e both reflection and refraction of the wave. Reflec- tion dominates when the refractive index changes rapidly over a height range, which is Chapter 7 Electromagnetic Waves small compared with the wavelength. Thus, reflection is the main mechanism for the low frequencies, where the refractive index changes more rapidly with real height and, more specifically, height measured in wavelengths. On the other hand, for the higher frequencies refraction dominates over reflection, even though the amount of refraction decreases as the frequency increases. If the angle between the ionospheric layer and the direction of propagation is a, the wave will follow a path whose radius of curvature, r, is given by equation 7.87. cosa dn (7.87) This radius of curvature might be sufficient to return the wave to Earth, depending on the angle, a, frequency, intensity of ionization, and other factors discussed later. lonospheric Attenuation Collisions occur between free electrons and gas molecules, which destroy the oscillatory energy acquired by the electron, converting it to random thermal energy. Thus, energy is taken from the wave that is consequently attenuated. At low heights, there are more gas molecules and SO more collisions (i.e., more attenuation). The number of collisions also depends on the amplitude of the electron vibrations, which increases with increasing wave- length and field strength. In the LF and MF bands (i.e., 150 kHz to 6 MHz), the D layer almost totally attenu- ates the wave SO that ionospheric propagation is mainly restricted to nighttime, when the D layer is absent. Then reflections take place off the lower edge of the E layer. This is rarely used as a means of communication, but it is a source of interference to MF broadcast services. At still lower frequencies, reflection off the lower edge of the ionosphere occurs before much attenuation takes place. Gyrofrequency-The Ordinary and Extraordinary Wave In the absence of a magnetic field, electrons would oscillate in the direction of the electric vector of the