lightning strikes, and this appears to be due to rapid changes in the orientation of the axes of the crystals as the electrostatic charge distribution changes. The occurrence of depolarization due to ice crystals and the magnitude of the effect on XPD vary with climate and geography, tending to be greatest in maritime climates. Further information on this subject can be obtained from CCIR (1990c) and Chu (1980). The Sky Wave Ultraviolet and X-radiation from the sun and, to a lesser extent, particle radiation from sunspots and galactic cosmic rays, cause the gases of the upper atmosphere to become ionized. The degree of ionization does not change uniformly with height, but there are rela- tively dense regions in layers above the Earth's surface. The properties of the four principal layers are D: About 50 to 90 km in altitude. This is greatly dependent on the sun's intensity and absorbs low-frequency radio waves on passage through the layer. E: About 90 to 130 km in altitude. It depends on the sun's intensity, with a small residual level at night, and exhibits a "sporadic" nature. F1: About 130 to 210 km in altitude. It merges with the F2 layer at sunset and depends on the sun's intensity. F2: About 210 to 400 km in altitude. It merges with the F1 layer at sunset and is influenced by Earth's magnetic field; and it depends on auroral zones, particularly in sunspot activity. Reflection and Refraction of Waves in the lonosphere Radio waves cause free electrons and ions to oscillate and reradiate, increasing the phase velocity of the wave but reducing the group velocity. Thus, the ionosphere behaves as a medium with a refractive index of less than 1. This value is formulated later, when other influences have been discussed, but for now, it is sufficient to know that the refractive index of the ionosphere deviates further from 1 for the lower frequencies and for the higher electron densities. The change of electron density with height results in a refractive index gradient, sym- bolized by dn/dh. This gradient will caus