The interpretation and synthesis of certain spread-Fconfigurations appearing on equatorial ionograms

On ionograms obtained near the magnetic equator the rectangular configuration called ‘equatorial spread F’ arises from scattering in the vertical plane (passing through the ionosonde) normal to thin, magnetic field-aligned irregularities located at or beneath the base of the F layer [Cohen and Bowles, 1961]. It is shown in the present paper that some strikingly different spread-F configurations on equatorial ionograms are due to irregularities of the same kind, but embedded in the F layer. Since these other configurations result from similar irregularities, the category of ‘equatorial spread F’ is generalized to include them. An ‘ionogram’ corresponding to scattering from an individual ‘equatorial spread F’ irregularity can be calculated. A composite ionogram resulting from a number of such scatterers can then be synthesized by superposition. By appropriate distributions of scattering centers in the east-west plane, many features of the ‘equatorial spread-F’ configurations observed on Huancayo ionograms can be simulated. This process of ionogram synthesis constitutes a new technique for the determination of (1) the height of patches of small-scale irregularities in the F region; (2) the horizontal distance of the patches from the ionosonde in the vertical east-west plane; (3) the thickness of the patches; (4) their-east-west extent; and (5) the east-west component of their drift velocity. The patches are measured to have an east-west extent of up to 300 km and a thickness between 10 and 50 km. Their drift velocity is eastward, gradually decreasing from 200 to 100 meters/sec after 2100 hours local time.