Lidar Observations of Predawn Thermosphere-Ionosphere Na (TINa) Layers Over Boulder (40.13 degrees N, 105.24 degrees W): Annual Phase Variations and Correlation With Sunrise and Tidal Winds

We have discovered that the peak phase time of predawn thermosphere-ionosphere Na (TINa) layers (& SIM;110-150 km altitude) undergoes clear annual variations with the earliest occurrence in summer and latest in winter over Boulder (40.13 & DEG;N, 105.24 & DEG;W), which are closely correlated to annual phase variations of sunrise and tidal winds. Such discoveries were enabled by the first characterization of 12 monthly composites of TINa layers from January through December using 7 years of lidar observations (2011-2017). Despite their tenuous densities, the predawn TINa layers have nearly 100% occurrence rate (160 out of 164 nights of observations). Monthly composites show downward-phase-progression TINa descending at similar phase speeds as Climatological Tidal Model of the Thermosphere tidal winds. These TINa layers occur in ion convergence but neutral divergence regions, modeled using tidal winds. These results support the formation mechanism (neutralization of converged TINa+ forming TINa) proposed previously and suggest that migrating tidal winds experience annual phase variations. With tons of cosmic dust falling on Earth every day, metallic atoms and ions (e.g., Fe, Na, K, and Ca+) are released via meteor ablation and sputtering into the upper atmosphere, forming permanent metal layers (& SIM;75-105 km) that have been known for nearly a century. What was unknown until 2011 was the existence of thermosphere-ionosphere metal (TIMt) layers that were discovered with high-sensitivity lidar observations first from Antarctica and then were observed globally. These neutral TIMt layers locate above the permanent layers with an upper reach to & SIM;200 km and exhibit intermittent occurrence. In 2021 surprising regular occurrence of TIMt layers in Na species (TINa) was reported for the first time from lidar observations over Boulder, Colorado, where TINa layers occur before dawn and after dusk nearly every night. By analyzing 7 years of lidar data, we have further discovered that the Boulder predawn TINa layers occur in earlier hours in summer than in winter. Such annual phase variations are correlated with sunrise and solar-heating-driven tidal winds. These TIMt layers are of great scientific interest as they provide unique tracers for making direct measurements in the least understood but crucially important "thermospheric gap" region of 100-200 km. First characterization of 12 monthly composites of Na mixing ratios reveals annual phase variations of predawn thermosphere-ionosphere Na (TINa) layers (110-150 km)Predawn TINa occur & SIM;2.5 hr earlier in summer than in winter, closely correlating to annual phase variations of sunrise and Climatological Tidal Model of the Thermosphere semidiurnal westward-propagating tidal windsTINa layers descend at similar rates as and in tidal-wind-modeled ion convergence regions, supporting neutralization of TINa+ forming TINa