Mid‐latitude Thermosphere‐Ionosphere Na (TINa) layers observed with high‐sensitivity Na doppler lidar over Boulder (40.13°N, 105.24°W)

We report the first lidar observations of regular occurrence of mid-latitude thermosphere-ionosphere Na (TINa) layers over Boulder (40.13 degrees N, 105.24 degrees W), Colorado. Detection of tenuous Na layers (similar to 0.1-1 cm(-3) from 150 to 130 km) was enabled by high-sensitivity Na Doppler lidar. TINa layers occur regularly in various months and years, descending from similar to 125 km after dusk and from similar to 150 km before dawn. The downward-progression phase speeds are similar to 3 m/s above 120 km and similar to 1 m/s below 115 km, consistent with semidiurnal tidal phase speeds. One or more layers sometimes occur across local midnight. Elevated volume mixing ratios above the turning point (similar to 105-110 km) of Na density slope suggest in situ production of the dawn/dusk layers via neutralization of converged Na+ layers. Vertical drift velocity of TINa+ calculated with the Ionospheric Connection Explorer Hough Mode Extension tidal winds shows convergent ion flow phases aligned well with TINa, supporting this formation hypothesis.