First Lidar Observations of Ionosphere‐Thermosphere‐Mesosphere (ITM) Na Transition Layer (∼120–95 km): Regular Occurrence Over Boulder (40.13°N, 105.24°W) and Possible Formation Mechanisms

Abstract Using lidar observations of 12 monthly composites of normalized Na density ratios, we have discovered an Ionosphere‐Thermosphere‐Mesosphere (ITM) Na transition layer from ∼120 to ∼95 km over Boulder, Colorado. The transition layer occurs regularly in all months with a distinct separation from the predawn TINa layer and exhibits coherent downward phase progression. The double‐layer structure in May–July differs from its single layer in other months. Annual oscillations dominate the descending phase speeds (maximizing ∼3.3 km/hr in June) and layer widths, reflecting seasonal variations in tidal wind forcing of Na + ions. The Na transition layer forms within regions of vertical wind convergence and modeled ion convergence, suggesting both vertical‐wind‐driven neutral convergence that directly concentrates neutral Na, and tidal‐wind‐converged Na + ions that recombine with electrons, contribute to the transition layer formation. These results establish the Na transition layer as a robust, dynamically‐chemically driven feature coupling the ITM.