Mesopause temperatures calculated from the O2(a1Δg) twilight airglow emission recorded at Maynooth (53.2°N, 6.4°W)
Mulligan, F. J.
Galligan, J. M.
Spectra of the O<sub>2</sub>(<i>a</i><sup>1</sup>Δ<i><sub>g</sub></i>)
airglow emission band at 1.27 µm have been recorded during twilight at Maynooth
(53.2°N, 6.4°W) using a Fourier transform spectrometer. Synthetic spectra have
been generated for comparison with the recorded data by assuming a particular
temperature at the emitting altitude, and modelling the absorption of each line
in the band as it propagates downward through the atmosphere. The temperature
used in generating the synthetic spectra was varied until an optimum fit was
obtained between the recorded and synthetic data; this temperature was then
attributed to the altitude of the emitting layer. Temperatures derived using
this technique for 91 twilight periods over an 18-month period exhibit a strong
seasonal behaviour with a maximum in winter and minimum in summer. Results from
this study are compared with temperatures calculated from the OH(3, 1) Meinel
band recorded simultaneously. In winter OH temperatures exceed O<sub>2</sub>
values by about 10 K, whereas the opposite situation pertains in summer; this
result is interpreted in terms of a possible change in the altitude of the
mesopause as a function of season. Estimates of the twilight O<sub>2</sub>(0, 0)
total band intensity indicate that its intensity is lower and that its decay is
more rapid in summer than in winter, in agreement with earlier observations.