ses. If the angle between the parameters in CCIR (1990c). ionospheric layer and the direction of propagation is a, the wave will follow a path whose radius of curvature r is given by Equation 6.87. 6.4.15.2 Depolarisation due to ice crystals (6.87) Depolarisation due to clouds of ice crystals can be considerable and can occur in the absence of significant attenuation and depolarisa- tion due to rain. Since the major axes of the crystals are normally This radius of curvature may be sufficient to return the wave to close to the horizontal, depolarisation tends to a minimum for Earth, depending upon the angle a, frequency, intensity of ionisa- horizontal or vertical polarisation. Abrupt changes in XPD have tion, and other factors discussed later. been observed to coincide with lightning strokes and this appears to be due to rapid changes in the orientation of the axes of the crystals 6.5.3 lonospheric attenuation as the electrostatic charge distribution changes. The occurrence of depolarisation due to ice crystals and the Collisions occur between free electrons and gas molecules, which magnitude of the effect on XPD vary with climate and geography; destroy the oscillatory energy acquired by the electron, converting tending to be greatest in maritime climates. Further information on it to random thermal energy. Thus, energy is taken from the wave this subject may be obtained from CCIR (1990c) and Chu (1980). which is consequently attenuated. At low heights, there are more gas molecules so more collisions i.e. more attenuation. The number of The sky wave collisions also depends on the amplitude of the electron vibrations, which increases with increasing wave length and field strength. 6.5.1 The structure of the ionosphere In the I.f. and m.f. bands, i.e. 150kHz to 1.6MHz, the D layer almost totally attenuates the wave, so that ionospheric propagation Ultraviolet and X-radiation from the sun and, to a lesser extent, is mainly restricted to night time, when the D layer is absent. Then, particle radiation from sunspots a