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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 article, the spatial relationship between propagation paths of subionospheric VLF/LF signals exhibiting sudden amplitude perturbations (Trimpi events) and time correlated cloud-to-ground lightning flashes was investigated.
Abstract: A study has been made of the spatial relationship between propagation paths of subionospheric VLF/LF signals exhibiting sudden amplitude perturbations (Trimpi events) and time correlated cloud-to-ground lightning flashes. On each of the 4 days examined the storm centers were located close to the signal path from the NAU transmitter (28.5-kHz) in Puerto Rico to Stanford (SU) and were at large distances from the propagation path of the 48.5-kHz transmitter signal from Nebraska to SU. Nevertheless, no Trimpi events were observed on the former path, while many were seen on the latter. Furthermore, the detected Trimpi perturbations of the 48.5-kHz signal received at Stanford were found to be associated with the lightning activity in the distant storm centers. Since the NAU-SU path lies entirely at L < 2 and the 48.5-SU path is located mostly at 2< L <3, the L dependent magnetospheric conditions which determine the level of lightning-induced electron precipitation are different along the two paths. Thus, the authors postulate that the observed difference in Trimpi occurence on the two paths was due to the different magnetospheric conditions. Hence the occurence of Trimpi events over the geographical region corresponding to L <3 may be more dominantly controlled by magnetosphericmore » conditions than the source lightning distribution.« less

23 citations

Journal ArticleDOI
TL;DR: The Geological Survey of Sweden has been collecting airborne tensor very low frequency data (VLF) over several decades, covering large parts of the country as mentioned in this paper, and the data has been an invaluable source o...
Abstract: The Geological Survey of Sweden has been collecting airborne tensor very low frequency data (VLF) over several decades, covering large parts of the country. The data has been an invaluable source o ...

23 citations

Journal ArticleDOI
TL;DR: In this article, a new VLF phenomenon named a proton whistler has been identified in the Alouette 1 and Injun 3 satellites, which is observed only after the reception of an upward-propagating electron whistler.
Abstract: A new VLF phenomenon named a proton whistler has been identified in the VLF data from Alouette 1 and Injun 3 satellites. Two independent analyses on 12% of the proton whistler Injun 3 data were conducted: a semimonthly sample of data for Injun 3's 10-month lifetime to determine the gross features of proton whistlers; and a local nighttime and local daytime study of proton whistlers. Six assertions are made from these experimental observations: (1) Proton whistlers are observed only after the reception of an upward-propagating electron whistler. (2) At a frequency termed the crossover frequency (ω12), the initial electron whistler frequency-time trace and the proton whistler trace are coincident in time. (3) Proton whistler traces show initially a rapid rise in frequency which starts at ω12 and which asymptotically approaches the proton gyrofrequency Ω1 (200–650 cps for altitudes of 2700–400 km). (4) Proton whistlers occur more frequently during local nighttime than during local daytime, and they have not been observed to occur below 442 km during local nighttime or below 640 km during local daytime. (5) The ratio of the crossover frequency to the proton gyrofrequency increases with decreasing altitude and approaches unity at an altitude of approximately 440 km around local midnight and approximately 640 km around local noon. (6) There is an apparent northern hemisphere-southern hemisphere asymmetry in the occurrence of proton whistlers; they occur 3 times more frequently in the northern than in the southern hemisphere. Also, there is a high-latitude boundary in the occurrence of proton whistlers that seems to correspond with the auroral zone. The theory describing proton whistlers due to Gurnett is summarized.

23 citations

Journal ArticleDOI
TL;DR: In this paper, the authors measured 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.
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.

23 citations

Journal ArticleDOI
TL;DR: In this paper, the phase and signal amplitude of VLF radio waves, propagated over distances varying between 1000 and 10000 km, which accompany solar flares have been studied for the period 1961-1964.
Abstract: : The observed changes of phase and signal amplitude of VLF radio waves, propagated over distances varying between 1000 and 10000 km, which accompany solar flares have been studied for the period 1961-1964. In general the phase of the received signal advances and the signal amplitude increases during the onset of a solar flare. However observations of the GBR transmissions at Rome (1500 km path) show that for this particular path the phase of the received signal retards in contrast to all other paths studied. Assuming that the conductivity of the lower ionosphere varies exponentially with height under both quiet and disturbed conditions, it is possible to evaluate the waveguide mode characteristics of VLF propagation over large distances. Using this theoretical approach it has been shown that the observations can only be interpreted if both the D region electron density profile is lowered and the gradient of ionisation density increased. The magnitude of the flare produced changes in ionisation density profiles of the lower ionosphere have been evaluated analytically from solar X-ray data. These profile changes are consistent with the lowering of the D region and increase in ionisation gradient that is required to explain the VLF radio wave observations. (Author)

23 citations


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