Spread-Fobservations by the Alouette Topside Sounder Satellite

Ionograms recorded by the Alouette topside sounder satellite exhibit a variety of spread-F echo configurations. The observed echoes are found to be consistent with three qualitative radio-propagation models involving aspect-sensitive scattering by thin magnetic-field-aligned irregularities, ducting along broad irregularities, and refraction within large-scale reductions of electron density. A large number of Alouette ionograms were examined for the three types of spread F to determine their frequency of occurrence. Spread F appeared on 54 per cent of the topside ionograms; the three types exhibit markedly different patterns of occurrence in geomagnetic latitude and local time.