Abstract First characterization of year‐round Na layers from 75 to 150 km is enabled with 7 years (2011–2017) of high‐detection‐sensitivity lidar observations over Boulder (40.13°N, 105.24°W). Clear annual and semiannual oscillations (AO and SAO) are revealed in the nightly‐mean thermosphere‐ionosphere Na (TINa) (∼105–150 km) number density and volume mixing ratio with the summer maximum but spring equinox (March/April) minimum. Such stark contrast to the summer minimum in the main Na layers (∼75–105 km) supports the theory of TINa formed via TINa + ion neutralization (). The SAO/AO amplitude ratio profiles (75–150 km) exhibit significant changes (∼0.06–2), linking TINa SAO to thermospheric density SAO and the minimal wave/eddy transport around midlatitude equinoxes which hinders TINa + ion production and upward transport via reduced diffusion of the main Na layer. Stronger TINa in autumn than in spring equinox is explained by the maximal (minimal) meteoric influx occurring in September (April).
Chu et al. (Tue,) studied this question.