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

Abstract : 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.