Pluner. SpuceSi~, Vol. 37, No. I, pp. 61-72. 1989 00324633/X9 $3 OO-tO.00 Printed in Great Britain. Pergamon Press plc zyxwvut BEHAVIOUR OF THE O2 INFRARED ATMOSPHERIC (O-O) BAND IN THE MIDDLE ATMOSPHERE DURING EVENING TWILIGHT AND AT NIGHT zyxwvutsrqponmlkjihgfed M. J. LOPEZ-GONZALEZ, J. J. LOPEZ-MORENO, M. A. LOPEZ-VALVERDE and R. RODRIGO Instituto de Astrofisica de Andalucia, Apartado 2144, 18080 Granada, Spain (Received 14 July 1988) Abstract-Measurements of the (M band of the Infrared Atmospheric System of the molecular oxygen during early night-time show a non-negligible fraction of the emission which is a remnant of the very high concentration of atmospheric 02(a’AJ during the day. The importance of this contribution decreases progressively after sunset. The complexity of the photochemistry during twilight, involving fast variation in the concentration of the atmospheric compounds and in the solar fluxes, makes it difficult to study the decay of the daytime emission, We have developed a detailed treatment of the different processes governing the production and loss mechanisms of the O,(a’$) at times near sunset and during the night. This model is capable of explaining the remnant observed in measurements carried out by rocket-borne instrumentation. 1. INTRODUCTION The ((M) band of the Infrared Atmospheric System (IAS) of molecular oxygen at 1.27 pm is one of the brightest features of the terrestrial airglow. This band is strongly absorbed by oxygen in the lower atmo- sphere. It was first observed in the day and twilight airglow by Noxon and Valiance Jones (1962) with an airborne spectrometer. In the nightglow, it was observed by Gush and Buijs (1964) using a balloon borne interferometer to measure the emission from an altitude of about 25 km. The dayglow emission intensity decreases very quickly after sunset. Typical intensities for the 1.27 pm band are 25 MR in the dayglow and 100 kR in the nightglow. It is now generally accepted that solar photolysis of ozone is the main source of dayglow and early twilight IAS emission (Valiance Jones and Gattinger, 1963; Evans et al., 1968). However, the processes respon- sible for the nightglow emission are not yet clear. Gattinger (1968) proposed that the observed night- glow zenith intensity could be explained, if O,(a’A,) were produced in three body recombination of atomic oxygen. Recent laboratory measurements by Ali et al. (1986) have shown that atomic oxygen recombination produces O,(a’A& with a total efficiency of 0.7, although they were not able to distinguish between the fraction of O,(a’4) resulting from direct atomic oxygen recombination and that produced through a process involving collisions of molecular oxygen with an unidentified precursor previously formed in atomic oxygen recombination. The radiative lifetime of the O*(a’A,) is too long (-3900 s) (Badger et al., 1965) to ensure that the daytime 02(a’$) concentration be relaxed to their steady-state night-time values in the first hours of the night and possible remnant contribution must be considered and calculated with precision. Altitude profile measurements of O,(a’A,) night- glow emission have been performed (Evans et al., 1972; Bishop et al., 1972; Han et al., 1973; Thomas and Young, 1981; Makino et al., 1983; Lopez- Moreno et al., 1984; Greer et al., 1986) and several excitation mechanisms have been proposed in attempts to explain the measurements. Lopez-Moreno et al. (1986, 1988), McDade et al. (1987) and McDade and Llewellyn (1988) have recently analyzed the IAS of 0, at night-time from rocket-borne measurements. McDade et al. (1987) and McDade and Llewellyn (1988) analyzed the rocket measurements of Greer ef al. (1986) and Thomas and Young (1981) while L6pez-Moreno et al. (1986, 1988) analyzed the rocket observations of Lopez-Moreno et al. (1984). They concluded that direct and indirect atomic oxygen recombination and the remnant dayglow emission can explain in a large part the nightglow profile, below 95 km. Other mechanisms were studied to achieve a better agreement between predictions and observations. Oxi- dation of vibrationally excited OH at levels v < 4 by atomic oxygen was considered by Lopez-Moreno et al. (1988) as a minor source of the night-time IAS. McDade et al. (1987) and McDade and Llewellyn (1988) discussed the existence of another additional source of O,(a’A,) to explain their observed nightglow profile above 95 km altitude. This additional source was not identified, but the altitude dependence of the 61