The availability of similar ionosondes at four locations in India covering the regions from the equator to the northern crest of the equatorial ionization anomaly (EIA) has provided a unique opportunity to study the role of the EIA and related processes in the occurrence of spread F. The study is conducted during equinoctial months of high solar activity period (Rz=145), when the probability of occurrence of spread F is maximum. This study also deals with vertical movement of the F layer due to both electric field and neutral winds and the occurrence of spread F. The present study shows that a well-developed anomaly beyond the latitude of 18°N is one of the base-level conditions for the generation of spread F. The total integrated vertical movement is the controlling factor for the onset of spread F and not the peak value of h′F alone. There seems to be no threshold value in the altitude of the F layer (h′F) for the occurrence of spread F at all stations. The direction and magnitude of meridional winds that are crucial in the generation of spread. F showed higher values of poleward wind on non-spread-F days, indicating that the irregularity growth is suppressed due to the poleward wind.
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