EOMF-16. Boulder TINa Layers (100–150 km) Detected by High-Sensitivity Na Lidar and Their Relationship to Tidal Winds and TIDs

Abstract
Following the discovery of thermosphere-ionosphere metal (TIMt) neutral layers in the 100–200 km at McMurdo (77.83°S, 166.66°E), Antarctica (Chu et al., 2011), many more TIMt layers have been reported in the polar and low-latitude regions, most of which occurred irregularly. This study presents the first lidar observations of regular occurrences of mid-latitude thermosphere-ionosphere Na (TINa) layers over Boulder (40.13°N, 105.24°W), Colorado. TINa layers (~0.1–1 cm-3) up to 150 km detected by high-sensitivity Na lidar exhibit dusk and dawn layers with downward phase, which is consistent with the semidiurnal tidal phase. To be specific, regularly occurred TINa layers descend from ~125 km after dusk and from ~150 km before dawn, while the downward-progression phase speeds are ~3 m/s above 120 km and ~1 m/s below 115 km. One or more layers occasionally appear around local midnight and are likely related to strong TIDs (Traveling Ionospheric Disturbance) observed in TEC (Total Electron Content). The increase in volume mixing ratio above the turning point of the Na density slope (~105–110 km) strongly suggests that the dawn/dusk layer is produced in situ by neutralizing converged Na+ layers. These new discoveries will provide new insights to plasma-neutral coupling, thereby furthering our understanding of the dynamics and chemistry of the upper atmosphere. It has been very challenging to study the plasma-neutral coupling in the E-region because there were almost no tracers for neutral atmosphere measurements. For such reasons, the major goal of ICON (Ionospheric Connection Explorer) mission is to study the ion-neutral coupling in the upper atmosphere and near-Earth space environment. The mission measures wind and temperature below 110 km throughout the day and night, and enables the measurement from 110 km to 300 km during daytime for latitudes from -10°S to 42°N. Unfortunately, the ICON mission cannot measure the E-region (110–150 km) during nighttime. Now TINa layers detected by the high-sensitivity Na lidar over Boulder likely provide a tracer to measure neutral winds and temperatures in the nighttime E-region (100–150 km). As they are closely related to the metallic ions, TINa layers provide a natural laboratory to study the metallic ion transport and investigate their roles in ion-neutral coupling. These new discoveries provide a great opportunity to study the plasma-neutral coupling and fill some data gaps of ICON mission.