Simulations of the ionospheric annual asymmetry: Sun-Earth distance effect

In this study, the physical mechanisms of the Sun-Earth distance effects on the ionospheric annual asymmetry are investigated using Thermosphere-Ionosphere Electrodynamics General Circulation Model simulations. The main findings are the following: (1) The Sun-Earth distance affects the ionospheric annual asymmetry mainly through photochemical processes.

(2) During the daytime, this photochemical process results from the combined effect of ionization rate of atomic oxygen and the recombination with neutral species. The solar irradiation variation between December and June directly leads to about 6% December-June electron density difference via ionization of atomic oxygen, whereas the recombination with neutral composition contributes to 12%-15% December-June electron density difference.