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.

Investigation of TEC and VLF space measurements associated to L'Aquila (Italy) earthquakes

Abstract: . In this study, we report on Total Electron Content (TEC) and Very Low Frequency (VLF) space measurements derived from Global Positioning System (GPS) and DEMETER satellites, respectively. These measurements are associated with the earthquake (EQ) of a magnitude of 6.3, which occurred on 6 April 2009, in L'Aquila (Italy). Anomaly features are derived from the analysis of TEC and VLF observations recorded two weeks before and after the seismic event occurrence. A TEC map with an interpolated regional pixel resolution of 1° × 1° × 15 min in latitude, longitude and time was generated, allowing for the checking of a possible presence of disturbances over the L'Aquila region. This analysis is combined with the study of the time profile associated to the VLF flux density variations recorded by the Instrument Champ Electrique (ICE) experiment on-board DEMETER satellite. We discuss, on the one hand, the combination efficiency of the electronic density and the VLF electromagnetic measurements and, on the other hand, the difficulty to distinguish between global effects and regional ones related to the earthquake.

Computational results of the effect of localized ionospheric perturbations on subionospheric VLF propagation

Abstract: The purpose of this paper is to investigate the effect of localized perturbations in the lower ionosphere on subionospheric VLF propagation by means of the finite element methos. Owing to the complexity of the method, we have analyzed only a two-dimensional case where a localized perturbation lies on the great circle path between the transmitter and receiver. The first-order mode is assumed to be incident into the waveguide, and we have found significant oscillations in the changes of amplitude and phase in the vicinity of the perturbation (distance less than 600 km), which are due to mode conversion (i.e., conversion to higher-order modes) and subsequent multiple-mode propagation. In this paper, we present many computational results of the spatial distribution of scattered fields, frequency dependence, effects of enhancement factor, horizontal and vertical scale, and altitude of the perturbation, etc. Some important findings are summarized as follows: (1) The study of scattered fields has suggested that a single-mode propagation model does not hold and that the geometrical structure of the perturbation, especially in the horizontal direction, plays an important role in the scattering. (2) The magnitude of the perturbation, that is, the enhancement factor, effects mainly the magnitude of the scattered field.

A comparison of performances of linearized and global nonlinear 2-D inversions of VLF and VLF-R electromagnetic data

Abstract: The performances of linearized (local) and global nonlinear joint 2-D inversions of very low frequency (VLF) and VLF resistivity electromagnetic measurements are analyzed. A stable iterative inversion scheme is used in linearized inversion while the very fast simulated annealing approach is used in global nonlinear inversion. Synthetic noise‐free and noisy data due to three different models in complexity and two field examples are considered. Synthetic examples show that linearized inversion reveals the subsurface structure better than global nonlinear inversion provided the model has only a few parameters under inversion. Both linearized and global nonlinear inversions must be performed combining all available data in order to obtain the most reliable estimates of the subsurface parameters. Complex models with a large number of parameters are better to invert using global nonlinear inversion although the CPU time needed is always much longer than the one used in linearized inversion. Contrary to global n...

A comparison of sferics as observed in the very low frequency and extremely low frequency bands

Abstract: A large number of sferics were photographically recorded in the very low frequency (VLF) and extremely low frequency (ELF) bands at a UCLA field station in Hawaii. From the characteristic VLF waveforms it was clear that the VLF signals were generated from lightning discharges. It was found that an observable ELF component (slow tail) followed the VLF component in almost all cases. It was also found that about one third of the sferics observed were ELF signals, similar in appearance to slow tails but not preceded by observable VLF oscillations. Peak amplitudes were measured for both the VLF and ELF components of almost 3000 sferics. The results were tabulated in groups according to (1) whether the sferics were recorded during the day or during the night, (2) whether the polarity of the initial excursion of the ELF signal was positive or negative, and (3) whether the VLF and ELF components appeared together or separately. Amplitude distribution histograms were plotted for all cases. For those sferics possessing both VLF and ELF components, the VLF to ELF peak amplitude ratios were also tabulated separately as in (1) and (2) above, and ratio-distribution histograms were plotted. The more important results obtained from the histograms were as follows. 1. No significant differences were found between the amplitude distributions for the ELF waveforms that were preceded by VLF oscillations and those that were not. Hence, it is probable that both groups were generated by lightning discharges. 2. For both daytime and nighttime sferics the median value of the ELF amplitude was greater for ELF waveforms of positive polarity than for waveforms of negative polarity. 3. For both daytime and nighttime sferics the median value of the VLF/ELF peak-amplitude ratio was greater for ELF waveforms of negative polarity than for waveforms of positive polarity. 4. The polarity of the ELF waveform was predominantly negative at night and positive during the day (verified by a count of the polarities of almost 6000 additional ELF waveforms). An attempt is made to explain the experimental results in terms of known properties of lightning discharges, and some of the difficulties in making such an interpretation are indicated.

New type of ensemble of quasi-periodic, long-lasting VLF emissions at the auroral zone

Abstract: . A new type of the series of quasi-periodic (QP) very low frequency (VLF) emissions in frequency range of 1–5 kHz, and not associated with geomagnetic pulsations, has been discovered at auroral latitudes (L = 5.3) during the Finnish VLF campaign (held in December 2011). At least five unusually spectacular events, each with a duration of several hours, have been observed during the night under conditions of quiet geomagnetic activity (Kp = 0–1), although QPs usually occur during the daytime. Contrary to the QP emissions typically occurring during the day, the spectral structure of these QP events represented an extended, complicated sequence of repeated discrete rising VLF signals. Their duration was about 2–3 min each, with the repetition periods ranging from ~1 min to ~10 min. Two such nighttime non-typical events are reported in this paper. The fine structure of the separated QP elements may represent a mixture of the different frequency band signals, which seem to have independent origins. It was found that the periodic signals with lower frequency appear to trigger the strong dispersive upper frequency signals. The temporal dynamics of the spectral structure of the QPs studied were significantly controlled by some disturbances in the solar wind and interplanetary magnetic field (IMF). This finding is very important for future theoretical investigations because the generation mechanism of this new type of QP emissions is not yet understood.