Nighttime FF-region morphology in the low and middle latitudes seen from DMSP F15 and TIMED/GUVI

We investigate the seasonal, longitudinal, and altitudinal variations of the FF-region morphology at 2100–2300 LT in the low- and middle-latitudes using the data collected in August, September, and November of 2003. The topside morphology is investigated using in situ measurements of the O+O+ fraction and total ion density by the Defense Meteorological Satellite Program (DMSP) F15 satellite. The morphology of the equatorial ionization anomaly (EIA) near the FF peak altitude is investigated using the OI 135.6-nm radiance maps provided by the Global Ultraviolet Imager (GUVI) on board the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite. The hemispheric asymmetries of the topside ionosphere at 840 km in the months near solstices can be characterized by the reduction of the O+O+ fraction and ion density in the winter hemisphere compared to those in the summer hemisphere. The minimum O+O+ fraction and ion density layers occur around 30∘30∘ magnetic latitude in the winter hemisphere. During the fall equinox, the hemispheric asymmetries are reversed in the regions of opposite magnetic declinations. From the comparison of the topside morphology with the global wind circulation pattern at 2200 LT predicted by the Horizontal Wind Model 93 (HWM93) we infer that hemispheric asymmetry of the topside ionosphere is created primarily by the retardation of the downward plasma diffusion in one hemisphere through the field-aligned equatorward winds. The global EIA morphology does not conform to the topside morphology. The complex seasonal-longitudinal variations of the EIA strength and asymmetry are not explained simply by considering the modulation of the FF-layer height by the winds. The magnetic declination is not a useful tool in understanding the global EIA morphology.