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.

On the generation of ELF/VLF waves for long‐distance propagation via steerable HF heating of the lower ionosphere

Abstract: [1] ELF/VLF radio waves (300 Hz to 30 kHz) have been successfully generated via modulated HF (3–10 MHz) heating of the lower ionosphere in the presence of natural currents, most recently with the HAARP facility in Alaska. Generation is possible via amplitude modulation or via two techniques involving motion of the HF beam during the ELF/VLF cycle, known as beam painting and geometric modulation, described and measured by Cohen et al. (2010b). In this paper, we describe a theoretical model describing the HF heating and ionospheric responses, followed by a full-wave calculation of ELF/VLF propagation, and utilize this end-to-end model to derive the predicted radiated ELF/VLF pattern up to 1000 km from the HF heater in the Earth-ionosphere waveguide. We quantitatively compare the generated ELF/VLF signals on the ground from various generation techniques and find it to be generally in agreement with earlier measurements. We apply a simplified ELF/VLF propagation model to quantify the contribution of the ELF/VLF phased array in the radiation pattern resulting from geometric modulation and find this contribution to be significant. We also use a limited HF heating model to quantify the degree to which the current power level of HAARP is sufficient for the beam painting technique, since this technique requires high HF power densities at high altitudes.

Very-Low-Frequency transmitters bifurcate energetic electron belt in near-earth space.

Abstract: Very-Low-Frequency (VLF) transmitters operate worldwide mostly at frequencies of 10-30 kilohertz for submarine communications. While it has been of intense scientific interest and practical importance to understand whether VLF transmitters can affect the natural environment of charged energetic particles, for decades there remained little direct observational evidence that revealed the effects of these VLF transmitters in geospace. Here we report a radially bifurcated electron belt formation at energies of tens of kiloelectron volts (keV) at altitudes of ~0.8-1.5 Earth radii on timescales over 10 days. Using Fokker-Planck diffusion simulations, we provide quantitative evidence that VLF transmitter emissions that leak from the Earth-ionosphere waveguide are primarily responsible for bifurcating the energetic electron belt, which typically exhibits a single-peak radial structure in near-Earth space. Since energetic electrons pose a potential danger to satellite operations, our findings demonstrate the feasibility of mitigation of natural particle radiation environment.

Are VLF rapid onset, rapid decay perturbations produced by scattering off sprite plasma?

Abstract: Rapid onset, rapid decay perturbations (RORDs) of subionospheric VLF propagation require highly localized or laterally structured plasma at low altitudes to explain the wide angle scattering observed and the rapid decay. Simultaneous occurrence of RORDs and red sprites, illustrated by a single event here, together with VLF phase and group delay measurements from a pair of spaced receivers suggest that RORDs are produced by scattering from conducting columns at the position and with the lateral shape of the sprite. The sprite luminosity decays much faster than the RORDs which depend on the sprite conductivity and so plasma density. Plasma is also produced near the sprite plasma by energetic electrons precipitated from the magnetosphere by ducted whistlers and after the expected whistler and electron propagation delay. This whistler-induced electron precipitation (WEP) plasma produces wide angle VLF scattering similar to that by sprite plasma, implying similar lateral fine structure. This suggests that the processes leading to sprites also produce whistler ducts in the magnetosphere.

ELF and VLF signals radiated by the “polar electrojet antenna”: Experimental results

Abstract: The heating facility at Ramfjordmoen near Tromso, Norway, has been used to modulate the auroral electrojet at frequencies in the range 223 Hz to 5.44 kHz. ELF/VLF signals have been received at Lycksele, Sweden, 554 km from the heating transmitter, over the whole frequency range with maximum amplitudes of ∼50 fT. Both azimuthal and radial magnetic field components were recorded and the ratio of these two components, commonly termed the polarization, was determined. The experimental results have been successfully modelled by using waveguide mode theory and assuming the heating source to be a point dipole located in the ionosphere at the height of the maximum ELF/VLF Hall current.

Overview of the Uranian radio emissions: Polarization and constraints on source locations

Abstract: Radio emission from Uranus was discovered a few days before the closest approach of Voyager 2 with Uranus on January 24, 1986. The planetary radio astronomy experiment recorded several types of emissions in the frequency range 1–900 kHz; they differ by their spectral, temporal, and polarization characteristics. The analysis of the observations led us to distinguish two smooth radio components, one occurring in the lowest frequency range (SLF) from 20 kHz to 347 kHz, and the other one reaching the highest observed frequencies, i.e., 865 kHz (SHF). The SLF component is left-hand polarized when observed in the northern magnetic hemisphere and right-handed in the southern magnetic hemisphere, without polarization reversal from dayside to nightside. The SHF component is only observed in the nightside of Uranus, and it is left-hand polarized. Several bursty emissions were observed: some of them are broadband bursts and occur in the 78- to 750-kHz range (b-bursty), others are narrow-band bursts and occur in the 40- to 270-kHz range (n-bursty). The broadband bursts were observed at low magnetic latitudes of the spacecraft and mainly when it was in the southern magnetic hemisphere; their polarization is left-handed. The narrow-band bursts were observed in the northern magnetic hemisphere and are not correlated with the magnetic latitude of the spacecraft; their polarization is right-handed. Sporadic bursts of short duration were observed mainly in the northern magnetic hemisphere with right-hand polarization. Two periodic events were observed, ahead of the inbound bow shock and inside the magnetosphere of Uranus. Very low frequency emissions at 1.2 and 20.4 kHz are also discussed; some of them are correlated with in situ phenomena. From the polarization study we deduce the possible source locations of the main components in the frame of our hypothesis. The source regions are characterized by the latitude and longitude of the footprints of the magnetic field lines through the source expressed in the Uranographic longitude system (ULS). The SLF source would be fixed in magnetic longitude at the northern magnetic pole in the range 13°< latitudeULS<38°, 26°