Ionospheric propagation of hf and vhf radio waves across the geomagnetic equator

Transequatorial propagation is influenced by many of the same equatorial F region events that have been identified by using other radio and optical sensors. The daytime equatorial ionosphere provides unusual long‐range propagation conditions because of the peculiar latitudinal profile created by vertical drift over the magnetic equator. Radio signals reflected from this configuration are narrowly spread (coherent) yet have maximum frequencies that at times may reach 60 MHz. The inhomogeneities in the postsunset F region lead to a different, scatterlike propagation. Because of the wide range of scale sizes and plasma densities in the nighttime equatorial ionosphere, frequencies throughout the HF and VHF band are significantly distorted, and reflections can occur as high as 100 MHz. The irregularities responsible for these conditions follow the diurnal and seasonal trends of spread F only with certain qualifications. Horizontal and vertical drift of the scattering centers is evident from the use of both directional antennas and Doppler analysis. The reflection mechanisms are only partially accounted for, and the causes of the F region irregularities at all scale sizes are as yet unknown.