sphere is split into charac- quency is defined in terms of an universal constant and the square teristic ordinary and extraordinary wave components of appropriate root of the electron density (Ratcliffe, 1959). Table 9.1 lists the power. At m.f. and above these components may be regarded as magnitude of the refraction and of other propagation parameters for travelling independently through the ionosphere with polarisations signals at a frequency of 100MHz which traverse the whole ionos- which remain related, but continuously change to match the chang- phere. ing propagation path and associated ionospheric conditions. The phase paths of the ordinary and extraordinary wave components 9.3.2 Change in phase-path length differ, SO that in the case of transionospheric signals when the components have comparable amplitudes, the plane of polarisation The phase-path length is given approximately as the integral of the of their resultant slowly rotates. This effect is known as Faraday refractive index with respect to the ray-path length. Ignoring spatial rotation. gradients, the change in phase-path length introduced by passage through the ionosphere to the ground of signals at v.h.f. and higher 9.3.5 Absorption frequencies from a spacecraft is proportional to the total electron content. This is the number of electrons in a vertical column of unit Absorption arises from inelastic collisions between the free elec- cross-section. trons, oscillating under the influence of the incident radio wave, and the neutral and ionised constituents of the atmosphere. The absorp- 9.3.3 Group delay tion experienced in a thin slab of ionosphere is given by the Apple- ton-Hartree equations (Ratcliffe, 1959) and under many conditions The group and phase velocities of a wave differ because the ionos- is proportional to the product of electron density and collision phere is a dispersive medium. The ionosphere reduces the group frequency, inversely proportional to the refractive index and inver- 9/6 The lonosphere and the Troposphere sely propor