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.

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