occurring whis- of plasma waves and artificial aurorae (CCIR, 1990c). tlers originating in lightning discharges. These are dispersed audio- So far, no mention has been made of the height region below about frequency trains of energy, ducted through the ionosphere and then 100km. As already noted, the D-region is characterised by a com- propagated backwards and forwards along the Earth's magnetic- plex structure and high collision frequencies which lead to large field lines to conjugate points in the opposite hemisphere. Whistler daytime absorption of h.f. and m.f. waves. This absorption may be 9/10 The lonosphere and the Troposphere measured using fixed-frequency vertical-incidence pulses (Apple- electron densities in the F2-region, in principle short-term predic- ton and Piggott, 1954), or by monitoring C.W. transmissions at tions based on ionospheric probing measurements or on correlations ranges of 200-500km, where there is no ground-wave component with geophysical indices should be of great value for real-time and the dominant signals are reflected from the E-region by day and frequency management. In practice, however, aside from the tech- the sporadic-E layer by night. There is then little change in the nical problems of devising schemes of adequate accuracy: raypaths from day to night SO that, assuming night absorption can be neglected, daytime reductions in amplitude are a measure of the 1. Not all systems are frequency agile (e.g. broadcasting). prevailing absorption. Multifrequency absorption data give infor- 2. Effective schemes may require two-way transmissions. mation on the height distributions of electron density (Beynon and 3. Only assigned frequencies may be used. Rangaswamy, 1969). Auroral absorption is often too great to be measured in such ways, but special instruments known as riometers An alternative to short-term predictions is real-time channel can be used (Hargreaves, 1969). These operate at a frequency sounding; certain procedures involve a combination of the two around 30MHz and record 