First Lidar Observations and WACCM-X-Na Simulations of the Ionosphere–Thermosphere–Mesosphere (ITM) Na Transition Layer (~120–95 km) over Boulder (40.13°N, 105.24°W)

The coupling between the Ionosphere, Thermosphere, and Mesosphere (ITM) is driven by complex interactions between neutral dynamics and plasma processes. While the main metal layers (75–105 km) and thermosphere-ionosphere metal (TIMt) layers (110–150 km) have been studied extensively, the altitude region connecting them has historically received less attention. Using 12 monthly composites of high-sensitivity resonance-fluorescence lidar observations over Boulder, Colorado (40.13°N, 105.24°W), we report the first discovery of regular occurrence of ITM Na transition layer spanning ~95–120 km.
This layer identifies as a distinct feature occurring between the TINa layer above and the main Na layer below, bridging two disparate environments: the lower thermosphere/E–F regions dominated by ion chemistry, and the upper mesosphere/D-region dominated by neutral metal chemistry. This altitude range overlaps with a transition region recently identified via the amplitude ratio of semi-annual to annual oscillation (SAO/AO) of Na density (Chu & Chen, 2025). Furthermore, this region corresponds to the transition between a well-mixed regime dominated by eddy diffusion and a diffusively separated regime controlled by molecular diffusion.
The transition layer exhibits a coherent downward phase progression with an annual oscillation in phase speeds peaking at ~3.27 km/hr in June, driven by seasonal migrating and non-migrating tidal forcing. To investigate the formation mechanisms, we utilize WACCM-X-Na simulations to evaluate the convergence of neutral species and metal ions. The model results demonstrate that the layer forms within regions of tidal vertical wind and metal ion convergence. This suggests a combined effect: 1) tidal-driven neutral vertical wind convergence that directly compresses neutral Na atoms and 2) vertically convergent ion transport that enables in-situ production of Na atoms via neutralization of converged Na⁺ ions. By bridging the gap between the lower thermosphere and the upper mesosphere, the Na transition layer serves as a sensitive tracer of tidal energy coupling and neutral-ion interactions. This study provides critical observational constraints for whole-atmosphere models and advances our understanding of vertical transport in the coupled ITM system.