 opaque to the lower production are hard X-rays with wavelengths less than about 2nm h.f. waves and partially reflecting at the higher frequencies. It and Lyman-a radiation at 121.6nm. Chemical reactions responsible results from a number of separate causes and may be classified into lonospheric effects on radio signals 9/5 different types (Smith and Matsushita, 1962), each with charac- velocity and introduces a group delay which for transionospheric teristic occurrence and other statistics. In temperate latitudes spo- signals at v.h.f. and higher frequencies, like the phase-path change, radic-E arises principally from wind shear, close to the magnetic is proportional to the total electron content. equator it is produced by plasma instabilities and at high latitudes it is mainly due to incident energetic particles. It is most common at Table 9.1 Effect of one way traverse of typical mid-latitude low latitudes where it is essentially a daytime phenomenon. ionosphere at 100MHz on signals with elevation angle Irregularities also develop in the D-region due to turbulence and above 60 degrees (CCIR, 1990a) wind shears and other irregularities are produced in the F-region. The F-region irregularities can exist simultaneously over a wide Effect Night Frequency range of heights, either below or above the height of maximum dependence electron density, and are referred to as spread-F irregularities. They are found at all latitudes but are particularly common at low lati- tudes in the evenings where their occurrence is related to rapid Total electron changes in the height of the F-region (Cohen and Bowles, 1961). content Faraday rotation 15 rotations 1.5 rotations Group delay 12.5us 1.2us lonospheric effects on radio signals Change in phase- 5.2km 0.5km A radio wave is specified in terms of five parameters: its amplitude, path length phase, direction of propagation, polarisation and frequency. The Phase change 7500 radians 750 radians principal effects of the ionosphere in modifying these parameters Phase stability 150 radia