The OI 135.6-nm nightglow images from GUVI on board the TIMED satellite provide a unique opportunity to investigate the global plasma distribution in the F region. We report that there exist large seasonal-longitudinal variations in the F-region plasma density which may explain some aspects of the equatorial spread F (ESF) morphology. We suggest that an occurrence of high plasma density is responsible for the strong ESF activity during equinox periods at most longitudes, whereas an occurrence of low plasma density is responsible for the suppressed ESF activity during northern summer in the American-Atlantic sector.