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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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Book ChapterDOI
01 Jan 1970
TL;DR: In this article, a phenomenological theory of discrete emissions has been advanced, based on cyclotron resonance between energetic electrons and narrow band whistler mode waves traveling along the static magnetic field.
Abstract: Very low frequency whistler mode noise from the magnetosphere frequently appears in the form of narrow band tones of variable frequency called discrete VLF emissions. These tones may appear spontaneously or they may be triggered by transmissions from ground based VLF stations (Helliwell, 1965). They can be observed from within the plasmasphere out to the magnetopause and at frequencies from 300 to 30000 Hz. A phenomenological theory of discrete emissions has been advanced, based on cyclotron resonance between energetic electrons and narrow band whistler mode waves traveling along the static magnetic field (Helliwell, 1967). An important feature of this theory is the maintenance of oscillations through feedback between the waves and the electrons over a path that is long compared with the wavelength.

34 citations

Journal ArticleDOI
TL;DR: The results of very low frequency (VLF) wave amplitude measurements carried out at the low latitude station Varanasi (geom. lat. 14∘55′N, long. 154∘E), India during solar flares are presented for the first time.
Abstract: The results of very low frequency (VLF) wave amplitude measurements carried out at the low latitude station Varanasi (geom. lat. 14∘55′N, long. 154∘E), India during solar flares are presented for the first time. The VLF waves (19.8 kHz) transmitted from the NWC-transmitter, Australia propagated in the Earth-ionosphere waveguide to long distances and were recorded at Varanasi. Data are analyzed and the reflection height H′ and the sharpness factor β are evaluated. It is found that the reflection height decreases whereas sharpness factor increases with the increase of solar flare power. The H′ is found to be higher and β smaller at low latitudes than the corresponding values at mid and high latitudes. The sunspot numbers were low during the considered period 2011–2012, being the rising phase of solar cycle 24 and as a result cosmic rays may impact the D-region ionosphere. The increased ionization from the flare lowers the effective reflecting height, H′, of the D-region roughly in proportion to the logarithm of the X-ray flare intensity from a typical mid-day unperturbed value of about 71–72 km down to about 65 km for an X class flare. The sharpness (β) of the lower edge of the D-region is also significantly increased by the flare but reaches a clear saturation value of about 0.48 km−1 for flares of magnitude greater than about X1 class.

33 citations

Journal ArticleDOI
TL;DR: In this article, the authors compare the VLF signals from modulated ionospheric heating experiments with the electric field pulsations observed by the STARE auroral radars, and show that the correlation between the VlF signals and electric field signals can provide a sensitive, high-time resolution indicator of the ionosphere electric field.
Abstract: Very low-frequency radio waves can be generated in the lower ionosphere by periodically modulating the conductivity, and thus natural currents, using high power HF radio waves. In conjunction with the heating facility near Tromso, Norway, such waves have been generated and detected on the ground using two orthogonal receiving antennas so that the polarization ellipse of the VLF signal could be reconstructed. Changes in the strength and direction of the electric field driving the modulated current are reflected in variations in the size and orientation of the VLF ellipse. During a natural Pc 5 pulsation event on October 16, 1981, a close correlation was found between the VLF signal and the electric field pulsations observed by the STARE auroral radars. This first comparison between the Heating and STARE experiments shows that VLF signals from modulated ionospheric heating experiments can provide a sensitive, high time resolution indicator of the ionospheric electric field.

33 citations

Journal ArticleDOI
TL;DR: In this article, the authors proposed a mechanism of proton interaction with a VLF wave propagating in pararesonance mode, which consists in particle crossing of multiple cyclotron resonances in an inhomogeneous magnetic field.

33 citations

B. C. Edgar1
01 Mar 1972
TL;DR: In this article, the magnetospherically reflected (MR) whistler was investigated and the gross features of MR whistler frequency-time spectrograms were explained in terms of propagation through a magnetosphere composed of thermal ions and electrons and having small density gradients across L-shells.
Abstract: Very low frequency (VLF) electromagnetic wave phenomenon called the magnetospherically reflected (MR) whistler was investigated. VLF (0.3 to 12.5 kHz) data obtained from the Orbiting Geophysical Observatories 1 and 3 from October 1964 to December 1966 were used. MR whistlers are produced by the dispersive propagation of energy from atmospheric lightning through the magnetosphere to the satellite along ray paths which undergo one or more reflections due to the presence of ions. The gross features of MR whistler frequency-time spectrograms are explained in terms of propagation through a magnetosphere composed of thermal ions and electrons and having small density gradients across L-shells. Irregularities observed in MR spectra were interpreted in terms of propagation through field-aligned density structures. Trough and enhancement density structures were found to produce unique and easily recognizable signatures in MR spectra. Sharp cross-field density dropoff produces extra traces in MR spectrograms.

33 citations


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