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An analysis has been made of spread-F data obtained from LG.Y. f-plots for several ionosonde stations grouped about longitude 75 W. to establish whether there is any connection between the severity of frequency-spreading (ilJ) and the time of day, season of the year, magnetic activity, height of the F layer, critical frequency of the F layer, and the latitude of the ionosonde station. The diurnal variations of the severity of frequency spreading are found to vary considerably with latitude and season and no clear pattern emerges. Magnetic activity affects the value of ilf but again in a complex way which varies with latitude. The magnitude of ilf seems to be greatest when the layer is high and descending at low and middle latitudes but not at high latitudes. At all latitudes the magnitude of ilf is greatest when the critical frequency is lowest. This is considered to be the dominant effect having a profound influence on the diurnal and seasonal distributions of ilf. These results are discussed in terms of the hypothesis that frequency spreading is due to the availability of a range of values of N IDJ1x. at the maximum of the F2 layer. This range of values is thought to correspond to a system of irregularities each involving an enhancement or a deficiency of electron density relative to the background ionization. The extra ionization involved in the irregularities is estimated to be of the order of 10' electrons/c.c. and is found to vary little with season, magnetic activity, and latitude.

Spread?F and the Perturbations of the Maximum Electron Density of the F Layer