Ground illumination by an h.f. transmitter through a twilight ionosphere

A theoretical investigation of the effect of the horizontal gradient in a model of the ionospheric twilight region on the pattern of illumination of the Earth by multihop propagation from an isotropic transmitter is described. A 2-dimenional, digital, ray-tracing program is used, and the signal losses due to absorption along the rays and spatial focusing or defocusing of the rays are computed. Focus points and nonilluminated gaps on the Earth´s surface are found. A graphical presentation is given which aids in understanding the form of the illumination patterns.