The electromagnetic field is characterized by the electric and magnetic fields which are vector quantities. The direction taken by the electric-field vector at each point along the path defines the polarization of the field. Faraday rotation of the electric field occurs when a signal propagates through the ionosphere. This rotation is readily observed at microwave frequencies and provides a useful way to measure the integrated electron density along the path.

We are concerned here with the more subtle changes in polarization that are caused by scattering in the lower atmosphere. Significant changes in polarization that are caused by wide-angle scattering in the troposphere are observed for scatter propagation beyond the horizon. By contrast, line-of-sight propagation is dominated by very-small-angle forward scattering. We have assumed so far that the change in polarization for this type of propagation is negligible. That assumption permits one to describe the propagation of light and microwaves in terms of a single scalar quantity. We now need to test this assumption by calculating the depolarization of the incident field.

If the transmitted wave is linearly polarized, we want to estimate how much the electric field rotates as the signal travels through the random medium. Two rather different descriptions of depolarization have been developed and apparently describe different aspects of the same phenomenon. The first is based on diffraction theory.