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.

A statistical study of ELF-VLF plasma waves at the magnetopause

Abstract: ISEE 1 plasma wave data are used to study the broadband ELF-VLF plasma waves at the magnetopause. Enhanced wave intensities are detected at 85 percent of all magnetopause crossings. Wave amplitudes vary from event to event and even with a single event. Wave spectra averaged over many passes, however, are similar at dawn, noon, and dusk local hours. The only parameter correlated with wave intensity is the magnitude of the Z component of the magnetosheath magnetic field. The results place strong constraints on any proposed generation mechanism for the broadband magnetopause boundary layer waves.

Longitudinal variations of very‐low‐frequency chorus activity in the magnetosphere: Evidence of excitation by electrical power transmission lines

Abstract: The distribution of VLF chorus in the high-altitude magnetosphere as detected by the Ogo 3 satellite was investigated. Chorus occurrence frequency over 4,668 samples of 5 min of broadband data was computed for Northern Hemisphere bins of 10 deg by 10 deg in dipole invariant latitude and longitude. Peaks in activity were noted over Alaska, the eastern U.S. and Canada, western Europe, and western Siberia, and were shown not to be the result of biased sampling. It is suggested that power line radiation of only about 1 W or less at a given harmonic frequency may be sufficient to stimulate the observed chorus activity and that such power levels can be expected in industrial areas or from large distribution networks. Preliminary results show that chorus starting frequencies are highly correlated with 60 Hz harmonics in the American sector and with 50 Hz harmonics in the European sector.

Recovery signatures of lightning‐associated VLF perturbations as a measure of the lower ionosphere

Abstract: A new model of the physical processes associated with subionospheric VLF signal perturbations caused by lightning-induced electron precipitation (LEP) bursts is developed to diagnose the state of the lower ionosphere (e.g., electron number density and rate coefficients for various chemical reactions) on the basis of measurements of VLF recovery signatures. The model accounts for the energy spectrum of the electron bursts precipitated by lightning-generated whistlers, the chemical relaxation of enhanced secondary ionization in the nighttime D region due to LEP bursts, and quantitatively treats the resultant effects on propagation of the VLF signal in the Earth-ionosphere waveguide. Application of the model to experimental data obtained for the VLF propagation path from NPM station (Hawaii) to Palmer station (Antarctica) indicates that effective electron detachment rate γ, enhanced secondary ionization profile (e.g., energy content of LEP bursts), as well as the ambient electron density distribution, may be estimated using observed subionospheric VLF recovery signatures. The effective detachment rate was identified as ∼10−18 N s−1, where N is total number density of neutrals. Model indicates in particular that the attachment-detachment processes play the dominant role in recovery of subionospheric VLF signal perturbations on timescales ∼ 100 s, and that the observed perturbations of the NPM-Palmer signal correspond to the LEP bursts consisting of relatively soft (< 250 keV) electrons.

Detection and location of red sprites by VLF scattering of subionospheric transmissions

Abstract: Scattering by the conductive columns of red sprites of VLF waves ("VLF sprites") traveling in the Earth- ionosphere waveguide is characterized by wide angle scattering (to 180o). This enabled the first VLF detection of sprite conductivity and is now used routinely for sprite detection and location by measurement of the phase and amplitude at both of the frequencies transmitted by NAA, NSS, NLK and NPM. In a blind test using only one VLF receiver and the first three of these transmitters, all of the VLF events fitting the criteria for VLF sprites were found to correspond to optical sprites and vir- tually all of the sprites observed optically corresponded to VLF events. Using only a single receiver in the high noise envi- ronment of local thunderstorms enabled the range to the sprite to be found to within 100 km and the direction to within 90 o. Use of a large number (-10) of suitably spaced antennas greatly increases the location accuracy and provides some information on the lateral structure of sprite conductivity.

Terrestrial VLF transmitter injection into the magnetosphere

Abstract: [1] Very Low Frequency (VLF, 3–30 kHz) radio waves emitted from ground sources (transmitters and lightning) strongly impact the radiation belts, driving electron precipitation via whistler-electron gyroresonance, and contributing to the formation of the slot region. However, calculations of the global impacts of VLF waves are based on models of trans-ionospheric propagation to calculate the VLF energy reaching the magnetosphere. Limited comparisons of these models to individual satellite passes have found that the models may significantly (by >20 dB) overestimate amplitudes of ground based VLF transmitters in the magnetosphere. To form a much more complete empirical picture of VLF transmitter energy reaching the magnetosphere, we present observations of the radiation pattern from a number of ground-based VLF transmitters by averaging six years of data from the DEMETER satellite. We divide the slice at ∼700 km altitude above a transmitter into pixels and calculate the average field for all satellite passes through each pixel. There are enough data to see 25 km features in the radiation pattern, including the modal interference of the subionospheric signal mapped upwards. Using these data, we deduce the first empirical measure of the radiated power into the magnetosphere from these transmitters, for both daytime and nighttime, and at both the overhead and geomagnetically conjugate region. We find no detectable variation of signal intensity with geomagnetic conditions at low and mid latitudes (L < 2.6). We also present evidence of ionospheric heating by one VLF transmitter which modifies the trans-ionospheric absorption of signals from other transmitters passing through the heated region.