LF Waves Research Articles

The effects of ions on low frequency and very low frequency propagation in an abnormally ionized atmosphere

The purpose of this letter is to point out the important role of ions in affecting LF and VLF sky wave transmission during periods of abnormal ionization of the region below the normal D layer such as occurs in a polar cap proton event. The approach is quite simplified and approximate; however, it is adequate to demonstrate the considerable—and in some cases dominant—role of the ions and their importance in interpretation of VLF and LF sky wave propagation effects (phase and amplitude) during periods of abnormal ionization well below the normal D layer. As a consequence of the effect of ions, deductions about the chemistry and ionization reactions in this region that are based on interpreting the propagation effects as due to electrons alone are subject to serious, if not gross, error.

Propagation of the Low-Frequency Radio Signal

The propagation of a radio signal and the propagation in the time domain is reviewed for linear amplitude systems. The particular case of the propagation of a ground wave pulse is considered in detail. A stretching in the form or shape of the pulse is noted as a result of the filtering action of the propagation medium. Theoretical transfer characteristics for the media of propagation of LF signals are introduced and methods of computation are considered. The particular case of a signal transmitted between two points on the earth's surface is considered from the viewpoint of propagation in the time domain. The field of LF waves propagated around the earth is, in large measure, influenced by the reflection and transmission processes at the ionosphere. Such processes are evaluated theoretically with the aid of Maxwell's equations together with an equation which describes the electron motion in the presence of a static magnetic field, a superposed electrodynamic field together with mechanical collisions between electrons and ions, such as the Langevin equation of motion of the electron. The use of full mathematical rigor in the application of these equations is feasible and indeed desirable at LF. Thus, the application of these equations to an electron-ion model plasma with arbitrary orientation of the superposed magnetic induction results in anisotropic transmission and reflection properties. The full rigor can be applied to model plasmas in which the electron density and collision frequency vary with altitude.