A STUDY OF APPARENT IONOSPHERIC MOTIONS ASSOCIATED WITH MULTIPLE TRAVELING IONOSPHERIC DISTURBANCES

The apparent undulatory motion of the ionospheric HF reflecting surface is studied using a method based on the relationship between angle-of-arrival and Doppler. The conditions are mid-latitude, daytime, F-region, winter/spring/summer. We usually find that multiple traveling ionospheric disturbances are active simulataneously, so that the HF Doppler evolves in a highly irregular and non-sinusoidal manner. We study the influence of this multiple-wave overlap on the apparent propagation velocity measured quasi-instantaneously. The apparent propagation velocity retains the key features observed by others, namely dominant propagation toward the E/SE, and virtually no propagation toward the W/NW. However, the apparent instantaneous velocity varies widely in both magnitude and azimuth over times as short as 500 s. This is explained in terms of multiple-wave overlap.