Showing papers on "Very low frequency published in 1998"

Subionospheric VLF signal perturbations possibly related to earthquakes

Abstract: A likely VLF subionospheric signal effect related to seismic activity was first reported by Hayakawa et al. [1996a, b] in association with the great Kobe earthquake. We have analyzed similar data during periods around 10 other great earthquakes (magnitude M>6) in order to understand the main features of such an effect. The following characteristics emerged from our analysis: The effect appears as a transient oscillation with a 5- to 10-day period, which is initiated a few days before a large earthquake and decays over a few days to weeks after it. It is mainly related to crustal earthquakes. It appears when resonant atmospheric oscillations with periods in a range of 5–11 days exist before the earthquake. The seismic influence on the VLF signal is probably explained by the generation of long-period gravity waves during the earthquake process and their intensification at heights of 70–90 km.

Ionospheric D region remote sensing using VLF radio atmospherics

Abstract: Lightning discharges radiate the bulk of their electromagnetic energy in the very low frequency (VLF, 3–30 kHz) and extremely low frequency (ELF, 3–3000 Hz) bands. This energy, contained in impulse-like signals called radio atmospherics or sferics, is guided for long distances by multiple reflections from the ground and lower ionosphere. This suggests that observed sferic waveforms radiated from lightning and received at long distances (>1000 km) from the source stroke contain information about the state of the ionosphere along the propagation path. The focus of this work is on the extraction of nighttime D region electron densities (in the altitude range of ∼70–95 km) from observed VLF sferics. In order to accurately interpret observed sferic characteristics, we develop a model of sferic propagation which is based on an existing frequency domain subionospheric VLF propagation code. The model shows that the spectral characteristics of VLF sferics depend primarily on the propagation path averaged ionospheric D region electron density profile, covering the range of electron densities from ∼100 to 103 cm−3. To infer the D region density from observed VLF sferics, we find the electron density profile that produces a modeled sferic spectrum that most closely matches an observed sferic spectrum. In most nighttime cases the quality of the agreement and the uncertainties involved allow the height of an exponentially varying electron density profile to be inferred with a precision of ∼0.2 km.

Impact of modulation frequency on cross-phase modulation effect in intensity modulation-direct detection WDM systems

Abstract: The impact of modulation frequency on the crossphase modulation (XPM) effect in intensity modulation (IM)-direct detection wavelength-division multiplexing systems is investigated theoretically and numerically. A simple expression for IM is derived, verified by simulation and its validity is presented. The dependence of XPM-induced IM on the fiber length, fiber dispersion, channel separation and pump modulation frequency is assessed. It is shown that at very low frequency the walkoff effect has almost no influence on the XPM-induced IM efficiency which increases with the square of the frequency; at higher frequency the IM efficiency can be reduced significantly by the walkoff and scales linearly with modulation frequency.

Very low frequency high voltage sinusoidal electrical testing method, systems and apparatus

Abstract: Very low frequency (VLF) high voltage (HV) sinusoidal electrical test waves provide for testing AC electrical power installations and equipment having large electrical capacitances. VLF HV sinusoidal electrical waves are suitable for testing characteristics and/or qualities of insulation on long, buried electric power cables. Capacitance of a load being tested by VLF HV sinusoidal waves is discharged during a second half of each positive half-wave and during a second half of each negative half-wave by using a sequence of resistive discharge paths. Each successive discharge path in a sequence has less resistance than its predecessor for creating a sequence of progressively-reduced discharge Time Constants. Solenoid-operated switch contacts bring successive discharge paths into action. Also, solenoid-operated switch contacts reverse polarity to create positive and negative half waves of the VLF HV sinusoidal electrical test waves. To avoid inadvertent switch closure, downward spring-bias requires solenoids be energized for overcoming spring force and gravity to close switches upwardly. AC voltage is amplitude-modulated sinusoidally by moving a brush along transformer taps arranged with sine-function voltage differences. Alternatively, such modulation is achieved by a revolving heart-shaped-cam cyclically operating in opposite directions a variable autotransformer brush. Stepping-up amplitude-modulated AC voltage to 40 kilovolts or more, then rectifying and polarity reversing successive half waves creates VLF HV sinusoidal test waves.

VLF signatures of ionospheric perturbations caused by lightning discharges in an underlying and moving thunderstorm

Abstract: A sequence of VLF events (first of early/fast type, then of LEP type), observed at Poitiers on the amplitude of the signal from the GQD/19.02 kHz transmitter in England, is analysed. VLF data, sampled at 100 ms intervals, are combined with sferics and METEORAGE data on lightning ground strikes detected within a ± 50 km belt centered on the VLF path. Both the slowly and rapidly recovering early/fast events are observed in association with intense positive CG flashes, located in the Baie de Seine (L=2.25), at a distance less than 35 km from the VLF path. The LEP events are observed when the storm, initially overlapping the path, moves southeastwards. Some early/fast events are also observed on the very short path HWU/18.3 kHz-Poitiers.

Remote sensing of the electrodynamic coupling between thunderstorm systems and the mesosphere / lower ionosphere

Abstract: In the past few years, dramatic experimental evidence has emerged, showing that tropospheric lightning discharges modify the mesosphere and the lower ionosphere through heating and ionization, producing gamma-ray bursts and optical emissions known as red sprites, blue jets, and elves. These transient electrodynamic coupling processes may have long-term effects such as chemical changes, persistent heating of ionospheric electrons, and increased production of mesospheric and stratospheric nitrogen oxides (NOy). In order to assess the regional and global effects of the intense electrodynamic coupling of thunderstorms to the middle atmosphere, the occurrence rate of Sprites needs to be known over large areas of the Earth. Since continuous optical monitoring of Sprite occurrence on large spatial scales is not practical, a continuous proxy indicator for Sprite occurrence is needed. Sprites are intense, transient luminous events in the mesosphere and lower ionosphere above thunderstorm systems. They extend from ∼40 to ∼90 km in altitude, are primarily red in color, and develop to full brightness in a few milliseconds. Sprites are nearly uniquely associated with positive cloud-to-ground lightning flashes, yet they occur in association with only a small subset of those flashes. The peak current of each flash, measured at high frequencies by the National Lightning Detection Network, is not a sufficient indicator of the likelihood of a positive cloud-to-ground lightning flash to produce a Sprite. In this work, remote sensing of the electrodynamic coupling between thunderstorm systems and the mesosphere and lower ionosphere is accomplished by measurement of radio atmospherics in the ELF (extremely low frequency, here 15 Hz 1.5 kHz) and VLF (very low frequency, here 1.5 22 kHz) ranges. Radio atmospherics (“sferics”) provide a unique signature of each lightning return stroke and propagate efficiently in the waveguide bounded by the Earth’s surface and the ionosphere. Novel digital signal processing techniques for ELF/VLF broadband measurements allow automated

A 25‐year record of 10 kHz sferics noise in Antarctica: Implications for tropical lightning levels

Abstract: We report measured levels of very low frequency (VLF) radio noise at ∼10 kHz, due to lightning sferics, observed at Halley Station, Antarctica (76°S, 27°W) between 1971 and 1996. The observed VLF noise levels at Halley are a product of the thunderstorm source function and the transfer function for propagation to the receiver in the waveguide formed by the Earth's surface and the ionosphere. Least squares fitting enables us to confirm the characteristic diurnal, annual and semi-annual periodicities found by the present authors in a separate paper. That method and also cross correlation of annual averaged 10 kHz VLF power with sunspot number shows a ∼4 dB peak-to-peak fluctuation at the ∼11-year solar cycle period, believed to be due to the influence of EUV flux on the the ionospheric D region. Finally we constrain any linear trend to 1.4±2.6 dB in 25 years. If a positive trend is present and is interpreted as a change in tropical South American lightning flash rate, it is less than 10%.

Radio terminal test equipment

Abstract: PROBLEM TO BE SOLVED: To produce a radio wave environment where arrival directions, amplitude and a delay time of a plurality of radio waves are equivalent to those in a real environment in the vicinity of the area of a radio terminal. SOLUTION: The radio terminal test equipment is provided with a plurality of antennas 3a-3c that are placed around a radio terminal 1, a signal source 4 that generates a signal equivalent to that by a base station, a distributor 5 that distributes the signal generated by the signal source into a number of the antennas, and fading generators 6a-6c that adjust each output of the distributor so that radio waves radiated from a plurality of the antennas have characteristics in a real environment in each direction of the antennas when viewing the radio terminal. COPYRIGHT: (C)1999,JPO

Single direction square wave modulation intermediate frequncy ionic imported apparatus

Abstract: The present invention relates to an unidirectional square wave modulated intermediate frequency ionic induction apparatus, which consists of a voltage drop rectification circuit, an intermediate frequency oscillation booster circuit, an inter-mediate frequency rectification voltage stabilizer circuit, an intermediate frequency switching-triode and an intermediate frequency square wave generator It can produce unidirectional intermediate frequency pulse voltage and current used in the medicine induction for treating chronic prostatitis It can also consist of a low frequency switching-triode,low frequency square ware generator, very low frequency square wave generator and a second low frequency square ware generator, very low frequency square wave generator and a second low frequency square wave generator It can produce various low frequency modulated unidirectional intermediate frequency square waves