Showing papers on "Very low frequency published in 2003"

Miniature radio-acoustic sounding system for low altitude wind and precipitation measurements

Abstract: The present invention is directed to a radio-acoustic sounding system for providing wind measurements at altitudes of 100 meters or less. Wind measurements are obtained by transmitting a pulse of audio frequency energy through one or more volumes corresponding to the coverage area of one or more radio frequency transceivers. The frequency of the audio pulse is selected to have a wavelength that is one-half the wavelength of the electromagnetic energy transmitted by the antenna or antennas. By monitoring a return radio frequency signal at selected times following the transmission of the audio pulse, wind data is obtained at selected altitudes. Wind speed and direction can be obtained by observing the Doppler frequency shift of return radio frequency signals, or by observing the amplitude of the return radio frequency signals. In accordance with an embodiment of the present invention, precipitation measurements may be made by transmitting a radio frequency signal at the same radio frequency as is used in connection with wind measurements, and observing return radio frequency signals.

On the correlated spectral and timing properties of 4U 1636-53: An atoll source at high accretion rates

Abstract: We analyze ∼600 ks data from the X-ray observatory Rossi X-ray Timing Explorer of the neutron star low mass X-ray binary and atoll source 4U 1636-53. These observations span almost three years, from April 1996 to February 1999. The color-color and hardness-intensity diagrams show significant secular shifts of the atoll track, similar to what is observed for some Z sources. These are most evident in the hardness-intensity diagram, where shifts in intensity up to ∼20% are observed. We find that the intensity shifts in the hardness-intensity diagram are responsible for the parallel tracks observed in the kilohertz quasi periodic oscillations frequency vs. intensity diagram. While the parallel tracks in the frequency vs. color plane partially overlap, systematic long term shifts are still evident. We also study the broad band power spectra of 4U 1636-53 as a function of the source position S a along the track in the color-color diagram. These power spectra are all characteristic of the lower and upper banana state in atoll sources, showing very low frequency noise, band limited noise, and sometimes one or two kHz QPOs. We find that the very low frequency noise in some intervals is not well described by a power law because of an excess of power between 20 and 30 mHz, which can be fitted by a Lorentzian. Also, the characteristic frequency of the band limited noise shows a trend to decrease with increasing S a at high S a values.

Near-Relativistic Solar Electrons and Type III Radio Bursts

Abstract: Recently it has been found that the inferred injection times of greater than 25 keV electrons are up to 30 minutes later than the start times of the associated type III radio bursts at the Sun. Thus it has been suggested that the electrons that produce type III bursts do not belong to the same population as those observed above 25 keV. This paper examines the characteristics and circumstances of 79 solar electron beam events measured on the ACE spacecraft. Particular attention is paid to the very low frequency emissions of the associated radio bursts and the ambient conditions at the arrival times of the electrons at the spacecraft. It is found that the inferred greater than 25 keV electron injection delays are correlated with the times required for the associated radio bursts to drift to the lowest frequencies. This suggests that the electrons responsible for the radio emission and those observed above 25 keV are part of a single population, and that the electrons both above and below 25 keV are delayed in the interplanetary medium. Further evidence for a single population is the general correspondence between electron and local radio intensities and temporal profiles. It is found that the delays increase with the ambient solar wind density consistent with the propagation times of the electrons being determined by the characteristics of the interplanetary medium. However it is known that particle arrival times at 1 AU are a linear function of inverse particle speed. Conventionally such a relationship is taken to indicate scatter-free propagation when inferred path lengths lie close to 1.2 AU, as they do for the electron events studied here. These conflicting interpretations require further investigation.

Early/fast VLF events produced by electron density changes associated with sprite halos

Abstract: [1] The magnitudes of scattered fields produced during early/fast very low frequency (VLF) events observed at 13 closely spaced (∼65 km) sites are compared with those expected for sprite halo disturbances using a numerical model of wave propagation within the Earth-ionosphere waveguide. Three different early/fast events of varying magnitudes are analyzed using three different nighttime ambient lower ionospheric electron density profiles. The electron density disturbances associated with sprite-halo events are determined using a full-wave electromagnetic (FWEM) model. Observed scattered field amplitudes of typical (VLF amplitude changes of 0.2 dB 1 dB) are found to be within a factor of seven for peak currents of causative lightning based on NLDN. However, in previous studies, some sprite-producing lightning flashes have exhibited large slow-tail components, indicating substantial continuing currents and implying charge removal up to 2–3 times larger than that inferred from the peak current reported by NLDN. For the cases discussed in this paper, scattered field calculations using disturbances caused by 2–3 times larger charge removal are found to be within a factor of two of the measured values. VLF scattering from electron density changes associated with sprite halos thus appear to be the underlying cause of at least some of the VLF perturbations observed as early/fast events.

Fast auroral snapshot satellite observations of very low frequency saucers

Abstract: Wave and charged particle observations of quasielectrostatic whistler emissions known as “VLF saucers” (very low frequency, or kilohertz radio range) from the Fast Auroral SnapshoT (FAST) satellite demonstrate that the majority (∼85%) of VLF saucer emissions are generated on flux tubes that carry antiearthward, energetic (>10 eV) electrons in the downward current region of the aurora. In most cases, the VLF saucers are nested, that is, they have two or more clearly discernible “arms” that indicate several distinct source regions at differing altitudes. These observations verify previous interpretations that the individual source regions are highly localized both in latitude and altitude. In some cases, the individual source regions are localized in three dimensions. An important new finding is that the FAST satellite frequently detects solitary structures identified as electron phase-space holes at the vertex of VLF saucers. Electron phase-space holes were identified in ∼79% of the VLF saucer events in th...

Selectivity of damped AC (DAC) and VLF voltages in after-laying tests of extruded MV cable systems

Abstract: The purpose of HV after-laying tests on cable systems on-site is to check the quality of installation. The test on extruded MV cable systems is usually a voltage test. However, in order to enhance the quality of after installation many researchers have proposed performance of diagnosis tests such as detection, location and identification of partial discharges (PD) and tan /spl delta/ measurements. Damped AC voltage (DAC) also called oscillating voltage waves (OVW) is used for PD measurement in after-laying tests of new cables and in diagnostic test of old cables. Continuous AC voltage of very low frequency (VLF) is used for withstand voltage tests as well as for diagnostic tests with PD and tan /spl delta/ measurements. Review on the DAC and VLF tests to detect defects during on-site after-laying tests of extruded MV cable systems is presented. Selectivity of DAC and VLF voltages in after-laying testing depends on different test parameters. PD process depends on type and frequency of the test voltage and hence, the breakdown voltage is different. The withstand voltage of XLPE cable insulation decreases linearly with increasing frequency in log scale. Experimental studies with artificial XLPE cable model indicate that detection of defects with DAC or VLF voltage can be done at a lower voltage than with DC. DAC voltage is sensitive in detecting defects that cause a breakdown due to void discharge, while VLF is sensitive in detecting defects that cause breakdown directly led by inception of electrical trees.

On the lithosphere‐atmosphere coupling of seismo‐electromagnetic signals

Abstract: [1] Employing borehole and terrestrial antennas, vertical electric field components of the naturally occurring very low frequency electromagnetic emissions at the frequency of 3 kHz have been monitored simultaneously at Agra (geographic latitude 27.8°N, longitude 78°E), India, from 15 March 1999 to 30 September 1999. This period of observation included a major seismic swarm activity in the months of March and April which occurred in the Chamoli hills of north India, the next two months of May and June being quiet with respect to local lightning and spheric activities, and the rest of the three months from July to September being highly disturbed due to local lightning and thunderstorm activities. The abnormal electric field changes occurred in the form of noise bursts of varying amplitude and duration and included three kinds of data: (1) noise bursts observed by borehole antenna only, (2) noise bursts observed by terrestrial antenna only, and (3) noise bursts observed by both the antennas. We find that the occurrence of the first kind of data is positively correlated with major seismic activities in the region over the period of observations. The third kind of data indicates coupling between the two antennas dominated overall by atmospheric emissions. The long distance propagation of the seismo-electromagnetic emissions through the middle layer crust working as waveguide or through seismic faults is found to be associated with large attenuation ≈13 dB/km. Hence, the observation of the emissions at Agra, about 400 km from Chamoli, is interpreted in terms of leakage to the atmosphere through “windows” of low conductivity in the skin layer near the epicenter, possibly produced by some geophysical formations as discussed by other workers and then propagation in the Earth-ionosphere waveguide.

VLF/LF High-Voltage Design and Testing

Abstract: : The U.S. Navy has had a requirement for long-distance communications from the beginning and it is natural that their interest in high-power wireless transmitting systems dates back to the beginning of radio. Until only recently, the Navy was still operating some of the very-low-frequency and lowfrequency (VLF/LF) antennas that were constructed not long after the beginning of radio. For example, the LF transmitting antenna at Chollas Heights in San Diego, torn down in 1997, dated back to 1915. Three of the towers in the VLF antenna at Annapolis are identical to towers in the original VLF antenna constructed in 1917. They were added to the original antenna between 1934 and 1937 as part of an upgrade. The Annapolis antenna was modified in 1969, keeping those three towers. The VLF station was closed in late 1997 and the antenna has been demolished. The design of high-power VLF/LF antennas has evolved over the years, but the design of the modern antennas still leans heavily on the early antenna designs. The existing VLF/LF system is the only visible remnant of the Navys original radio system constructed at the beginning of this century, and it is appropriate that this report begin with a brief review of the history of the Navys involvement in radio communications.

VLF testing and diagnosis on medium voltage power cables

Abstract: The advantages on VLF (very low frequency) on site testing compared to conventional testing at power frequency are shown in practical and dielectrical aspects. Furthermore VLF test voltages enable the possibility of diagnosis on site. The two well established diagnosis methods, dissipation factor (DF) measurement and partial discharge (PD) measurement are explained. Where the DF diagnosis gives a tool for judgement of the ageing status especially of plastic cables, the PD diagnosis enables detection of single defects within a cable system. The economic advantages of diagnosis are shown in practical examples. An outlook for application on VLF high voltage on site test systems concludes the paper.

Enhancing VLF data for qualitative interpretation: An example of massive chalcopyrite exploration

Abstract: Economic ores usually consist of some combination of semiconductor sulfide minerals that respond to applied electric and electromagnetic fields. Consequently, electric and electromagnetic methods are the main exploration tools to locate the sulfide minerals. Models obtained from two- or three-dimensional inversion of induced polarization (IP) or controlled source audio-magnetotellurics (CSAMT) data can provide a good description of orebodies (Basokur et al., 1997). However, these methods are cumbersome in field procedures, survey costs are relatively high, and acquisition systems are expensive. For these reasons, they are not suitable for reconnaissance surveys and for exploration of small targets. The very low frequency (VLF) method, on the other hand, is appropriate under these conditions. It is fast, can often be implemented by 1–2 people, and the receiver unit is less expensive than those used with other ground EM methods. Moreover, the method uses existing VLF radio transmitters as electromagnetic so...

Submarine floating antenna model for LORAN-C signal processing

Abstract: An electromagnetic model of the floating antenna used by submarines for LORAN-C radionavigation and very low frequency (VLF) communications is proposed. We present a description of the floating antenna and analyze its electric characteristics. The current in the horizontal wire induced by the lateral wave is then computed and we suggest a model that yields to a nonrational transfer function. A parameter estimation algorithm based on parallel-extended Kalman filters is then proposed and validated on real data.

Experience with high potential testing hydro generator multi turn stator coils using 60 Hz AC, DC, and VLF (0.1 Hz)

Abstract: There are many different methods of employing a high potential test on a stator winding. Three such methods that this paper will explore with reference to one another are the AC (50-60 Hz), DC, and very low frequency (VLF) (0.1 Hz). Some users choose the AC high potential test knowing that this test best simulates the voltage stress on the winding while in service. Other users prefer the DC high potential test largely due to ease in performing the test. However, the DC voltage does not stress the stator coils the same way as when they are in service and may result in overly pessimistic results due to the influence of surface contaminants in the end windings. Finally, the VLF test, due to recent advances in technology, is becoming more practical for use in field conditions. However, the present standard governing the test is almost 40 years old and there is significant interest in what VLF voltage level best correlates with the AC and DC high potential tests. This paper reports preliminary test results on three generator windings that were destructively tested using the AC, DC, and VLF methods as part of an ongoing effort to provide a database upon which to set the appropriate VLF hipot level for modern synthetic resin-based stator insulation systems.

A new design for a very low frequency space borne radio interferometer

Abstract: The non transparency and severe propagation effects of the terrestrial ionosphere make it impossible for Earth based instruments to study the universe at low radio frequencies. An exploration of the low frequency radio window with the resolution and sensitivity essential to meet the scientific objectives will necessarily require a dedicated satellite based interferometer operating at these frequencies. Such missions have been proposed in the literature for about past fifteen years. Today, the steady and impressive advances in technology and computing resources have brought us on the brink of a quantum jump in the performance and capabilities of such missions, increasing their scientific desirability many-fold. This paper presents the concept design which emerged from a study to investigate the feasibility of a low frequency satellite based interferometer operating in the frequency range 0.1 -- $40 MHz$ titled {\em PrepAration for Radio Interferometry in Space (PARIS)}. The salient features of the design are: an on-board correlator to reduce the data volumes to be transmitted to the Earth by about two orders of magnitude; use of three orthogonal dipoles in place of two to achieve better polarisation characteristics; direct digitisation of the entire radio frequency band of interest which eliminates the need for a local oscillator system; an overlap in the observing frequency range with upcoming ground based instruments for aid in imaging and calibration and an all-sky imaging capability. The most constraining bottle neck for the present design is the large intra-constellation telemetry requirement. It is expected that technological solutions to meet this requirement will be found in near future as other formation flying missions which share this requirement emerge.

Daytime very low frequency (VLF) emissions observed at Maitri station, Antarctica

Abstract: The present paper deals with the study of naturally occurring Very Low Frequency (VLF) radio waves at Indian Antarctic Station, Maitri (70° 46'S, 11° 44'E) on the basis of routine continuous observation made during day time From the detail analysis of daytime data on the basis of continuous observation made at the ground station, we have presented a few of VLF phenomena such as rising types of emissions - discrete and periodic emissions These recent observations at Maitri, Antarctica may reveal propagational characteristic for the various VLF emissions generated during daytime

Observation of very low frequency emissions at Indian Antarctic station, Maitri

Abstract: Recently, we have succeeded in recording VLF emissions at the Indian Antarctic station, Maitri (geom. lat. 62°S, geom. long. 57.23°E,L = 45) using a T-type antenna, pre/main amplifiers and digital audio tape recorder. VLF hiss in the frequency ranges 11–13 kHz and 13–14.5 kHz and some riser-type emissions in the frequency range 3–5 kHz and magnetospheric lines at about 6.2, 8.0 and 9.2 kHz are reported for the first time. The generation and propagation mechanism of these emissions are discussed briefly.

Initial results of VLF emissions observations on the ground station at Maitri, Antarctica

Abstract: The ground based VLF database from Indian Antarctic station Maitri (70°46¢ S, 11° 44¢ E, L ~ 4.6) has been searched for very low frequency (VLF) emissions during January-February of the years 1998 and 1999. The VLF recording system was successfully installed during the XVII Indian Scientific Expedition to Antarctica (ISEA) 1997-98. The spectrograms of analysed data have revealed the common occurrence of various emission events like chorus rising emissions, emission of rising type, falling emission of hook type and multiple emissions. Also the observed emissions (periodic and quasi-periodic) are found to be risers and fallers triggered from the top of hiss band.

Extremely Low Frequency (ELF) Electromagnetic Wave Propagation

Abstract: Extremely low frequency (ELF) electromagnetic waves are currently the lowest frequency waves used routinely for wireless communication. Their ability to cover the globe with low transmitted power and to penetrate deeply into electrically conducting material makes them indispensable in a variety of applications, including communication with submerged submarines. ELF waves are also used in a variety of scientific applications, from geological remote sensing to the study of lightning, which is a strong natural ELF wave source. Transmitting controlled electromagnetic power at such long wavelengths is tricky and requires antennas rather different from those used at higher frequencies. ELF waves transmitted near the surface of the Earth are strongly reflected by the ground and the ionosphere, and thus propagate as guided waves in this spherical waveguide. The article discusses ELF transmission and reception, ELF applications, and describes in detail techniques for mathematically modeling, with varying degrees of simplicity and approximation, ELF electromagnetic wave propagation in this Earth-ionosphere waveguide. Keywords: ELF; extremely low frequency; VLF; very low frequency; earth-ionosphere waveguide; waveguide mode theory; submarine communication