Decay of O-2(a(1)Delta(g)) in the evening twilight airglow: Implications for the radiative lifetime

Time-resolved spectra of the decay of the O-2 infrared atmospheric (a(1) Delta(g)-->X(3) Sigma(g)(-)) (0,0) band emission in the evening twilight at 41.7 degrees 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 (a(1) Delta(g), v=0) in the late twilight, these observations support a radiative lifetime of about one hour.