Production of lower hybrid waves and field-aligned plasma density striations by whistlers

Two different source mechanisms that can cause the spectral broadening of VLF radio signals traversing the ionosphere, a phenomenon first observed by Bell et al. (1983), are proposed. The first is a nonlinear scattering of the whistler-mode VLF signals by preexisting ionospheric density fluctuations that render a mode conversion to lower hybrid waves. In the absence of ionospheric irregularities, a second mechanism, that involves a parametric instability, can excite the lower hybrid waves, as was proposed by Lee and Kuo (1984). Since both types of spectra were recorded in experiments, it is concluded that the two suggested source mechanisms contribute additively to the observed spectral broadening of injected VLF waves.

Spectral broadening of VLF radio signals traversing the ionosphere

A parametric instability at the lower hybrid frequency which is known as the ion quasi-mode instability may be excited in the ionosphere. The instability considered is a three-wave interaction in which an incident whistler mode wave near the lower hybrid frequency decays into a lower hybrid wave and an ion acoustic type of oscillation. Threshold calculations are made at intervals along the L = 4 magnetic field line starting at the earth's surface (the geomagnetic position of Siple Station, Antarctica) and proceeding to the equatorial plane. A model is developed for the ionospheric plasma parameters needed to evaluate the expression for threshold field strength. The fields directly radiated by the Siple transmitter are not strong enough to meet the requirement for threshold. However, it is possible that the threshold can be met for Siple transmissions which are amplified by natural processes and also for natural emissions.

The possible production of lower hybrid parametric instabilities by VLF ground transmitters and by natural emissions

Numerous phenomena that occur in the mesosphere, ionosphere, and the magnetosphere of the Earth are caused by the sources located in the lower atmosphere and on the ground. We describe the effects produced by lightning activity and by ground-based transmitters operated in high frequency (HF) and very low frequency (VLF) ranges. Among these phenomena are the ionosphere heating and the formation of plasma density inhomogeneities, the excitation of gamma ray bursts and atmospheric emissions in different spectral bands, the generation of ULF/ELF/VLF electromagnetic waves and plasma turbulence in the ionosphere, the stimulation of radiation belt electron precipitations and the acceleration of ions in the upper ionosphere. The most interesting results of experimental and theoretical studies of these phenomena are discussed below. The ionosphere is subject to the action of the conductive electric current flowing in the atmosphere–ionosphere circuit. We present a physical model of DC electric field and current formation in this circuit. The key element of this model is an external current, which is formed with the occurrence of convective upward transport of charged aerosols and their gravitational sedimentation in the atmosphere. An increase in the level of atmospheric radioactivity results in the appearance of additional ionization and change of electrical conductivity. Variation of conductivity and external current in the lower atmosphere leads to perturbation of the electric current flowing in the global atmosphere–ionosphere circuit and to the associated DC electric field perturbation both on the Earth’s surface and in the ionosphere. Description of these processes and some results of the electric field and current calculations are presented below. The seismic-induced electric field perturbations produce noticeable effects in the ionosphere by generating the electromagnetic field and plasma disturbances. We describe the generation mechanisms of such experimentally observed effects as excitation of plasma density inhomogeneities, field-aligned currents, and ULF/ELF emissions and the modification of electron and ion altitude profiles in the upper ionosphere. The electrodynamic model of the ionosphere modification under the influence of some natural and man-made processes in the atmosphere is also discussed. The model is based on the satellite and ground measurements of electromagnetic field and plasma perturbations and on the data on atmospheric radioactivity and soil gas injection into the atmosphere.

Atmosphere–Ionosphere Electrodynamic Coupling

Two-probe techniques have been implemented to measure the wavenumber-frequency spectrum of plasma turbulence in a laboratory plasma. The turbulence is driven by injected waves from an in-plasma crossed-dipole antenna. The methods find application in examining a number of theories relevant to the generation of lower hybrid waves in the earth's ionosphere. These waves play a key role in the bandwidth expansion of VLF waves traversing the ionosphere and in the source mechanism producing frequency-upshifted plasma lines (HFPLs). Thesis Supervisor: Prof. Min-Chang Lee Title: Leader, Ionospheric Plasma Research Group, Plasma Science and Fusion Center

Two-probe techniques to measure wavenumber-frequency spectrum of VTF plasma

We observed variations of the fluxes of electrons spilled out of the magnetosphere and short-period oscillations of the earth's magnetic field, stimulated by pulsed signals from a land- based VLF transmitter. (AIP)

I. A. Zhulin

Papers

Artificial injection of very low frequency (VLF) waves into the ionosphere and the magnetosphere of the earth