less than about 10nm and to ultraviolet radiation with wavelengths near 100nm; the other is at about 170km and is pro- duced by radiation of wavelengths 0-80nm. Countering this production, the free electrons tend to recombine Transmitter with the positive ions and to attach themselves to neutral molecules Dead range to form negative ions. Electrons can also leave a given volume by Skip range diffusion or by drifting away under the influences of temperature and pressure gradients, gravitational forces or electric fields set up by the movement of other ionisation. The electron density at a given height is governed by the so-called Continuity equation in terms of Figure 9.1 High frequency propagation paths via the the balance between the effects of production and loss. ionosphere at high latitudes: (a) sample distribution of Night-time electron densities are generally lower than in the electron density (arbitrary units) in northern hemisphere daytime because the rates of production are reduced. Figure 9.2 high-latitude ionosphere (adapted from Buchau, 1972); (b) gives examples of a night-time and a daytime height distribution of raypaths for signals of constant frequency launched with electron density. The ionisation is continuous over a wide height different elevation angles range, but there are certain height regions with particular charac- teristics, and these are known, following EV Appleton, by the letters Figure 9.1 escape whereas rays 2 are reflected back to ground. Rays D, E and F. The E-region is the most regular ionospheric region, 3 escape because they traverse the ionosphere at latitudes where the exhibiting a systematic dependence of maximum electron density electron density is low (the plasmapause trough (Muldrew, 1965)). on solar-zenith angle, leading to predictable diurnal, seasonal and Irregularities in the F-region are responsible for the direct backscat- geographical variations. There is also a predictable dependence of tering of rays 4. The low-elevation rays 5 are reflected to ground its electro