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 signatures of ionospheric heating by HIPAS

Abstract: The amplitude and phase of subionospherically propagating VLF signals are known to be sensitive indicators of the electron density and temperature at D region altitudes. In this paper we present new observations at a station in Alaska positioned to provide measurements of VLF signals that have propagated beneath the ionospheric region heated by the high-power auroral stimulation (HIPAS) HF heating facility near Fairbanks, Alaska. Analysis of data from HIPAS campaigns conducted in fall 1992 and spring 1993 has shown that in roughly 60% of the cases analyzed, the amplitude of the 23.4-kHz signal from the NPM transmitter in Hawaii as observed in Fort Yukon, Alaska, exhibited a measurable change in amplitude with the same on/off modulation pattern as that of the HIPAS HF transmissions at 2.85 MHz. In almost 70% of the cases analyzed, the same signal exhibited similar measurable changes in phase. The amplitude changes ranged from −0.2 dB to +0.5 dB, and the sensitivity of the measurement was approximately ± 0.02 dB. The phase changes ranged from −4.5° to −0.3°, and the sensitivity of the measurement was typically ± 0.4°. It is demonstrated that the phase and amplitude changes can be used as diagnostic tools to determine characteristics of the ambient electron density profile above the HIPAS facility.

On the impedance of a long buried horizontal wire antenna

Abstract: Simple engineering expressions for the external impedance per unit length of a long buried horizontal electric antenna are derived by employing finitely conducting earth image-theory techniques. The results should be applicable to horizontal wire antennas employed in the extremely low frequency (ELF) and very low frequency (VLF) bands.

Simulation on the Electromagnetic Environment in the Carrier of Airborne VLF Wire Antennas

Abstract: The airborne VLF (Very low frequency) transmitting system is commonly used in underwater communication with a very high power, which produces a complement electromagnetic field. It may threaten to the equipment and personnel on the aircraft. In this paper, the electromagnetic environment inside the aircraft with airborne DTWA (Dual Trailing Wire Antenna) is studied based on the numerical simulation software FEKO. The aircraft model and the VLF transmitting system model are established. By calculating the radiation near field of the DTWA, the electromagnetic field distribution and the induced current of the cables inside the aircraft cabin are analyzed. The conclusions are helpful for the further research on the application of airborne VLF communication system.

Multispectral analysis for electromagnetic wave field components in a magnetoplasma: application to narrow-band VLF emissions

Abstract: The multispectral analysis of the wave field components of an electromagnetic wave observed at a fixed point in a magnetoplasma is performed using a maximum entropy method. The validity of the analysis is a posteriori tested comparing the propagation characteristics of the waves, as derived from the spectral matrix estimates, to the propagation characteristics predicted by the magneto-ionic theory. The techniques so developed are applied to the analysis of ground-based transmitter signals observed on GEOS 1. Comparisons with results obtained from the Welch (1967) method show the superiority of the maximum entropy method of spectral analysis, at least in the case of narrow-band electromagnetic emissions. 34 references.

Evidence of critical dynamics in various electromagnetic precursors

Abstract: A wide variety of electromagnetic phenomena possibly related with earthquake (EQ) preparation processes have been reported in the literature during the last few decades. An interesting aspect in their study is the time series analysis of the related observables aiming at the investigation of any embedded dynamics. In this review article we focus on the study of fracto-electromagnetic emissions (fracto-EME) at the MHz band, the ultra-low frequency (ULF) magnetic field variations (<3 Hz) and the subionospheric very low frequency (VLF) propagation anomalies. We present recent analysis results for these electromagnetic signals using two independent methods which are known for their ability to uncover critical dynamics, the recently proposed method of critical fluctuations (MCF) and the natural time (NT) analysis method. Our results show that all three considered electromagnetic signals present critical characteristics from a few weeks up to a few days before the main shock occurrence. On the other hand, signatures for the departure from the critical (highly symmetrical) state towards a low symmetry state, a state during which there is high localization of the EQ preparatory process, have been identified in specific cases for the MHz fracto-EME as well as for the ULF magnetic field variations. Based on a multidisciplinary analysis, a four-stage model of EQ dynamics by means of fracto-EME in the MHz and kHz bands has recently been proposed. The hypothesis that the precursors considered in this article emerge during the spatially extensive phase of EQ preparation, which corresponds to the first stage of the abovementioned four-stage model, as well as their relation with the foreshock seismic activity are discussed.