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.

Reduction of the VLF Signal Phase Noise Before Earthquakes

Abstract: In this paper we analyse temporal variations of the phase of a very low frequency (VLF) signal, used for the lower ionosphere monitoring, in periods around four earthquakes (EQs) with magnitude greater than 4. We provide two analyses in time and frequency domains. First, we analyse time evolution of the phase noise. And second, we examine variations of the frequency spectrum using Fast Fourier Transform (FFT) in order to detect hydrodynamic wave excitations and attenuations. This study follows a previous investigation which indicated the noise amplitude reduction, and excitations and attenuations of the hydrodynamic waves less than one hour before the considered EQ events as a new potential ionospheric precursors of earthquakes. We analyse the phase of the ICV VLF transmitter signal emitted in Italy recorded in Serbia in time periods around four earthquakes occurred on 3, 4 and 9 November 2010 which are the most intensive earthquakes analysed in the previous study. The obtained results indicate very similar changes in the noise of phase and amplitude, and show an agreement in recorded acoustic wave excitations. However, properties in the obtained wave attenuation characteristics are different for these two signal parameters.

First Results of the Wave Measurements by the WHU VLF Wave Detection System at the Chinese Great Wall Station in Antarctica

Abstract: A Very Low Frequency (VLF) wave detection system has been designed at Wuhan University (WHU) and recently deployed by the Polar Research Institute of China at the Chinese Great Wall station (GWS, 62.22°S, 58.96°W) in Antarctica. With a dynamic range of ∼110 dB and timing accuracy of ∼100 ns, this detection system can provide observational data with a resolution that can facilitate space physics and space weather studies. This paper presents the first results of the wave measurements by the WHU VLF wave detection system at GWS to verify the performance of the system. With the routine operation for 3 months, the system can acquire the dynamic changes of the wave amplitudes and phases of various ground-based VLF transmitter signals emitted in both North America and Europe. A preliminary analysis indicates that the properties of the VLF transmitter signals observed at GWS during the X-class solar flare events are consistent with previous studies. As the HWU-GWS path crosses the South Atlantic Anomaly region, the observations also imply a good connection in space and time between the VLF wave disturbances and the lower ionosphere variation potentially caused by magnetospheric electron precipitation during the geomagnetic storm period. It is therefore well expected that the acquisition of VLF wave data at GWS, in combination with datasets from other instruments, can be beneficial for space weather studies related to the radiation belt dynamics, terrestrial lightning discharge, whistler wave propagation, and the lower ionosphere disturbance, etc., in the polar region.

Generation of extra and very low-frequency (ELF/VLF) radiation by ionospheric electrojet modulation using high-frequency (HF) heating waves

Abstract: Extra and very low-frequency (ELF/VLF) wave generation by modulated polar electrojet currents is studied numerically. Through Ohmic heating by the amplitude-modulated high-frequency heating wave, the conductivity and thus the current of the electrojet are modulated accordingly to set up the ionospheric antenna current. Stimulated thermal instability, which can further enhance the electrojet current modulation, is studied. It is first analysed analytically to determine the threshold heating power for its excitation. The nonlinear evolutions of the generated ELF/VLF waves enhanced by the instability are then studied numerically. Their spectra are also evaluated. The field intensity of the emission at the fundamental modulation frequency is found to increase with the modulation frequency in agreement with the Tromso observations. The efficiency enhancement by the stimulated thermal instability is hampered by inelastic collisions of electrons with neutral particles (mainly due to vibration excitation of N 2 ), which cause this instability to saturate at low levels. However, the electron inelastic collision loss rate drops rapidly to a low value in the energy regime from 3.5 to 6 eV. As the heating power exceeds a threshold level, significant electron heating enhanced by the instability is shown, which indeed causes a steep drop in the electron inelastic collision loss rate. Consequently, this instability saturates at a much higher level, resulting to a near step increase (of about 10–13 dB depending on the modulation wave form) in the spectral intensity of ELF radiation. The dependence of the threshold power of the HF heating wave on the modulation frequency is determined.

Excitation of electromagnetic whistler waves due to a parametric interaction between magnetosonic and lower oblique resonance modes in a cold, magnetized plasma

Abstract: We have studied the parametric interaction between a fast magnetosonic (FM) mode and a lower oblique resonance (LOR) mode in a cold, magnetized plasma using a kinetic, three dimensional Particle-in-Cell simulation code called the Large Scale Plasma. The FM mode is excited with a loop antenna driven at a frequency below the lower hybrid frequency (ωLH), while the LOR is excited at a frequency above ωLH. For historical purposes explained in the Introduction, we call the antennas which drive the FM mode and LOR mode Extremely Low Frequency (ELF) and Very Low Frequency (VLF) antennas, respectively. The antennas are modeled as magnetic dipoles (ρant = 0) and are assigned a time varying current density within a finite sized current loop. The VLF and ELF antennas are driven at 10 A and 3 A, respectively. The parametric interaction is excited with a combined ELF/VLF antenna (which we call a parametric antenna) and includes both antennas driven simultaneously in the same simulation domain. We show that the parametric antenna non-linearly excites electromagnetic (EM) whistler waves to a greater extent than the VLF antenna alone. We also show that the parametric excitation of EM whistler waves leads to greater emitted EM power (measured in Watts) compared with a VLF antenna alone.We have studied the parametric interaction between a fast magnetosonic (FM) mode and a lower oblique resonance (LOR) mode in a cold, magnetized plasma using a kinetic, three dimensional Particle-in-Cell simulation code called the Large Scale Plasma. The FM mode is excited with a loop antenna driven at a frequency below the lower hybrid frequency (ωLH), while the LOR is excited at a frequency above ωLH. For historical purposes explained in the Introduction, we call the antennas which drive the FM mode and LOR mode Extremely Low Frequency (ELF) and Very Low Frequency (VLF) antennas, respectively. The antennas are modeled as magnetic dipoles (ρant = 0) and are assigned a time varying current density within a finite sized current loop. The VLF and ELF antennas are driven at 10 A and 3 A, respectively. The parametric interaction is excited with a combined ELF/VLF antenna (which we call a parametric antenna) and includes both antennas driven simultaneously in the same simulation domain. We show that the paramet...

Simplified VLF/LF Mode Conversion Program with Allowance for Elevated, Abritrarily Oriented Electric Dipole Antennas.

Abstract: : This report presents an updated version of an earlier simplified mode conversion program for VLF/LF propagation in the earth-ionosphere waveguide. The new program includes the provision for calculating at an arbitrary height within the guide all three electric field components generated by an electric dipole of arbitrary orientation and height within the guide. The program is designed for treating air to air, ground to air or air to ground VLF/LF problems involving a waveguide channel which is horizontally inhomogeneous along the direction of propagation. (Author)