Solar zenith angle and solar activity dependences of vertical profile of electron number density in the nightside auroral region

Solar zenith angle and solar activity dependences of electron number density in the nightside auroral region from the topside ionosphere to the magnetosphere within a geocentric radial distance of 2.6 R _E were statistically investigated based on analysis of 7-years of plasma wave data measured by the plasma wave instrument onboard the Akebono (EXOS-D) satellite. The results are summarized as follows: (1) Electron number density N _e changes depending on solar zenith angle and solar activity: N _c in sunlight is about 3 times larger than that in darkness, and N _c during solar maximum is about 10 times larger than that during solar minimum. (2) During solar maximum, geopotential scale height is almost constant within a range from 250 km to 400 km. During solar minimum, geopotential scale height is drastically changes at a geopotential height around 2000-2500 km, or an actual height of 3000-4000 km: Geopotential scale height is 250-400 km below the transition height and larger than 500 km above the transition height. In order to discuss the auroral phenomena in various seasonal and solar activity conditions, the variations of ambient electron number density, as obviously shown in this study, should be taken into consideration in future studies.