Seasonal variation of thermospheric density and composition

Thermospheric neutral density and composition exhibit a strong seasonal variation, with maxima near the equinoxes, a primary minimum during northern hemisphere summer, and a secondary minimum during southern hemisphere summer. This pattern of variation is described by thermospheric empirical models. However, the mechanisms are not well understood. The annual insolation variation due to the Sun‐Earth distance can cause an annual variation, large‐scale interhemispheric circulation can cause a global semiannual variation, and geomagnetic activity can also have a small contribution to the semiannual amplitude. However, simulations by the National Center for Atmospheric Research (NCAR) Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model (TIE‐GCM) indicates that these seasonal effects do not fully account for the observed annual/semiannual amplitude, primarily because of the lack of a minimum during northern hemisphere summer. A candidate for causing this variation is a change in composition, driven by eddy mixing in the mesopause region. Other observations and model studies suggest that eddy diffusion in the mesopause region has a strong seasonal variation, with eddy diffusion larger during solstices than equinoxes, and stronger turbulence in summer than in winter. A seasonal variation of eddy diffusion compatible with this description is obtained. Simulations show that when this function is imposed at the lower boundary of the TIE‐GCM, neutral density variation consistent with satellite drag data and O/N 2 consistent with measurements by TIMED/GUVI, are obtained. These model‐data comparisons and analyses indicate that turbulent mixing originated from the lower atmosphere may contribute to seasonal variation in the thermosphere, particularly the asymmetry between solstices that cannot be explained by other mechanisms.