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

摘要

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 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 A 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 Te 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 Te 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 A N_e 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 A 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.