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Very low frequency

About: Very low frequency is a research topic. Over the lifetime, 1540 publications have been published within this topic receiving 24233 citations. The topic is also known as: VLF.


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Proceedings ArticleDOI
05 Mar 2011
TL;DR: In this paper, the authors present the technical requirements for OLFAR and first order estimates of data rates for space-based radio astronomy based on the proposed scalable distributed correlator model.
Abstract: Recently new and interesting science drivers have emerged for very low frequency radio astronomy from 0.3 MHz to 30 MHz. However Earth bound radio observations at these wavelengths are severely hampered by ionospheric distortions, man made interference, solar flares and even complete reflection below 10 MHz. OLFAR is Orbiting Low Frequency ARray, a project whose aim is to develop a detailed system concept for space based very low frequency large aperture radio interferometric array observing at these very long wavelengths. The OLFAR cluster could either orbit the moon, whilst sampling during the Earth-radio eclipse phase, or orbit the Earth-moon L2 point, sampling almost continuously or Earth-trailing and leading orbit. The aim of this paper is to present the technical requirements for OLFAR and first order estimates of data rates for space based radio astronomy based on the proposed scalable distributed correlator model. The OLFAR cluster will comprise of autonomous flight units, each of which is individually capable of inter satellite communication and down-link. The down-link data rate is heavily dependent on distance of the cluster from Earth and thus the deployment location of OLFAR, which are discussed.

54 citations

Journal ArticleDOI
TL;DR: In this article, the authors conducted a survey of extremely low frequency (ELF) and very low frequency radio noise since February 1985 and found that the noise amplitudes vary seasonally by up to a factor of 4 in some of the sixteen frequency bands in the 10-Hz to 32-kHz frequency range.
Abstract: Our laboratory has been conducting a global survey of extremely low frequency (ELF) and very low frequency (VLF) radio noise since February 1985 Eight measurement stations around the world record the instantaneous noise amplitude in each of sixteen narrow-frequency bands in the 10-Hz to 32-kHz frequency range, and we have calculated the monthly averages of these amplitudes for the four stations with the longest times of operation The period, amplitude, and phase of temporal variations in the averages are important indicators of the sources and propagation characteristics of the noise in the various frequency bands Furthermore, since the principal source of ELF/VLF radio noise is lightning, long-term variations of the noise must relate to changes in global thunderstorm activity We find that the noise amplitudes vary seasonally by up to a factor of 4 in some of the sixteen frequency bands; in addition, many of the variations correlate quite well with global lightning flash rates

53 citations

Journal ArticleDOI
TL;DR: The observation of subionospheric very-low-frequency (VLF) propagation between the transmitter NWC (Australia) and its three receiving sites in Japan (Chofu, Chiba, and Kochi) is used to find a possible precursor of ionospheric perturbations to the Sumatra earthquake in late December 2004.
Abstract: The observation of subionospheric very-low-frequency (VLF) propagation between the transmitter NWC (Australia) (frequency 19.8 kHz) and our three receiving sites in Japan (Chofu, Chiba, and Kochi) are used to find a possible precursor of ionospheric perturbations to the Sumatra earthquake in late December 2004. Night-time fluctuations in VLF propagation data at all three sites have exhibited significant enhancements from 4 days before the earthquake, which is likely to be a precursor to the earthquake.

52 citations

Journal ArticleDOI
TL;DR: It is suggested that in CHF, enhanced peripheral chemoreceptor activity may facilitate slow oscillations in the cardiorespiratory signals.
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.

52 citations

Journal ArticleDOI
TL;DR: In this paper, a full wave model for trans-ionospheric propagation is presented, which includes wave polarization, incidence angle, bearing, ground conductivity, horizontal distance from the source, and ionospheric profile.
Abstract: [1] Accurate models of trans-ionospheric propagation are needed to assess the role of Earth-originating very low frequency (VLF) electromagnetic waves in radiation belt dynamics. Recent studies have called the relatively crude early trans-ionospheric models into question, finding that they underestimate the attenuation by 20–100 dB. A full wave model that includes all of the relevant physics has recently become available and experimentally verified to within a few decibels via comparison to more extensive satellite data. Using this model, we discuss the importance of wave polarization, incidence angle, bearing, ground conductivity, horizontal distance from the source, and the ionospheric profile, all of which are demonstrated to play a significant role in the trans-ionospheric propagation. Trans-ionospheric attenuation estimates are provided both for the case of vertical incidence of a whistler-mode wave and for the case of magnetospheric injection from a dipolar terrestrial VLF source. These estimates agree with observation to within ˙6 dB. The remaining discrepancy may be attributable to ionospheric variation and/or factors not captured by our horizontally stratified model. On the basis of the full wave treatment presented herein, we find that the earlier work showing a >20 dB overestimation by traditional models results from the unrealistic simplifying assumption that the wave is vertically incident onto the ionosphere, exacerbated by the fact that most of the satellite data used for comparison came at large horizontal distances (hundreds of kilometers) from the source.

52 citations


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Performance
Metrics
No. of papers in the topic in previous years
YearPapers
202320
202232
202156
202048
201942
201852