Mid-latitude spread-F structure

Observations of spread-F at frequencies of 1.98, 3.84, and 5.80 MHz and multiple angles of arrival have been obtained with an HF radar near Brisbane. It is suggested that the spreading of the ionogram trace is due to a spread in the arrival angles of echoes, and that the reflection process may involve total specular reflection rather than scattering. The results indicate that the structure cannot be purely frontal with purely linear movement. The velocities are found to be much less than for coexisting traveling ionospheric disturbances.