First Characterization of Pre-Dawn Thermosphere-Ionosphere Na (TINa) Layers from January through December Using 7 Years of Lidar Observations over Boulder: Seasonal Variations

After more than 30 years of lidar observations at mid-latitudes, only intermittent occurrence of thermosphere-ionosphere Na (TINa) layers had been reported from a few observations following the first discovery of these layers in Antarctica in 2011. Since then, the first discovery of regularly occurring mid-latitude TINa layers has been made over Boulder (40.13°N, 105.24°W), Colorado. For the first time, Na layers have been observed regularly descending from ~150 km before dawn every day.
Based on such regular occurring observations, we report the first characterization of pre-dawn TINa layers from January through December using 7 years of high sensitivity lidar data over Boulder. Despite their tenuous densities, the University of Colorado Boulder STAR lidar observations reveal pre-dawn TINa layers with nearly 100% occurrence rate (160 out of 164 nights of observations). Monthly composite contours show clear downward phase progression of the pre-dawn TINa layers through all 12 months of a year at this mid-latitude site, and the various starting time of pre-dawn TINa layers shows positive correlation with sunrise time from January through December. Utilizing a harmonic fitting method, we characterize the seasonal variations of these pre-dawn TINa layers with their annual and semi-annual oscillations quantified for the first time.
Such systematic characteristics of TINa layers will help study the sources and formation mechanisms, possibly providing new tracers to profile the neutral temperature and winds in the E and lower F regions (~100 to 200 km altitude) where neutral measurements are very rare but extremely important. These layers themselves also pose intriguing questions about the mechanisms of ion transport and plasma-neutral coupling in the E to F regions, as leading theories hypothesize that such neutral thermosphere-ionosphere metal (TIMt) layers are formed via the neutralization of converged TIMt+ ion layers. The intermittent occurrence of TIMt layers and their correlations to various ionospheric, geomagnetic, and solar activities present as a natural laboratory to test our understandings of the metal chemistry, ionic and neutral transport, and plasma-neutral coupling.