Impact of the lower thermospheric winter‐to‐summer residual circulation on thermospheric composition

Gravity wave forcing near the mesopause drives a summer‐to‐winter residual circulation in the mesosphere and a reversed, lower thermospheric winter‐to‐summer residual circulation. We conducted modeling studies to investigate how this lower thermospheric residual circulation impacts thermospheric composition (O/N 2 ). We found that the upwelling associated with the residual circulation significantly decreases O/N 2 in winter and the downwelling in summer slightly increases O/N 2 . Consequently, the residual circulation reduces the summer‐to‐winter latitudinal gradient of O/N 2 , which causes the simulated latitudinal gradient of O/N 2 to be more consistent with observations. The smaller summer‐to‐winter latitudinal gradient of O/N 2 would decrease the ionosphere winter anomaly in model simulations, which would bring the simulated winter anomaly into better agreement with ionospheric observations. The lower thermospheric residual circulation may be a process that has been largely ignored but is very important to the summer‐to‐winter latitudinal gradients, as well as annual/semiannual variations in the thermosphere and ionosphere.