Zonal wind [m/s] at 250 km for 12 UT and geographic latitude of 30o for simulations a. with tides at the lower boundary (LB), b. without tides at the LB, c. differences between a. and b. which isolates the effect of the LB tides, indicating the strong changes due to upward propagating tides after DOY 232.
Frontiers in Astronomy and Space Sciences: The vertical coupling of the lower and upper atmosphere via atmospheric solar tides varies on time scales from days, seasons to interannually, and modifies the thermosphere and ionosphere system. The Ionospheric Connection (ICON) explorer is designed to study the vertical coupling. In this study we use ICON data from the 220-270 Day Of Year (DOY), 2020 time period when large changes in the migrating semidiurnal tide (SW2) and the zonal and diurnal mean zonal wind occur within 8 days after DOY 232. We use the thermosphere-ionosphere-electrodynamics general circulation model (TIEGCM) driven by observationally fitted tides via the Hough Mode Extension (HME) method to isolate the effect of the changing upward propagating tides on the dynamics, composition, and plasma distribution . We find that associated with the distinct SW2 changes, the zonal and diurnal mean zonal wind at 250 km undergoes a similar dramatic change in the latitudinal structure. The analysis of SW2 reveals that the antisymmetric HMEs become more prevalent after DOY 232 compared to before. Similar latitudinal and temporal changes to SW2 are also found in the migrating diurnal, terdiurnal and quad-diurnal tides (DW1, TW3, QW4, respectively) at 250 km. Ter- and quad-diurnal tides are not included in the HME lower boundary forcing and are generated internally in the model. Especially, TW3 is strong in the thermosphere (almost 2/3 of SW2) and most likely caused by nonlinear tidal interaction between DW1 and SW2 above 130 km. Surprisingly, the solar in-situ forcing of TW3 and SW2 in the upper thermosphere is not nearly as important as their upward propagating tidal component, which contradicts previous findings during other seasons and solar cycle conditions. The strong dynamical changes during the study period lead to an approximately 15-20\% zonal and diurnal mean NmF2 decrease, which has a major contribution from the roughly 10\% decrease in the $O/N_2$ at 300 km during the same period. These changes are stronger than general seasonal behavior which are eliminated in the reported numbers due to using differences of simulations.