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.

Chapter 11 - The Oblique Reflection of CW Low-Frequency Radio Waves from the Ionosphere

Abstract: Propagation factors peculiar to the l.f. and v.l.f. band have resulted in their continued use over the years. In particular these waves are not adversely affected during periods of ionospheric disturbance even when communication at h.f. over high latitude paths is disrupted; and the phase stability of transmission permits frequency comparison within a few parts in 10 10 or better, and long range radio navigation utilizing phase comparison between spaced phase locked transmitters. In this paper we will give an account of some measurements made on waves of low and very low frequency reflected from the ionosphere at oblique incidence, specifically, we will discuss the amplitude, and phase variations of waves propagated over distances up to 2500 km over middle to high latitude paths. Both normal and abnormal propagation phenomena (such as SID's, PCD's, ionospheric storms, and nocturnal anomalies) will be summarized.

Effects of aging on partial discharge patterns in voids under very low frequency excitation

Abstract: As the use VLF diagnostic testing is a promising trend, it is important to investigate PD characteristics during aging at very low frequency range. Test samples of solid insulation with voids inside fabricated by a 3D printer are subjected to VLF excitation for PD measurements. PD characteristics at different aging states of test samples are investigated under power frequency (50 Hz) and VLF excitation (0.1 Hz). PD patterns show that behaviors of discharges at 0.1 Hz and 50 Hz can be similarly categorized into 2 stages. In the first stage, electrical discharges tend to happen at high instantaneous voltage value and have large discharge magnitudes due to lack of initial free electrons at the beginning of the test. Along the testing period in the first stage, discharge magnitudes gradually decrease and PD activities happen at lower voltage value as more free electrons are available. In the second stage, PD characteristics are hardly changed. However, there is a substantial difference in time duration in the first stage between the frequency of 0.1 Hz and 50 Hz. The first stage at 50 Hz excitation is much shorter than that under VLF excitation. This could be explained by significant dependence of physical conditions such as surface conductivity of the void and charge decay mechanisms on applied frequency. At VLF, charge decay rate is higher which may cause the discharge process taking longer to finish the first stage.

Plasma heating near a VLF antenna

Abstract: The collisionless plasma heating due to a strong parametric instability of VLF oscillations, excited in the near zone of an antenna in the ionospheric plasma, is considered. An expression for the temperature is derived.

The effect of lightning-induced electromagnetic waves on the electron temperatures in the lower ionosphere

Abstract: In this study, the heating of the nighttime lower ionosphere due to electromagnetic radiation in the Very Low Frequency (VLF) band that are transmitted by cloud-to-ground (CG) lightning return strokes is investigated. For this purpose, the temperature of electrons in the lower ionosphere is calculated by using the electron energy balance equation, which is obtained by using Maxwellian distribution. In the result of calculations, in the 10 V/m of the electrical field for all modes of EM wave, it was observed that the electron temperatures increased by about 9000-11000 K at an altitude of about 85 -90 km. With an increase in the electric field, it was observed that the altitude where the maximum temperatures were reached shifted higher. The Right-Handed mode of the EM wave unlike the other modes was not return based-state to an altitude of 95 -100 km. To fully determine the effect of lightning induced electromagnetic waves on the lower ionosphere, considering the effects of polarized modes (Right and Left) can also provide more information about this region.

Ducted whistler-mode signals received at two widely spaced locations

Abstract: Whistler-mode signals from a single VLF transmitter that have propagated in the same duct, have been observed simultaneously at Faraday, Antarctica (65°S, 64°W) and Dunedin, New Zealand (46°S, 171°E). The signals received have group-delay times that differ in the order of 10 ms, which can be explained by the differences in southern-hemisphere sub-ionospheric propagation time from duct exit region to receiver for the two sites. This difference has been used to determine the location of the duct exit region, with confirmation provided by arrival-bearing information from both sites. The whistler-mode signals typically occur one or two days after geomagnetic activity, with Kp\geq5. The sub-ionospheric-propagation model, LWPC, is used to estimate the whistler-mode power radiated from the duct exit region. These results are then combined with estimated loss values for ionospheric and ducted transmission to investigate the role of wave-particle amplification or absorption. On at least half of the events studied, plasmaspheric amplification of the signals appears to be needed to explain the observed whistler-mode signal strengths.