Scattering

The direction of radio waves can also be altered through scattering. The effect seen by a beam of light attempting to penetrate fog is a good example of light-wave scattering. Even on a clear night, a highly directional search light is visible due to a small amount of atmospheric scattering perpendicular to the beam. Radio waves are similarly scat-

tered when they encounter randomly arranged objects of wave- length size or smaller, such as masses of electrons or water droplets. When the density of scattering objects becomes great enough, they behave more like a propagating medium with a characteristic refractive index.

If the scattering objects are arranged in some alignment or or- der, scattering takes place only at certain angles. A rainbow pro- vides a good analogy for field-aligned scattering of light waves. The arc of a rainbow can be seen only at a precise angle away from

the sun, while the colors result from the variance in scattering across the light-wave frequency range. Ionospheric electrons can be field-aligned by magnetic forces in auroras and under other unusual circumstances. Scattering in such cases is best perpen- dicular to the Earth’s magnetic field lines.