Evidence for an OH(υ) excitation mechanism of CO2 4.3 μm nighttime emission from SABER/TIMED measurements -: art. no. D09307

The SABER instrument on board the TIMED satellite, successfully launched on 7 December 2001, measures the CO2 4.3 mum atmospheric emission at day and night, from the troposphere up to the thermosphere, with a near global latitude coverage and with a very high signal-to-noise ratio. SABER has also three channels near 15 mum for the measurements of the pressure-temperature structure and two channels around 2.0 and 1.6 mum, mainly sensitive to the OH(upsilon less than or equal to 9) overtone radiation from levels upsilon = 8-9 and upsilon = 3-5, respectively. In this paper we analyze the measurements of SABER in channel 7, centered near 4.3 mum, taken at night in the upper mesosphere and lower thermosphere under quiet (nonauroral) conditions. The measurements of the 4.3 mum radiance in this region are much larger than expected under local thermodynamic equilibrium (LTE) and show a strong correlation with the OH channel signal. It was proposed by Kumer et al. [1978] that the CO2(upsilon(3)) levels, responsible for the emission at 4.3 mum, were excited from OH(upsilon) via vibrational-vibrational energy transfer with N-2(1) and hence to CO2(upsilon(3)). SABER data (measuring simultaneously pressure, temperature, CO2 4.3 mum emission, and OH(upsilon) near-IR emission) offer an unprecedented data set for understanding the non-LTE excitation mechanisms of CO2(upsilon(3)) in the nighttime mesosphere. We have investigated the SABER 4.3 mum radiances with the help of a non-LTE radiative transfer model for CO2 and found that the large radiances can be explained by a fast and efficient energy transfer rate from OH(upsilon) to N-2(1) to CO2(upsilon(3)), whereby, on average, 2.8-3 N-2(1) vibrational quanta are excited after quenching of one OH(upsilon) molecule. A series of alternative excitation mechanisms that may enhance the nighttime 4.3 mum limb radiance were considered and found to be insignificant. The mechanism(s) whereby the energy is transferred from OH(upsilon) to N-2(upsilon) is (are) still uncertain. The populations of OH(upsilon) are not significantly affected by incorporation of this fast transfer since N-2 quenching of OH(upsilon) is negligible when compared to O-2 quenching.