Statistical behavior of the longitudinal variations of daytime electron density in the topside ionosphere at middle latitudes

Electron density in the topside ionosphere has significant variations with latitude, longitude, altitude, local time, season, and solar cycle. This paper focuses on the global and seasonal features of longitudinal structures of daytime topside electron density (Ne) at middle latitudes and their possible causes. We used in situ Ne measured by DEMETER and F-2 layer peak height (h(m)F(2)) and peak density (NmF2) from COSMIC. The longitudinal variations of the daytime topside Ne show a wave number 2-type structure in the Northern Hemisphere, whereas those in the Southern Hemisphere are dominated by a wave number 1 structure and are much larger than those in the Northern Hemisphere. The patterns around December solstice (DS) in the Northern Hemisphere (winter) are different from other seasons, whereas the patterns in the Southern Hemisphere are similar in each season. Around March equinox (ME), June solstice (JS), and September equinox (SE) in the Northern Hemisphere and around ME, SE, and DS in the Southern Hemisphere, the longitudinal variations of topside Ne have similar patterns to h(m)F(2). Around JS in the Southern Hemisphere (winter), the topside Ne has similar patterns to NmF2 and h(m)F(2) does not change much with longitude. Thus, the topside variations may be explained intuitively in terms of h(m)F(2) and NmF2. This approach works reasonably well in most of the situations except in the northern winter in the topside not too far from the F-2 peak. In this sense, understanding variations in h(m)F(2) and NmF2 becomes an important and relevant subject for this topside ionospheric study.