latter is due to precipitation associated with the whistler. We also find that small amplitude changes can be obtained under less restrictive ionospheric perturbation conditions.

Stimulated wave-particle interactions during high-latitude ELF wave injection experiments

Abstract: Interactions in the magnetosphere between man-made whistler mode waves and electrons can produce either an enhancement of the wave or the generation of waves at a different frequency. Furthermore, natural emissions can be frequency shifted or modified by a nearby transmitter signal. Injection of extremely low frequency waves into the outer magnetosphere was initiated in 1978 in Norway by both the University of Paris and The Aerospace Corporation. The antenna is a 10.6-km power line tuned to the transmitted frequency. The University of Paris used a 1-kW transmitter to drive the antenna with a maximum current of 8 A. The Aerospace Corporation used the transportable very low frequency transmitter with a typical antenna current between 20 and 45 A. Experiments were conducted at times when the GEOS 2 and SCATHA satellites were near the magnetic meridian of the transmitter. The transmissions consisted of either a keyed fixed frequency or continuous waves swept in frequency. Emissions correlated with the transmissions were detected by the satellite receivers on several dates. Although the satellites were within a few degrees of the magnetic meridian of the transmitter, emissions influenced by the transmissions are seen during only a small fraction (<10%) of the total transmission time. With the first type of transmission, signals similar to power line harmonic radiation were recorded. Artificially stimulated emissions are also likely to have been triggered. Both types of transmissions triggered or enhanced hiss at a constant frequency during fixed-frequency transmissions and at a variable frequency during a swept-frequency transmission. During swept-frequency transmission there occurred two examples of natural emissions shifting in frequency by the man-made signal. Electron cyclotron harmonic emissions also appear to have been shifted in frequency by the man-made signal.

Plasma Waves Stimulated by Electron Beams in the Lab and in the Auroral Ionosphere

Abstract: Energetic electron beams are frequently used as active probes of space plasmas Often the assumed test particle nature of these electrons is violated when the electron beam stimulates plasma wave emissions Such complex phenomena have been observed on rockets and satellites and are being modeled in laboratory plasmas The large vacuum chamber at NASA Johnson Space Center in Houston, Texas has been used for modeling F-region type ionospheric plasmas A VLF receiver has been flown into an auroral plasma and the spectra from this flight will be compared to VLF spectra obtained in the NASA/JSC laboratory chamber The electron beam is believed to have produced beam plasma discharge (BPD) on the rocket similar to that seen in the lab At times during the rocket flight the electron beam was operated at 4 kilovolts and the electron current modulated at 3 kilohertz from 0 to 80 milli-amps This resulted in the beam pulsing in and out of BPD and a variety of propagating wave modes

Characteristics of a power line used as a VLF antenna

Abstract: : The 100-kW Transportable Very-Low-Frequency (TVLF) transmitter system was used at Kafjord, Norway, for transmissions to the SCATHA and GEOS spacecraft. A 22-kV 10.6-km long transmission line was used as an antenna. Modifications were made in the line to reduce telephone interference. Components were designed and installed to reduce the resonant frequency and increase the antenna current. The final practical operating current was 45 amperes at 1280 Hz. This resulted in power dissipation of 72 kW and an estimated radiated power of 0.17 to 0.79 watts. (Author)

Whistler mode interactions with plasma sheet electrons

Abstract: We present a unique set of wave and particle measurements taken by the S3-3 satellite on August 2, 1976, from which we infer that signals from a ground-based VLF transmitter were ducted up to the equatorial plasma sheet region by a detached region at L ∼ 4.5. The waves resonated with the ∼ 1 keV plasma sheet electrons, producing narrowband emissions that were received in the conjugate hemisphere by the S3-3 satellite at L ∼ 3.

Very low frequency atmospheric radio noise field intensities

Abstract: Simultaneous observations of the field strength of atmospheric radio noise at three harmonically related frequencies in the VLF band for a period of two years from October 1976 to September 1978 have been analyzed to explore the frequency spectrum of atmospherics The results have been shown for different months and seasons of the year and interpreted in terms of source activity and propagation effect

Medium, Long and Very Long Wave Propagation (at Frequencies Less than 3000 kHz)

Abstract: : These Proceedings include the papers and discussions presented at the AGARD Electromagnetic Wave Propagation Panel Symposium on 'Medium, Long and Very Long Wave Propagation (at Frequencies less than 3000 kHz)' held in Brussels, Belgium in September 1981. The Meeting reviewed propagation information at ELF and VLF frequencies. It was intended to summarize the current state of knowledge in this frequency band in the areas of propagation, antennas, and radio communications technology, with speculation on trends and future use. There were 37 papers presented, 9 on the propagation medium, 4 on ELF propagation, 6 on VLF propagation, 4 on LF propagation 4 on MF propagation, 3 on numerical modelling of the propagation medium, and 7 on applications. (Author)

Very low frequency (vlf) hiss in jupiter's magnetosphere

Abstract: The particle energy required to generate the observed VLF hiss in the Jovian magnetosphere has been computed under longitudinal and transverse resonance condition. It is shown that the minimum energy required by electrons to generate VLF hiss under the longitudinal resonance condition lies in the range of 100eV–1keV for the wave frequencies of 2–10 kHz, while the corresponding energy range for the transverse resonance condition for the same frequency range comes out to be 8 keV–40 keV. Further, the average radiated power by the Cerenkov process in the Jupiter's magnetosphere atL=5.6 Rj by electrons of energy 10 eV, 100 eV, and 1 keV for the wave frequency of 5 kHz has also been computed.

The point-hemisphere discharge gap as a partial discharge reference source at low frequency (0.1 Hz)

Abstract: For more than a decade the point-hemisphere discharge gap has been accepted and successfully used as a known source of partial discharges In 1966 this source was carefully assessed for use at frequencies between 50 and 1250 Hz by Hogg and Walley With the increasing interest shown both academically and commercially in the use of very low frequency (VLF) 01 Hz experiments, it is now essential that the point-hemisphere discharge source must be reappraised The results of the tests undertaken demonstrate that the point-hemisphere discharge gap is sufficiently reliable to be employed as a secondary standard at very low frequencies, provided it is used under controlled ambient conditions

Laboratory Observation of a Very Low Frequency Instability in an Argon Plasma

Abstract: Very low frequency (50 to 500 Hz) self-excited electrostatic waves are detected in a cylindrical argon plasma column with a weak axial magnetic field (10 to 100 G). The waves propagate azimuthally with a phase velocity in the electron diamagnetic drift direction, but with a speed at least an order of magnitude less than the ion acoustic speed. A number of known plasma instabilities are considered as possible explanations, though most of them do not seem to account for the observed characteristics of the waves.

High-Power Transportable VLF Transmitter Facility

Abstract: : A 100-kW, transportable, very-low-frequency (TVLF) transmitter facility has been used for magnetospheric wave-injection experiments from sites in Alaska, New Zealand and Norway. A unique feature of the TVLF facility is the antenna which is a conducting cable lofted to an altitude of 1000 m by a 1000 cu m3 helium balloon. The antenna is driven at its base as a monopole above a ground plane. The antenna cable also serves as the balloon tether. The lowest operating frequency in this configuration is 6.6 kHz at which the radiated power is 100 W. At the highest operating frequency used in the experiments, 21 kHz, the radiated power is 10 kW. In Norway power lines were used as antennas. The minimum operating frequency was then 1 kHz and the radiated power is estimated to be about 0.5 W. In this report we describe the components and performance of the TVLF as used for these magnetospheric experiments.