A scattering theory of VHF transequatorial propagation

Transequatorial propagation in the VHF band can occur when the ionosphere does not support such propagation by refraction. The phenomenon is closely associated with the scattering phenomenon known as spread- F. This scattering is caused by the presence of irregularities in the ionization density of the F-layer. An inverse power-law spectrum of irregularities, with scales [wavelength/(2π)] spread from tens of kilometers down to the electron gyroradius, is employed. A model of scattering from long curved irregularities, aligned along the earth's magnetic field, is developed. For transequatorial propagation, the curvature of the irregularities causes the divergence of rays emanating from the transmitter in the magnetic meridian to be replaced by convergence of the scattered rays in the magnetic meridian in many situations of practical interest. The theory leads to the conclusion that single partial reflections from field-aligned irregularities are all that are required to explain observations of VHF transequatorial propagation, and that multiple total reflections in a field-aligned duct are not needed.