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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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Journal ArticleDOI
TL;DR: In this paper, the authors analyzed night-side measurements of the DEMETER spacecraft related to lightning activity and found that the penetration into the ionosphere occurs at nearly vertical wave vector angles (as was expected from coupling conditions) at distances of 100-900 km from the source lightning.
Abstract: (1) We analyze nightside measurements of the DEMETER spacecraft related to lightning activity. At the 707 km altitude of DEMETER, we observe 3-D electric and magnetic field waveforms of fractional-hop whistlers. At the same time, the corresponding atmospherics are recorded by a very low frequency (VLF) ground-based station located in Nancay (France). The source lightning strokes are identified by the METEORAGE lightning detection network. We perform multidimensional analysis of the DEMETER measurements and obtain detailed information on wave polarization characteristics and propagation directions. This allows us for the first time to combine these measurements with ray-tracing simulation in order to directly characterize how the radiation penetrates upward through the ionosphere. We find that penetration into the ionosphere occurs at nearly vertical wave vector angles (as was expected from coupling conditions) at distances of 100-900 km from the source lightning. The same distance is traveled by the simultaneously observed atmospherics to the VLF ground station. The measured dispersion of fractional-hop whistlers, combined with the ionosonde measurements at the Ebro observatory in Spain, allows us to derive the density profile in the topside ionosphere.

46 citations

Journal ArticleDOI
TL;DR: The conventional resting state metrics were not markedly affected by different TRs, and cardiorespiratory signals showed strongest aliasing in central brain regions in sTR 1–2 s, but in QPP analyses, the repeatability of the VLF pulse detection becomes linearly reduced with increasing sTR.
Abstract: Low image sampling rates used in resting state functional magnetic resonance imaging (rs-fMRI) may cause aliasing of the cardiorespiratory pulsations over the very low frequency (VLF) BOLD signal fluctuations which reflects to functional connectivity (FC). In this study, we examine the effect of sampling rate on currently used rs-fMRI FC metrics. Ultra-fast fMRI magnetic resonance encephalography (MREG) data, sampled with TR 0.1 s, was downsampled to different subsampled repetition times (sTR, range 0.3-3 s) for comparisons. Echo planar k-space sampling (TR 2.15 s) and interleaved slice collection schemes were also compared against the 3D single shot trajectory at 2.2 s sTR. The quantified connectivity metrics included stationary spatial, time, and frequency domains, as well as dynamic analyses. Time domain methods included analyses of seed-based functional connectivity, regional homogeneity (ReHo), coefficient of variation, and spatial domain group level probabilistic independent component analysis (ICA). In frequency domain analyses, we examined fractional and amplitude of low frequency fluctuations. Aliasing effects were spatially and spectrally analyzed by comparing VLF (0.01-0.1 Hz), respiratory (0.12-0.35 Hz) and cardiac power (0.9-1.3 Hz) FFT maps at different sTRs. Quasi-periodic pattern (QPP) of VLF events were analyzed for effects on dynamic FC methods. The results in conventional time and spatial domain analyses remained virtually unchanged by the different sampling rates. In frequency domain, the aliasing occurred mainly in higher sTR (1-2 s) where cardiac power aliases over respiratory power. The VLF power maps suffered minimally from increasing sTRs. Interleaved data reconstruction induced lower ReHo compared to 3D sampling (p < 0.001). Gradient recalled echo-planar imaging (EPI BOLD) data produced both better and worse metrics. In QPP analyses, the repeatability of the VLF pulse detection becomes linearly reduced with increasing sTR. In conclusion, the conventional resting state metrics (e.g., FC, ICA) were not markedly affected by different TRs (0.1-3 s). However, cardiorespiratory signals showed strongest aliasing in central brain regions in sTR 1-2 s. Pulsatile QPP and other dynamic analyses benefit linearly from short TR scanning.

46 citations

Journal ArticleDOI
TL;DR: Clark et al. as discussed by the authors compared a wave resolution time-dependent Boussinesq model with the observed bathymetry and incident random, directionally spread waves for dye plume observations.
Abstract: [1] A model that accurately simulates surf zone waves, mean currents, and low‐frequency eddies is required to diagnose the mechanisms of surf zone tracer transport and dispersion In this paper, a wave‐resolving time‐dependent Boussinesq model is compared with waves and currents observed during five surf zone dye release experiments In a companion paper, Clark et al (2011) compare a coupled tracer model to the dye plume observations The Boussinesq model uses observed bathymetry and incident random, directionally spread waves For all five releases, the model generally reproduces the observed cross‐shore evolution of significant wave height, mean wave angle, bulk directional spread, mean alongshore current, and the frequency‐dependent sea surface elevation spectra and directional moments The largest errors are near the shoreline where the bathymetry is most uncertain The model also reproduces the observed cross‐shore structure of rotational velocities in the infragravity (0004 < f < 003 Hz) and very low frequency (VLF) (0001 < f < 0004 Hz) bands, although the modeled VLF energy is 2–3 times too large Similar to the observations, the dominant contributions to the modeled eddy‐induced momentum flux are in the VLF band These eddies are elliptical near the shoreline and circular in the mid surf zone The model‐data agreement for sea swell waves, low‐frequency eddies, and mean currents suggests that the model is appropriate for simulating surf zone tracer transport and dispersion

45 citations

Journal ArticleDOI
TL;DR: In this article, the authors present a possible use of VLF/LF (very low frequency (3-30 kHz)/low frequency (30-300 kHz) radio sounding of seismo-ionospheric perturbations.

45 citations

Journal ArticleDOI
TL;DR: In this article, a self-consistent treatment of this process enables us to compute the frequency spectrum of these waves, assuming that the energy diffusion of electrons occurs in a characteristic time much larger than the time required for pitch-angle diffusion.

45 citations


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