Modulation of Thermospheric Circulation by Lower-Thermospheric Winter-to-Summer Circulation: The Atmosphere Gear Effect

This study investigates the impact of the lower-thermospheric winter-to-summer circulation on the thermosphere's thermal structure and meridional circulation. Using NCAR TIE-GCM, we compare simulations with and without the lower-thermospheric circulation, finding that its inclusion enhances summer-to-winter thermospheric circulation by 40% in the summer hemisphere but decelerates it in the winter thermosphere. Meanwhile, vertical wind exhibits stronger upward motion poleward of $\pm 30°$ latitude above ${10}^{-6}$ hPa ($\sim$174 km) when lower-thermospheric circulation is incorporated. This dynamic coupling functions as an atmospheric “gear mechanism,” accelerating momentum and energy transfer to higher altitudes. Including lower-thermospheric circulation improves agreement between the nudged run and NRLMSIS 2.1 in intra-annual variability (IAV) of mass density. This suggests lower-thermospheric circulation is a key factor in modulating IAV in the coupled thermosphere-ionosphere system. This study reveals a new coupling mechanism between the lower atmosphere, thermosphere, and ionosphere, with significant implications for understanding upper-atmospheric dynamics and improving space weather models.