Daytime longitudinal structures of electron density and temperature in the topside ionosphere observed by the Hinotori and DEMETER satellites

Daytime longitudinal structures of the electron density ( N e ) and temperature ( T e ) in the topside ionosphere observed by Hinotori and DEMETER are examined under various conditions of solar flux, local time, and seasons. Results from both satellites show a similar longitudinal N e structure in the morning from July to October, although the value of N e observed by Hinotori is higher than that of DEMETER owing to higher solar flux. This result implies that the longitudinal structure of N e may appear in any solar cycle. Further, a negative correlation between N e and T e in the longitudinal structures appears in the morning when N e is low, while a positive correlation appears around the magnetic equator when N e is sufficiently enhanced during noontime in the high solar flux. A spectrum analysis performed on the DEMETER data reveals that wave numbers 1–2 for N e and T e are dominant and nondominant. The observed wave numbers 3–4 for N e are dominant during November–May and June–October, while they are dominant for T e during October–June and July–September. Both N e and T e show the largest power of wave number 3 in December and wave number 4 in September. Further, observed annual variations of wave numbers 3–4 for N e and T e also differ from wave numbers 3–4 generated by waves in the lower thermosphere. It can be interpreted as discrepancies between the longitudinal distributions of N e and T e caused by difference in the condition of zonal winds driving E region dynamo and meridional winds modulating the ionospheric plasma structures.