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A study has been carried out on the occurrence of bottom side equatorial\udspread F (ESF) and its dependence on the polarity and magnitude of the\udthermospheric meridional wind just prior to ESF occurrence during summer,\udwinter and equinox seasons of solar maximum (2002) and minimum years (1995),\udusing ionosonde data of Trivandrum (8.5° N, 76.5° E, dip=0.5° N)\udand SHAR (13.7° N, 80.2° E, dip ~5.5° N) in the Indian\udlongitude sector. In this study, we have examined the changes in the\udthreshold height of the base of the F layer for the triggering of ESF,\udirrespective of the magnitude and polarity of the meridional winds during\udthe above periods. The study indicates that the threshold height above which\udESF triggering is entirely controlled only by the collisional R-T\udinstability is least for summer months, with higher values for winter and\udequinox, during the solar minimum period, whereas for the solar maximum period the\udthreshold height is least for winter, with higher values for summer and\udequinox. But the range over which the threshold height varies is very narrow\ud(<15 km) for solar minimum in relation to the large range of variation\ud(>50 km) in the solar maximum epoch. Further to this, the study also\udreveals a clear-cut increase in threshold height with solar activity for all\udseasons. Clear-cut seasonal variability is also observed in the threshold\udheight, especially for solar maximum. The study quantifies the level of the base of\udthe F layer below which neutral dynamical effects play a decisive role in\udthe triggering of ESF during different seasons and solar epochs

On the seasonal variations of the threshold height for the occurrence of equatorial spread F during solar minimum and maximum years