Investigation of dynamic behind the seasonal variations of Es and sporadic Na layer near the turbopause of aurora free zone

The dearth of experimental observations, as well as dynamics study, near the turbopause (100-120 km) is the culprit for the mystery of this critical layer in the lower thermosphere. The sporadic E layer (Es) and the sporadic Na layer, occurring within this region, provide unique tracers for such topic. It is believed that the downward transporting of Es, which is full with metal ions (such as Na+), enriches the Na reservoir in the lower E region that causes the occurrence of sporadic Na layer within. In this paper, a statistic study show strong positive correlation between Es and sporadic Na layer above 100 km, measured by Utah State University Na lidar at Logan, Utah and CADI (ionosonde) at Bear Lake Observatory nearby. Both of these two turbopause features indicate strong seasonal variation with peak occurrence rate in the summer and minimum during the winter. To explain the dynamics behind the variations in the aurora free zone, HAMMONIA model produced monthly zonal wind climatology and Climatological Tidal Model of the Thermospehre (CTMT) are joining together to reproduce the hourly zonal wind variation within the turbopause, along with the temperature prediction from the two models. Using the well accepted wind shear theory of the Es formation, we conclude that such seasonal behaviors of Es and sporadic Na layer are due to large negative zonal wind shear driven by tidal wave peaking near the turbopause during summer time in the early evening.