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.

Cone structure and focusing of VLF and LF electromagnetic waves at high altitudes in the ionosphere

Abstract: The frequency and angle dependencies of the electric field radiated by an electric dipole E = E(sub 0) cos omega(t) are studied through numerical calculations of absolute value of E in the VLF and LF frequency bands where F is less than or equal 0.02 to 0.05 f(sub b) in a model ionosphere over an altitude region of 800-6000 km where the wave frequency and electron gyrofrequency varies between F approximately 4-500 kHz and f(sub b) is approximately equal (1.1 to 0.2) MHz respectively. It is found that the amplitudes of the electric field have large maxima in four regions: close to the direction of the Earth magnetic field line B(sub 0) (it is called the axis field E(sub 0), in the Storey E(sub St), reversed Storey E(sub RevSt), and resonance E(sub Res) cones. The maximal values of E(sub 0), E(sub Res), and E(sub RevSt) are the most pronounced close to the lower hybrid frequency, F approximately F(sub L). The flux of the electric field is concentrated in very narrow regions, with the apex angles of the cones Delta-B is approximately (0.1-1) deg. The enhancement and focusing of the electric field increases with altitude starting at Z greater than 800 km. At Z greater than or equal to 1000 up to 6000 km, the relative value of absolute value of E, in comparison with its value at Z = 800 km is about (10(exp 2) to 10(exp 4)) times larger. Thus the flux of VLF and LF electromagnetic waves generated at high altitudes in the Earth's ionosphere are trapped into very narrow conical beams similar to laser beams.

Spectral Characteristics of VLF Sferics Associated With RHESSI TGFs

Abstract: We compared the modeled energy spectral density of very low frequency (VLF) radio emissions from terrestrial gamma ray flashes (TGFs) with the energy spectral density of VLF radio sferics recorded by Duke VLF receiver simultaneously with those TGFs. In total, six events with world wide lightning location network (WWLLN) defined locations were analyzed to exhibit a good fit between the modeled and observed energy spectral densities. In VLF range the energy spectral density of the TGF source current moment is found to be dominated by the contribution of secondary low-energy electrons and independent of the relativistic electrons which play their role in low-frequency (LF) range. Additional spectral modulation by the multiplicity of TGF peaks was found and demonstrated a good fit for two TGFs whose VLF sferics consist of two overlapping pulses each. The number of seeding pulses in TGF defines the spectral shape in VLF range, which allows to retrieve this number from VLF sferics, assuming they were radiated by TGFs. For two events it was found that the number of seeding pulses is small, of the order of 10. For the rest of the events the lower boundary of the number of seeding pulses was found to be between 10 to 103.

The Concept of an Airborne VLF Transmitter with Vertical Electric Dipole Antenna

Abstract: This paper presents the idea of a Very Low Frequency facility based on an airborne, mobile transmitter equipped with a symmetric-dipole antenna. The antenna, to be flown under an aerostat to high altitudes (between 12 and 60 km), is to present, in comparison to ground facilities, less financial issues and modified field intensity. The antenna concept was proven during three experiments flown in 2014 and 2015, with the longest aerial having the length of 200 m. The prototype transmitter is planned to be launched in 2019.

Experimental evidence of the simultaneous occurrence of VLF chorus on the ground in the global azimuthal scale – from pre-midnight to the late morning

Abstract: . Night-time VLF (very low frequency) chorus bursts lasting about one hour have been recorded at Finnish temporal station Kannuslehto (CGM: 64.2°; 107.9°, L = 5.3) during two VLF campaigns (on 25 February–4 March 2008 and 27 March–17 April 2011). The chorus bursts were associated with substorm development. They were accompanied by riometer absorption enhancements, which occurred simultaneously within as large longitude areas as from pre-midnight (Sodankyla, ~22:00 MLT) to the late morning (Tixie, ~03:00 MLT and Gakona, ~08:00 MLT) longitudes. It was found that the pre-midnight chorus observed on the ground occurred simultaneously with VLF chorus emissions recorded in the late morning on the low-altitude DEMETER satellite crossing the similar geomagnetic latitudes on the opposite local time sector. For the first time some evidence of simultaneous chorus burst generation in the global longitudinal scale was found (from pre-midnight to the late morning) by using direct comparison with satellite data as well as using non-direct indicator–azimuthally extended riometer absorption enhancements.

A log-likelihood ratio for improved receiver performance for VLF/LF communication in atmospheric noise

Abstract: Receiver-improving techniques are important for submarine communication at the LF/VLF bands, due to very demanding reception conditions. At the surface, the interference environment is dominated by the atmospheric noise, which is highly impulsive in character and may impede the reception of the radio signals. To handle the demanding channel and the impulsive interference environment, error correction and an adapted receiver are necessary. In this paper, we propose a log-likelihood ratio (LLR) as soft output from the demodulator suitable for atmospheric noise. The radio system is assumed to use minimum shift keying (MSK) and a low parity density check (LDPC) code. It is shown that the proposed interference-adapted LLR improves the performance substantially in atmospheric noise compared to when the LLR is designed for AWGN. The performance is also compared to a solution, where the soft output from the demodulator is simplified to a limiter, and to a solution when a larger system bandwidth is used in combination with a limiter. It is concluded that the proposed interference-adapted LLR achieves the best performance in the comparison, although the performance could probably be further improved, when compared to the obtained Shannon capacity of this particular interference.