Showing papers on "Very low frequency published in 1997"

Chemoreceptor dependence of very low frequency rhythms in advanced chronic heart failure.

Abstract: Factors responsible for very low frequency oscillations (VLF; cycle > 30 s) in the cardiovascular system remain obscure. We tested the hypothesis that increased peripheral chemosensitivity is important in the pathogenesis of VLF oscillations in patients with chronic heart failure (CHF). Fourteen male patients with stable, moderate to severe CHF (age 60 +/- 1.1 yr, ejection fraction 23 +/- 11%) and reproducible VLF oscillations in heart rate underwent a protocol consisting of three consecutive 20-min phases during which they breathed air, hyperoxia (O2 via mask, 60% O2 concn), and air again. Autoregressive spectral analysis of R-R intervals, blood pressure, and respiration was used to quantify total oscillatory power (TP), VLF, low (0.04-0.15 Hz)- and high (0.15-0.40Hz)-frequency power, and the coherence between these signals. Peripheral chemosensitivity was studied by assessing the ventilatory response to hypoxia using transient inhalations of pure N2. Discrete VLF rhythms were seen in R-R intervals in all 14 patients, in blood pressure in 7 of 14, and in respiration in 8 of 14 patients. A significant coherence (> 0.5) between heart rate and systolic blood pressure within the VLF band with mean phase value of -140 degrees, suggesting an antibaroreflex relationship, was seen in six subjects. Transient hyperoxia abolished the VLF oscillations in most subjects (12 of 14 in R-R intervals) and decreased R-R variability power within the VLF band. This response significantly correlated with peripheral chemoreceptor sensitivity (r = 0.77, P = 0.014). This study suggests that in CHF, enhanced peripheral chemoreceptor activity may facilitate slow oscillations in the cardiorespiratory signals.

Lightning and ionospheric remote sensing using VLF/ELF 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) frequency ranges. 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. These two facts suggest that observed sferic waveforms radiated from lightning and received at long distances (>1000 km) from the source stroke contain a great deal of information about both the state of the ionosphere along the propagation path and the dynamics of the current in the lightning return stroke. The aim of this dissertation is to develop and implement the necessary techniques to use sferic observations to determine the characteristics of the ionosphere and lightning. In order to accurately interpret observed sferic characteristics, a detailed model of sferic propagation is required. Such a model is developed, based on a frequencydomain subionospheric VLF and ELF propagation code, and with it the detailed spectral characteristics of VLF (>1.5 kHz) sferics are shown to depend primarily on the propagation-path-averaged ionospheric D region electron density profile, in the range of electron densities of 10–10 cm−3. To infer this D region density from VLF sferic observations, a model ionosphere is iteratively varied to find the model spectrum that agrees best with an observed sferic spectrum composed of the average of many individual sferic spectra. In most nighttime cases, the quality of the agreement allows the height of an exponentially-varying electron density profile to be inferred with a precision of 0.2 km. Since the general sferic waveform depends on the source current-moment waveform as well as the ionospherically-controlled propagation, the former quantity can

Temporal evolution of very strong Trimpis observed at Darwin, Australia

Abstract: Very strong phase and amplitude perturbations (Trimpis) of the very strong VLF transmission from NWC received in Darwin, Australia, enabled accurate measurement of the amplitude and phase of the scattered signal and of the time variation of these. The amplitude of the scattered signal decays as the logarithm of time, quite at odds with the exponential decay observed on classic Trimpis. During the amplitude decay, the phase of the scattered signal decreased at a decreasing rate. This is shown to be consistent with scattering from a bundle of sprite-like, conducting columns extending some 50 km below the base of the ionosphere.

VLF remote sensing of high‐energy auroral particle precipitation

Abstract: Ground-based measurements of VLF transmitter signals propagating in the Earth-ionosphere waveguide can be used to determine the location of nighttime high-energy (≥100 keV) auroral particle precipitation. When the region of auroral particle precipitation passes over a VLF propagation path, disturbances in the D region of the ionosphere created by the high-energy particles perturb the amplitude of VLF signals propagating below in a characteristic manner. Continuous nighttime observations of the amplitude of the signal from the NLK transmitter (24.8 kHz, Jim Creek, Washington) were made in Gander, Newfoundland, during November 1993 and January 1994. Simultaneous images of atmospheric X rays created by auroral particle precipitation taken by the AXIS instrument on the UARS satellite were examined for times when large-scale auroral particle precipitation extended over the NLK-Gander propagation path. Quantitative characteristics of the precipitation-associated NLK signal perturbations are established from days which clearly exhibit good correlation between the AXIS images and VLF data, and a larger data set from the months of November 1993 and January 1994 is examined statistically to determine the effectiveness of the VLF technique in capturing particle precipitation events. The number of particle precipitation onsets seen in the AXIS images that can be readily identified in the VLF amplitude data is found to be almost 94%. VLF propagation model calculations show that the observed VLF amplitude decreases are consistent with propagation under conditions of enhanced lower ionosphere electron density caused by auroral electron precipitation and suggest that electrons with energies greater than 100 keV are responsible for the VLF amplitude perturbations.

Decay of a vertical plasma column: A model to explain VLF sprites

Abstract: VLF sprites are identified by high angle scattering of VLF transmissions by the conducting plasma columns which appear as the luminous columns of “red sprites”. VLF sprites are “early/fast Trimpis” and probably vice versa. Recently discovered properties of early/fast Trimpis are the logarithmic decay of the amplitude of the scattered signal and monotonic variation of its phase. These properties are explained in terms of scattering from a vertical column or set of columns extending from 50 km (or lower) altitude to about 80 km.

Very low frequency (0.2–10.0 Hz) seismoacoustic noise below the seafloor

Abstract: The sources and propagation of VLF (0.2 → 10 Hz) ambient noise on and within the deep ocean crust at Deep Sea Drilling Project (DSDP) Hole 534B in the Blake Bahama Basin are shown to be related to the surface sea state and local lithology. This study represents the first experiment where ambient noise is measured simultaneously at several depths below the seafloor. The low-frequency microseism power spectral density (PSD) peak at 0.3 Hz is nearly invariant with depth between 0 and 100 m below the seafloor. PSD levels of the peak are 65 and 75 dB (rel 1 (nm/s2)2/Hz) for the vertical and horizontal components, respectively, and both horizontal and vertical components of the ocean bottom seismometer and borehole array compare favorably. Above 0.5 Hz the noise levels decrease with depth and increasing frequency. At 1.0 Hz, 100 m below the seafloor the noise level is 10 and 20 dB below the levels observed at the seafloor for vertical and horizontal components, respectively. There is evidence that amplification in some frequency bands may make deeper sites noisier than shallower sites in the same well. Temporal variation of the noise shows nonlinear interaction of local water-borne gravity waves to be the dominant source mechanism and that the passing of a local storm generates interface waves and increases the noise level (∼10 dB) from 0.3 to 1.5 Hz and 5 to 64 Hz. Between 1.5 and 5 Hz the spectrum is not strongly affected by the passing storm, indicating that the ocean wave spectrum may be saturated.

EXOS-D (AKEBONO) very low frequency plasma wave instruments (VLF)

Abstract: The Akebono (EXOS-D) satellite has been successfully observing the Earth's magnetosphere since it was launched on February 21, 1989. The objectives of VLF instruments on board the satellite were to investigate the behavior of plasma waves associated with accelerated auroral particles, wave-particle interaction mechanisms, and propagation characteristics of whistler-mode waves in the magnetosphere. The instruments measured not only the dynamic spectra of VLF waves up to 15 kHz by a wideband receiver, but also their absolute field intensities, wave normal vectors, and Poynting vectors. Two electric and three magnetic components with a bandwidth of about 50 Hz up to 12.5 kHz are sent to measure the wave normal vectors and Poynting vectors. The antenna impedance is measured to determine the correct absolute electric field intensities. The instruments have successfully measured the wave spectra, the wave normal vectors. Poynting vectors, the precise wave intensities, the antenna impedance, etc. The present paper describes the unique features of the instruments, especially the Poynting flux analyzers in more detail. Obtained scientific results are also reviewed.

Delineation of near-surface structures using VLF and VLF-R data; an insight from the joint inversion results

Abstract: Very low frequency (VLF) electromagnetic measurements are generally used to study the variations in electrical conductivity in the upper few hundred meters of the Earth’s crust The VLF magnetic field mode, in which the tilt angle and ellipticity of the magnetic polarization ellipse, ie, the real and imaginary anomalies respectively, are measured, has been widely used for more than 30 years The VLF resistivity mode (VLF-R), in use since the late 1960s, is, in principle, nothing but a high frequency magnetotelluric method in which the apparent resistivity and phase are recorded

Quasiperiodic 5-60 s fluctuations of VLF signals propagating in the Earth-ionosphere waveguide: A result of pulsating auroral particle precipitation?

Abstract: Subionospheric very low frequency and low-frequency (VLF/LF) transmitter signals received at middle-latitude ground stations at nighttime were found to exhibit pulsating behavior with periods that were typically in the ∼5–60 s range but sometimes reached ∼100 s. The amplitude versus time shape of the pulsations was often triangular or zigzag-like, hence the term “zigzag effect.” Variations in the envelope shape were usually in the direction of faster development than recovery. Episodes of zigzag activity at Siple, Antarctica (L ∼4.3), and Saskatoon, Canada (L ∼4.2), were found to occur widely during the predawn hours and were not observed during geomagnetically quiet periods. The fluctuations appeared to be caused by ionospheric perturbations at the ∼ 85 km nighttime VLF reflection height in regions poleward of the plasmapause. We infer that in the case of the Saskatoon and Siple data, the perturbations were centered within ∼500 km of the stations and within ∼ 100–200 km of the affected signal paths. Their horizontal extent is inferred to have been in the range ∼50–200 km. The assembled evidence, supported by Corcuffs [1996] recent research at Kerguelen (L ∼3.7), suggests that the underlying cause of the effect was pulsating auroral precipitation. The means by which that precipitation produces ionospheric perturbations at 85 km is not yet clear. Candidate mechanisms include (1) acoustic waves that propagate downward from precipitation regions above the ∼ 85 km VLF reflection level; (2) quasi-static perturbation electric fields that give rise to E×B drifts of the bottomside ionosphere; (3) secondary ionization production and subsequent decay at or below 85 km. Those zigzag fluctuations exhibiting notably faster development than recovery probably originated in secondary ionization produced near 85 km by the more energetic (E >40 keV) electrons in the incident electron spectrum.

Radiation of ring sources of the very low frequency band in a magnetic plasma medium in the presence of a cylindrical plasma channel

Abstract: A rigorous solution to the problem of the radiation of given harmonic ring electric and magnetic currents in the magnetic plasma medium in the presence of a cylindrical plasma waveguide channel oriented along the external magnetic field is obtained. Expressions for the wave fields and the radiation pattern are analyzed as applied to the very low frequency (VLF) band and the total radiated power is calculated. It is shown that the presence of a plasma channel of an elevated density results in a considerable increase in the radiation power of ring sources.

VLF wave generation using VLF heating: a pointer to a new technique for ELF wave production?

Abstract: The generation of very-low-frequency (VLF) waves via the cubic nonlinearity of the ionosphere using a VLF 'heater' at 16.4 kHz /spl plusmn/25 Hz (JXN Norway in FSK mode) and an adjacent 'probe' VLF transmitter (Omega Norway) on 12.1 kHz is reported. The transmitters are located near Mo I Rana, on the west coast of Norway. This experiment relies on what may be a unique configuration of these two VLF transmitters, in that they are separated on the Earth's surface by less than half of their mean VLF wavelength. Their signals can thus be expected to interact in the ionosphere. Clear signals have been observed on 20.65 kHz and 20.75 kHz at Lycksele and Uppsala in Sweden, at a mean range of 330 and 760 km from the two VLF transmitters. It is believed that the signals are generated in the ionosphere above the VLF by the cubic nonlinearity mechanism. This mechanism may have possibilities for the generation of extra-low-frequency (ELF) waves from two adjacent VLF transmitters whose cubic nonlinearity difference frequency is in the ELF range. It is estimated that were Omega Norway to be retuned to an appropriate frequency (32.9 kHz) then the ionosphere above Aldra should simulate an ELF antenna with an output power of about 10 mW. VLF transmitters at lower magnetic latitudes, such as those operating in Hawaii, if suitably retuned may cause the ionosphere to radiate significantly more ELF energy, typically 10 W at 100 Hz.

Studies of Electrostatic Waves, VLF-wave Particle Interactions, and Propagations in the Ionosphere

Abstract: : The long-term goal of this project is to study waves, irregularities, and propagation in the earth s ionosphere, particularly with respect to GPS. The project has two foci: first, to understand how the ionosphere affects GPS signals and second, how GPS can be used for remote sensing of the ionosphere. In association with these goals, we additionally conduct in situ experiments on satellites and sounding rockets to directly measure ionospheric waves and irregularities that affect signal propagation.