Evidence for non‐local‐thermodynamic‐equilibrium rotation in the OH nightglow

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Abstract
Improved ground‐based Fourier transform infrared spectroscopic measurements of the OH Meinel (M) Δν= 2 and 3 nightglow emissions revealed unexpectedly intense transitions from high rotational levels. The rotational development in the P branches of the OH M (3,1), (6,3), and (7,4) bands has been followed to the P1(N″) transitions with N″ = 10, 12, and 13, respectively. These measurements indicate that ∼10% of the OH(X, ν′ = 7) column rotational population is typically in rotational levels with N′ ≥7, in sharp contrast with the corresponding local thermodynamic equilibrium estimate of ∼1%. The excess high‐N′ population also persists in the lower vibrational states of OH(X), as evidenced by the readily detectable high‐P1(N″) transitions in the (3,1) and (6,3) bands and by the presence of observable R branch heads in the (4,2) and (5,3) bands. The R heads in the (4,2) and (5,3) bands form at moderately high N′ and are typically much more intense than expected on the assumption of complete rotational‐translational equilibration throughout the OH M source region. These new results provide strong evidence for incomplete R‐T equilibration of OH(X²Π,3≤ν′≤7,J′) on a column basis under typical nightglow conditions.