Absorption

Abstract

This chapter deals with the effect of absorption on reflection properties. The absorption, or dissipation of electromagnetic energy within the medium, can be due to conductivity (as in metals, and in the ionosphere). However, good insulators can also be absorbers at high frequencies, where the electromagnetic field energy is converted to heat via molecular or electronic excitations. The absorption is included in the Maxwell equation (1.2) by allowing the dielectric function ε to take complex values. In general, the curl of B is the sum of terms proportional to ∂ E/∂t and to the total current density. For non-magnetic media, and fields with the time variation e^−iωt, the form of (1.2) is retained, with the imaginary part of ε now proportional to the conductivity divided by the frequency (Born and Wolf, 1970, Section 13.1). The simplest model for conducting media is that of an electron gas, with mean free time between collisions τ. This leads to the dielectric function (see for example Kittel 1966, Booker 1984, Budden 1985)

$$\varepsilon \left( {\omega ,z} \right) = 1- \frac{{{\omega ^2}}}{{{\omega ^2} + {{i\omega } \mathord{\left/ {\vphantom {{i\omega } \tau }} \right. \kern- \nulldelimiterspace} \tau }}}$$

(1)

where ω _p is the plasma frequency. In the ionosphere, for example, ε is a function of height z through the proportionality of $\omega _p^2$ to the electron density, as well as through the dependence of τ on the electron, ion, and neutral species densities.