tional to the square of the wave frequency. The absorp- take place at heights of reduced ionisation between the E- and tion is referred to as non-deviative or deviative depending on F-regions, leading in some cases to round-the-world echoes (Fen- whether or not it occurs where the refractive index is close to unity. wick and Villard, 1963). Ducting can also occur within regions of Normal absorption is principally a daytime phenomenon. At fre- field-aligned irregularities above the maximum of the F-region. quencies below 5MHz it is sometimes SO great as to completely suppress effective propagation. The absorptions of the ordinary and 9.3.9 Scintillation extraordinary waves differ, and in the range 1.5-10MHz the extra- ordinary wave absorption is significantly greater. Ionospheric irregularities act as a phase-changing screen on transio- nospheric signals from sources such as Earth satellites or radiostars. 9.3.6 Amplitude fading This screen gives rise to diffraction effects with amplitude, phase and angle-of-arrival scintillations (Ratcliffe, 1956). If the ionosphere were unchanging the signal amplitude over a fixed path would be constant. In practice, however, fading arises as a consequence of variations in propagation path, brought about by 9.4 Communication and monitoring movements or fluctuations in ionisation. The principal causes of systems relying on ionospheric propagation fading are: Ionospheric propagation is exploited for a wide range of purposes, Variations in absorption. the choice of system and the operating frequency being largely 2. Movements of irregularities producing focusing and defocus- determined by the type and quantities of data to be transmitted, the path length and its geographical position. 3. Changes of path length among component signals propagated via multiple paths. 4. Changes of polarisation, such as for example due to Faraday 9.4.1 Communication systems rotation. Radio communication at very low frequencies (v.l.f.) is limited by These various causes lead to different depths of fadi