Non-local thermodynamic equilibrium in H<inf>2</inf>O 6.9 μm emission as measured by the improved stratospheric and mesospheric sounder

A detailed analysis of the H<inf>2</inf>O 6.9 μm limb radiance measured by the improved stratospheric and mesospheric sounder on board the Upper Atmosphere Research Satellite has been conducted using a non-local thermodynamic equilibrium LTE) radiative transfer model. There are important non-LTE effects in the measured radiance, both in the pressure modulator (sensitive only to the lines of the ν<inf>2</inf> fundamental band of H<inf>2</inf>O) and the wideband signal (similar to that of a conventional radiometer) above about 50-55 km. They show no variability with the solar zenith angle and a strong dependence on mesospheric temperatures. We have found that these effects are due to the departure from LTE of the (010) and (020) levels of H<inf>2</inf>O in the mesosphere. A value between 1.0 and 3.0×10-12 cm3 s-1, close to the lower limit of the measurements, has been deduced for the rate coefficient of the vibrational exchange between the ν<inf>2</inf> vibrational mode of H<inf>2</inf>O and O<inf>2</inf>(1). Also, evidence of the excitation of O<inf>2</inf>(1), with a quantum yield between 2 and 6, after photolysis of O<inf>3</inf> in the Hartley band has been found in the measurements. Copyright 1998 by the American Geophysical Union.