Showing papers on "Very low frequency published in 1999"

Ionization of the lower ionosphere by γ-rays from a magnetar : Detection of a low energy (3-10 keV) component

Abstract: A gigantic periodic flare from the soft γ repeater SGR 1900+14 produced enhanced ionization at ionospheric altitudes of 30 to 90 km, which was observed as unusually large amplitude and phase changes of very low frequency (VLF) signals propagating in the Earth-ionosphere waveguide. The VLF signals remained perturbed for ∼5 min and exhibited the 5.16 s periodicity of the giant flare detected on the Ulysses spacecraft [Hurley et al., 1999]. Quantitative analysis indicates the presence of an intense initial low energy (3–10 keV) photon component that was not detectable by the Ulysses instrument.

Scattering Pattern of Lightning-Induced Ionospheric Disturbances Associated with Early/Fast VLF Events

Abstract: Simultaneous observations of early/fast Very Low Frequency (VLF) events at nine closely spaced (65 km) sites are used together with a numerical model of the propagation and scattering of VLF signals in the earth- ionosphere waveguide to directly measure the scattering pat- tern of associated ionospheric disturbances. In cases when the causative lightning is within 700 km of the north-south array of observing sites, early/fast VLF events are typically observed at no more than 2 or 3 sites, which indicates a nar- row beam of the scattered signal in the forward direction. In the dierent cases studied, forward scattering patterns exhibit 15 dB beamwidths of less than 30 consistent with horizontal extent of 90 30 km.

Investigating seismoionospheric effects on a long subionospheric path

Abstract: We examine the possibility of earthquake precursors influencing the subionospheric propagation of VLF transmissions. We consider the long (12 Mm) path from northeastern United States to Faraday, Antarctica (65°S, 64°W), during 1990–1995 and investigate the subionospheric amplitude variation of signals from the NAA communication transmitter (24.0 kHz, 1 MW) in Cutler, Maine, with particular emphasis on possible changes induced by seismic events occurring in South America. We have analyzed the changes in timing of modal minima generated by the passage of the sunrise terminator over the Andes, i.e., the “VLF terminator time” (TT) method. The anomalous variations in timing throughout the year are of a size and occurrence frequency similar to those previously reported, i.e., ±0.5–1 hour and 1–2 per month. However, we find that in these anomalous cases, the time of the sunrise modal minimum does not change significantly, but rather, the minimum becomes insufficiently deep to be detected, and the time of the next nearest minimum is logged. Our analysis indicates that the occurrence rate of successful earthquake predictions using the TT method cannot be distinguished from that of chance. Additionally, the level of false earthquake prediction using the TT method is high.

Subionospheric VLF signatures of oblique (nonducted) whistler‐induced precipitation

Abstract: Subionospheric very low frequency (VLF) signal perturbations observed at multiple sites in association with individual lightning discharges exhibit onset delays which steadily increase with increasing L-value. The timing of the onsets and the inferred spatial extent (∼1000 km) of the ionospheric disturbance are consistent with those expected for electron precipitation induced by obliquely propagating (i.e., nonducted) whistlers.

Modeling of subionospheric VLF signal perturbations associated with earthquakes

Abstract: It has been reported that propagation of very low frequency (VLF) waves in the Earthionosphere waveguide might provide an indication of imminent earthquakes [Hayakawa et al., 1996; Molchanov et al., 1998]. Narrow-band data from Inubo, Japan, suggested that transmissions from Omega Japan, 1000 km away, might be influenced by pre-earthquake processes. The terminator time (TT) was defined as the time where a minimum occurred in the received phase (or amplitude) during sunrise and sunset. A few days before an earthquake the TT was observed to deviate significantly from the monthly averages, producing a longer “VLF day.” The TT effect has been explained through some rather simple modeling by a 1–2 km drop in the VLF reflection height at the lower ionospheric boundary. In this study we apply more realistic propagation models to show that the changes in VLF reflection height associated with earthquakes would have to be considerably larger (∼4–11 km) than those suggested previously in order to produce the reported effect. If the reported TT changes were caused by alterations in the VLF reflection height associated in some manner with an imminent earthquake, these effects would be commensurate with the effects of a solar flare. However, this would lead to changes in received amplitude (or phase) that would be significant at all times, and not just during the day/night transition. Hence it is not at all clear that a simple height-lowering explanation for the TT effect is correct.

Radio atmospheric propagation on Mars and potential remote sensing applications

Abstract: We theoretically analyze the propagation of very low frequency (VLF) and extremely low frequency (ELF) electromagnetic energy in the spherical waveguide formed by the ground and ionosphere of Mars to investigate the possibility of using such signals to remotely sense Martian ground conductivity and ionospheric parameters. This energy is presumed to be radiated from an electrical discharge in a Martian dust storm. Using a synthesis of observed and modeled Martian ionospheric and ground parameters and assuming a discharge current similar to that in a terrestrial lightning discharge, we calculate radio atmospheric spectra and waveforms for a variety of discharge orientations and observed electromagnetic field components. A number of characteristics of the modeled Martian radio atmospherics (sferics) are found to be significantly different from those of terrestrial sferics, and we discuss how these features could provide information regarding the Martian ionosphere. We also show that measurements of VLF and ELF attenuation rates could provide a measurement of large-scale ground conductivity to a depth of a few kilometers or more, thereby providing a technique for probing large-scale subsurface geological features which affect bulk conductivity, such as areas containing ice or water.

Ionospheric effects of relativistic electron enhancement events

Abstract: The relativistic electron population as measured both at geosynchronous orbit and at low altitudes at subauroral latitudes exhibits pronounced fluctuations in association with magnetospheric substorm and solar activity. A ground-satellite correlative study based on amplitude and phase measurements of VLF signals propagating in the earth-ionosphere waveguide indicates that the relativistic electron enhancements are accompanied by similar enhancements in nighttime ionospheric conductivity produced by associated enhanced precipitation. VLF signal amplitudes are found to exhibit >10 dB changes, showing the same 27 day cycle and 2–3 day rise and fall time pattern as relativistic electron enhancement events recorded by GOES 7 and SAMPEX, and indicating that the nighttime lower ionospheric electron density at subauroral latitudes is detect ably affected by 27-day periodicity in solar rotation.

Modeling the relaxation of red sprite plasma

Abstract: . Red sprites consist of multiple ionised columns extending above a thunderstorm from ~30 km to ~90 km. Electron densities in these columns are very much larger than the ambient background, perhaps fives times at 70 km. These highly ionized structures cause observable perturbations in subionospheric VLF transmissions known as "VLF Sprites". Three models of initial sprite electron density are considered, and using a realistic ionization relaxation model the time dependence of electron density is derived. A 3-D Born propagation code of is used to compute the time profile of a VLF sprite. Two profiles show good agreement with the time signature experimentally observed, in that scattered amplitude and phase decrease linearly log(t). These simulations provide insight into the nature and structure of sprite columns, and indicate an additional constraint which should be applied to red sprite creation models.

Spectral Law and Polarization Properties of the Low‐Frequency Waves at the Magnetopause

Abstract: The magnetic fluctuations, at the magnetopause and in the adjacent magnetosheath, exhibit power law spectra which are very reminiscent of turbulent spectra. In prospect of future modelizations of such a turbulence, new information is brought about the experimental properties of these fluctuations. The power laws spectra previously obtained in the ULF range are shown to hold also in VLF, up to the lower hybrid frequency. Concerning the polarization, 1) the direction with respect to the static magnetic field is shown to be dominantly perpendicular at low frequencies, consistently with Shear Alfven modes in this range, and 2) no right-hand sense of rotation can be evidenced at frequencies higher than the proton gyrofrequency, although one could expect the fast magnetosonic mode to be dominant in this range. The physical implications of this last observation for the non linear effects at work in the turbulence are briefly discussed.

Investigations into electromagnetic emissions from power system arcs

Abstract: Impulsive electromagnetic radiation can originate from many diverse sources in power systems when arcing occurs. The emission of radio wave energy in the form of a power system sferic in a similar way to lightning induced sferics is related to the non-linear properties of the plasma arc. This paper describes research work investigating the electromagnetic emissions in the very low frequency (VLF) and very high frequency (VHF) frequency bands from power system arcing events. Sources of power system sferics and the inducing mechanism are detailed. The characterisation of different types of power system arcing events recorded during experimentation are also discussed. The principles allowing power system arcing fault induced sferics to be discriminated from other impulsive noise sources and the location of the sferic's origin are outlined. Finally the practical implementation of a real-time fault monitoring system and recorded results are presented.

Magneto-inductive on-command fuze and firing device

Abstract: A fuze is enabled, armed, and fired while indicating its status to remote command/receiver stations so that interconnected line charges and other ordnance items can be detonated with increased safety and reliability from a safe man-weapon separation distance. The fuze is responsive to remotely transmitted magneto-inductive command signals in the extremely low frequency (ELF) to very low frequency (VLF) range to change its status and to transmit magneto-inductive status signals in the ELF to VLF range confirming its status to at least one of the remote stations. Transmission and reception of magneto-inductive signals in the ELF to VLF range allow for a unique communication method that provides safe and reliable communication suitable to effect fuzing of explosive devices on the beach through seawater, air, earth, buildings, vegetation and sediment or any combination of these conditions.

High voltage VLF test equipment with sinusoidal waveform

Abstract: The problems associated with testing field aged XLPE cable using HVDC are now well documented. AC test equipment for field tests at power frequency also presents problems. The equipment required for such tests is large, heavy and expensive even with resonant test sets. Equipment producing very low frequency (VLF) high voltage has been used to some extent in the past. Typically this equipment produced 0.1 Hz voltage having a variety of waveforms. Squarewaves, triangular waves, etc. have been available. Such nonstandard waveforms complicate efforts to correlate VLF techniques with the results of power frequency tests. This paper describes AC test equipment utilizing a new approach to high voltage VLF in the range of 0.02 to 0.1 Hz. The equipment generates a sinusoidal waveform directly, without the additional complexity, cost and weight of HV filters. This not only saves weight and reduces cost but also simplifies its use in diagnostic tests such as partial discharge detection and site location. The design concept and application of the equipment to both voltage withstand testing and partial discharge testing are discussed.

Observational detection of meteor-produced VLF electromagnetic radiation

Abstract: In November 1998, Croatian Physical Society's expedition to Mongolia was undertaken. The goal was to make measurements of the puzzling electrophonic sounds and very low frequency (VLF) radio emission from meteors during the anticipated Leonid meteor storm. During the night of 16/17 November 1998, an extremely high fireball activity of the Leonid meteors occured. During this period, we performed measurements of the VLF radiation from meteors. Here we present a positive signal which consists of a sequence of sharp, short VLF bursts, coincident with the appearance of meteor that was recorded by the video camera. This is the first completely controlled instrumental recording of such an event.

Time signal repeating installation and time correction system

Abstract: PROBLEM TO BE SOLVED: To enable to receive a repeating radio wave regardless of the arrangement position of a radio wave correction timepiece without requiring a troublesome labor. SOLUTION: This time signal repeating installation 2 is composed so as to receive a standard time radio wave signal S1 including a time code having a prescribed frequency (40 kHz) transmitted from a key station 1 after receiving AM modulation, and to correct an internal clock to the time corresponding to the time code included in the received standard time radio wave signal S1, and in first and second transmission time zones determined beforehand, to generate a first-intensity time radio wave signal S2a and a second-intensity time radio wave signal S2b, having different electric field intensities, having respectively a frequency 40 kHz included in the same frequency zone as the standard time radio wave signal, having the same format as a base band signal, and including the time code based on the internal clock after correction, and to transmit the signals to a radio wave correction clock 3 installed at close range or on a far position. COPYRIGHT: (C)2000,JPO