Theory of wave polarization of radio waves in magnetospheric cavities

Abstract

When interpreting observations of radio waves of low frequency in magnetospheric cavities, it is often assumed that the electron concentra­tion is small enough for the ray paths to be treated as straight, but great enough to ensure that a wave that starts as a pure ordinary (or extraordinary) wave has a wave polarization close to that of an ordinary (or extraordinary) wave at each point of the path. This polarization changes because the magnitude and direction of the planetary magnetic field change along the path. But the change of the magnetic field also introduces coupling between the ordinary and extraordinary waves. If the electron concentration is small or zero, this coupling may be cumulative so that it restricts or prevents the change of polarization. A full-wave integration of the governing differential equations is used to study this problem. It is similar to the problem of limiting polarization for a radio wave emerging from the ionosphere into free space. It is concluded that the polarization of an initially ordinary (or extraordinary) wave remains close to that of a locally produced ordinary (or extra­ordinary) wave with the same wave-normal direction, provided that the plasma frequency exceeds a minimum value. This value depends on the direction of the path. Some typical examples are given.