Electromagnetic transmission properties of a model ionosphere in the frequency range .001-.1 HZ.

Abstract

An attempt is made to determine the effect of the ionosphere upon the character of the power spectra of measured terrestrial magnetic field variations in the micropulsation region of the electromagnetic spectrum. A theoretical of the earth's ionosphere is developed for situations approximating the physical conditions at mid-geomagnetic latitudes. The model is constructed so that it is possible to compute the electromagnetic transmission properties in the frequency range .001 - .1 Hertz. The earth is assumed to be an infinite half-plane with the electrical properties of a perfect conductor. The structure of the ionosphere is taken to be plane layered with the planes parallel to the surface of the earth. The electrical properties of the ionosphere are approximated by the properties of an elementary plasma composed of ions, electrons, and neutral particles of a single type. At a given altitude, the mass of the heavy particles is assumed equal to the mean mass of particles expected to exist at that altitude. Plane waves are postulated at an altitude of 2000 km, incident at arbitrary angles. The analysis is conducted so that the problem reduces to a one dimensional problem of transmission through an inhomogeneous medium. Differential equations are derived for the reflection and transmission coefficients of the reduced one dimensional transmission path. These equations are integrated directly to yield the coefficients. After applying the necessary boundary conditions, a resulting magnetic field transmission coefficient is obtained. Two models are taken to represent daytime and nighttime conditions during minimum sunspot activity. The results show that varying the angle of incidence up to 60¿ from normal produces effects which are small in comparison to the effects from other causes. On the basis of the models chosen, the ionosphere can be considered transparent during the nighttime. During the daytime, the transmission character depends upon the type of wave mode propagating. In general, the effects are moderate compared to the dynamic character of observed power spectra of terrestrial geomagnetic fields. It is concluded that the ionosphere below 2000 km does not significantly contribute to the character of observed terrestrial geomagnetic power spectra.