Seasonal variations of the electron density distribution in the polar region during geomagnetically quiet periods near solar maximum

Abstract

Meridional electron density distributions above 45° invariant latitude (ILAT) during geomagnetically quiet periods are statistically studied. Electron density data were obtained from plasma waves observed by the Akebono satellite from March 1989 to February 1991 (near solar maximum) in an altitude range of 274–10,500 km. Field‐aligned electron density profiles were fitted by the sum of exponential and power law functions. The transition height, where the power law term equals the exponential term, is highest in the summer (at low solar zenith angle (SZA)) at ∼4000 km and lowest in the winter (at high SZA) at ∼1800 km in a region of ILAT ≥ 70°; this is caused by the larger scale height in the summer (∼550 km) than that in the winter (∼250 km). The largest seasonal variation and SZA dependence of the electron density are found at an altitude of ∼2000 km with a factor of ∼50 (∼104 /cc in the summer, ∼103 /cc in the winter) in the trough, auroral, and polar cap regions. The seasonal variation and SZA dependence are smaller, about a factor of 5–10, above ∼5000 km. Day‐night asymmetries in each season (within a factor of 5) are smaller than the seasonal variation. The scale height is larger in the dayside than in the nightside in each season. These results indicate that photoionization processes in the ionosphere strongly control electron density distributions up to at least ∼5000 km in the trough, auroral, and polar cap regions.