Topic
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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TL;DR: In this article, a hypothesis associating hydromagnetic waves with emitting particles is offered to account for the observed properties of the pulsations, which exhibits a latitude dependence, becoming longer at higher latitudes.
Abstract: Pulse trains with periods of the order of a minute occur in the natural VLF signals emitted in the exosphere. Composed of otherwise ordinary VLF emissions, the long-period pulses always rise in frequency. At magnetically conjugate points the long-period pulse signals are received simultaneously. The period of the pulsation events exhibits a latitude dependence, becoming longer at higher latitudes. On the basis of available data, a hypothesis associating hydromagnetic waves with emitting particles is offered to account for the observed properties of the pulsations.
50 citations
TL;DR: In this paper, the authors derived the night-to-night variations of the midlatitude ionospheric D region electron density profiles using broadband lightning-generated VLF signals.
Abstract: [1] Lightning discharges radiate most of their electromagnetic energy in the very low frequency (VLF, 3–30 kHz) and extremely low frequency (ELF, 3–3000 Hz) bands and are, consequently, an effective tool for remotely sensing the nighttime ionospheric D region electron density profile. Using broadband lightning-generated VLF signals, we derived the night-to-night variations of the midlatitude ionospheric D region electron density profiles. For 16 nights, between 1 July and 4 August 2004, we examined measured VLF data from lightning occurring near the United States east coast (∼530–860 km away from Duke University) and received at Duke University from 0400 to 0600 UT. From these observed VLF radio atmospherics (sferics), we extracted the nighttime D region electron density profiles covering the range of electron densities from 100 to 103 cm−3, in the altitude range of approximately 70–95 km, using a two-dimensional, laterally homogenous model of VLF propagation in the Earth-ionosphere waveguide. The inferred electron density profile variabilities were in good agreement with those from past nighttime rocket experiments at similar latitudes. Using the rocket-measured profiles in our propagation simulations, we determined that the two-parameter exponential D region electron density profiles we inferred were the best exponential fit, in the electron density range of ∼3 to ∼500 cm−3, to the rocket-measured D region electron density profiles. In an initial effort to determine the sources of the observed variabilities, we compared the SAMPEX precipitating electron measurements to the electron density profiles inferred during July 2000. The results indicate that high-energy electron precipitation might account for at least part of the night-to-night variations of the D region electron densities at the midlatitudes.
50 citations
TL;DR: In this article, the authors conducted VLF walking trace whistlers and Doppler shifts in fixed frequency transmissions identified on OGO midlatitude spectrographic records, and found that they were correlated with the DoP shifts.
Abstract: Nonducted VLF walking trace whistlers and Doppler shifts in fixed frequency transmissions identified on OGO midlatitude spectrographic records
50 citations
TL;DR: In this article, an earlier explanation of sunrise fading on long VLF paths is confirmed by examining data taken over a wider range of frequency and directions of propagation. But it is found that there is an appreciable dependence of the fading period on the magnetic direction of propagation, which is qualitatively in agreement with what would be expected from a sharply bounded ionosphere and a transverse magnetic field.
Abstract: : An earlier explanation of sunrise fading on long VLF paths is confirmed by examining data taken over a wider range of frequency and directions of propagation. It is found, however, that there is an appreciable dependence of the fading period on the magnetic direction of propagation. It is shown that this dependence is qualitatively in agreement with what would be expected from a sharply bounded ionosphere and a transverse magnetic field.
49 citations
TL;DR: In this article, a ship-borne receiver in the geomagnetic conjugate hemisphere is used to detect 2-hop signals, up to 10-hop, with the signal dispersing and evolving in shape, indicative of reamplification and retriggering of emissions during successive traversals of the equatorial interaction regions.
Abstract: [1] Modulated heating of the lower ionosphere with the HAARP HF heater is used to excite 1–2 kHz signals observed on a ship-borne receiver in the geomagnetic conjugate hemisphere after propagating as ducted whistler-mode signals. These 1-hop signals are believed to be amplified, and are accompanied by triggered emissions. Simultaneous observations near (∼30 km) HAARP show 2-hop signals which travel to the northern hemisphere upon reflection from the ionosphere in the south. Multiple reflected signals, up to 10-hop, are detected, with the signal dispersing and evolving in shape, indicative of re-amplification and re-triggering of emissions during successive traversals of the equatorial interaction regions.
49 citations