Observations and Simulations of Eddy Diffusion and Tidal Effects on the Semiannual Oscillation in the Ionosphere

We use the National Center for Atmospheric Research TIEGCM (Thermosphere Ionosphere Electrodynamics General Circulation Model) model to investigate the eddy diffusion and tidal effects on the ionosphere SAO (semiannual oscillation). We also use the COSMIC (Constellation Observing System for Meteorology, Ionosphere, and Climate) satellite GPS radio occultation observations to validate the simulation results. The TIEGCM is driven at the 97 km lower boundary by tidal and gravity wave (eddy diffusion coefficient) inputs. The eddy diffusion input can be constant or with an SAO modulation, and the tidal input has on and off options. The TIEGCM simulation with an SAO modulated eddy diffusion (with tidal input) agrees better with the COSMIC observation than that without the SAO. Turning off the tides at the lower boundary makes the TIEGCM-simulated ionospheric density much higher than the COSMIC observation. The simulations showed two results: (1) the need to add the SAO modulation to the eddy diffusion and (2) how tides reduce the ionospheric density and SAO. As to how much of the SAO should be added to the eddy diffusion is dependent on the amplitudes of the tides since both can have effects on the ionospheric density. The TIEGCM results also demonstrate that the ionospheric density diurnal signal is mostly in situ excited, while the semidiurnal signal comes from lower atmosphere.