Equatorial spread F statistics in the American longitudes: Some problems relevant to ESF description in the IRI scheme

Ground based ionograms (followed by radio wave scintillation and satellite in situ measurements) provide the most abundant data base for generating equatorial spread F (ESF) irregularity occurrence statistics. For practical purposes it is necessary that the statistics of the spread F occurrence distribution (from ionograms) be based, separately, on the range spreading and frequency spreading components of the spread F. The well known features of spread F include its day-to-day, seasonal and solar cycle variations. There seems to exist general consensus on the geomagnetic declination-, longitude- and solar- control of seasonal occurrence of spread F, whereas solar flux seems to produce superimposed effects, including the solar activity cycle in its occurrence features. The day-to-day variability is certainly the least understood of all the known spread F variabilities. Empirical description of the spread F occurrence statistics in the IRI (International Reference Ionosphere) prediction scheme requires detailed examination of the available statistics not only over the magnetic equator but necessarily also over low latitudes extending to equatorial anomaly location. We have produced, and are continuing to improve upon, such statistics based on spread F data covering the solar activity minimum and maximum pears for the two Brazilian stations, Fortaleza and Cachoeira Paulista. In this paper we present a comparative study of these results with the published statistics available by other technique and for other longitude sectors, and discuss the findings in terms of their implications for formulating an empirical description of the ESF for possible incorporation into the IRI model. (C) 1999 COSPAR. Published by Elsevier Science Ltd.