Role of Eddy Diffusion in the Delayed Ionospheric Response to Solar Flux Changes

Abstract. Simulations of the ionospheric response to solar flux changes driven by the twenty-seven days solar rotation have been performed using the global 3-D Coupled Thermosphere/Ionosphere Plasmasphere electrodynamics (CTIPe) physics- based numerical model. Using the F10.7 index as a proxy for solar EUV variations in the model, the ionospheric delay at the solar rotation period is well reproduced and amounts to about 1 day, which is consistent with satellite and in-situ measurements. From mechanistic CTIPe studies with reduced and increased eddy diffusion, we conclude that the eddy diffusion is a primary factor that influences the delay of the ionospheric total electron content (TEC). We observed the peak response time of atomic oxygen to the molecular nitrogen ratio to solar EUV flux changes quickly during the increased eddy diffusion compared with weaker eddy diffusion. These results suggest that an increase in the eddy diffusion leads to faster transport processes and an increased loss rates resulting in a decrease of the ionospheric time delay. Furthermore, we found that an increase in solar activity leads to an enhanced ionospheric delay. At low latitudes, the influence of solar activity is stronger, as EUV radiation drives ionization processes that lead to composition changes. Hence, the combined effect of eddy diffusion and solar activity lead to longer delay in the low and mid latitude region.