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 (Ne) and temperature (Te) 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 Ne structure in the morning from July to October, although the value of Ne observed by Hinotori is higher than that of DEMETER owing to higher solar flux. This result implies that the longitudinal structure of Ne may appear in any solar cycle. Further, a negative correlation between Ne and Te in the longitudinal structures appears in the morning when Ne is low, while a positive correlation appears around the magnetic equator when Ne 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 Ne and Te are dominant and nondominant. The observed wave numbers 3–4 for Ne are dominant during November–May and June–October, while they are dominant for Te during October–June and July–September. Both Ne and Te 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 Ne and Te 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 Ne and Te caused by difference in the condition of zonal winds driving E region dynamo and meridional winds modulating the ionospheric plasma structures. Key Points Annual variations of longitudinal structure of Ne and Te in the ionosphere Wave‐4 and wave‐3 dominate in December and September Zonal and medrional wind affect longitudinal structure of Ne and Te