Decay of O 2 (a¹Δ g ) in the evening twilight airglow: Implications for the radiative lifetime

Time‐resolved spectra of the decay of the O 2 infrared atmospheric ( a¹Δ g → X ³Σ g − ) ( 0,0) band emission in the evening twilight at 41.7°N have been obtained using the Fourier transform infrared (FTIR) technique. The measurements were made during a two‐month summer‐fall period which included the autumnal equinox. The late twilight variations were well described by single exponentials with decay time constants which ranged from (44.4±1.6) min in early August to (61.1±2.1) min near the equinox. In this connection, a strong positive trend in the time constant was observed during late August and early September. When combined with available data for collisional quenching and for fall‐winter, upper‐mesospheric altitude distributions of O 2 (a¹Δ g , ν=0) in the late twilight, these observations support a radiative lifetime of about one hour.