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.

VLF observations of auroral beams as sources of a class of emissions

Abstract: AN interesting very low frequency (VLF) phenomenon which I shall call auroral V-emissions (AVEs) has been known for a number of years, although few reports have been published. The phenomenon has also been known informally as “short saucers” and “splashes”. The dynamic spectra of a collection of AVEs are shown in Fig. 1a. The AVEs shown here have a fairly low frequency minimum, near 0.5 kHz, and have time scales of the order of 10 s. The broader form at the upper part of the record may perhaps be an example of an AVE, but in this paper it will not be considered as an AVE. The name auroral V-emission is based on the region of observation and the V-shaped or hyperbolic spectral appearance. The hyperbolic shape is often quite symmetrical, particularly for the AVEs with shorter time scales. AVEs have only been observed in satellites, usually near the auroral zone. Noise bands with distinct intensity minima at harmonics of the local proton frequency are often observed at the centre of an AVE, especially for observations at relatively high altitude (3,000 km) and for AVEs with a low frequency minimum. The phenomena reported here have a much shorter time scale and a more well defined spectral appearance than the “V-type VLF hiss”1,2.

Very low‐frequency electromagnetic interpretation using tilt angle and ellipticity measurements

Abstract: During this study, a plane‐wave computer modeling approach was used to gain more insight into the response of vertical sheet conductors at very low frequency (VLF). The medium which surrounds the conductors is assumed to be a dissipative one. Magnitude and shape of the anomalous field caused by a vertical “thin” conductor at VLF is found to be dependent mainly on (1) conductivity‐thickness product of conductor, (2) resistivity of host medium, (3) depth to the top of conductor, and (4) overlying conductive overburden. Other variables being the same, a shift in frequency in the 17 to 25 kHz range does not produce an appreciable change in the overall response of modeled conductors. The lateral distance between the maximum and minimum on a tilt angle profile is related to conductivity and thickness of conductor, resistivity of the host rock, and depth to the top of the conductor in a complex manner. An interpretation scheme has been proposed to determine the conductivity‐thickness product and depth of vertica...

Low‐frequency electromagnetic exploration for groundwater on Mars

Abstract: Water with even a small amount of dissolved solids has an electrical conductivity orders of magnitude higher than dry rock and is therefore a near-ideal exploration target on Mars for low frequency, diffusive electromagnetic methods. Models of the temperature- and frequency-dependent electrical properties of rock-ice-water mixtures are used to predict the electromagnetic response of the Martian subsurface. Detection of ice is difficult unless it is massively segregated. In contrast, liquid water profoundly affects soundings, and even a small amount of adsorbed water in the cryosphere can be detected. Subcryospheric water is readily distinguishable at frequencies as low as 100 Hz for fresh water to 10 mHz for brines. These responses can be measured using either natural or artificial sources. Ultra low frequency signals from solar wind and diurnal-heating perturbations of the ionosphere are likely, and disturbances of regional crustal magnetic fields may also be observable. Spherics, or extremely to very low frequency signals from lightning discharge, would provide optimal soundings; however, lightning may be the least likely of the possible natural sources. Among the active techniques, only the time-domain electromagnetic (TDEM) method can accommodate a closely spaced transmitter and receiver and sound to depths of hundreds of meters or more. A ground- or aircraft-based TDEM system of several kilograms can detect water to a depth of several hundred meters, and a system of tens of kilograms featuring a large, fixed, rover- or ballistically deployed loop can detect water to several kilometers depth.