TITLE:
  Non-local thermodynamic equilibrium in H2O 6.9 mu emission as
  measured by the improved stratospheric and mesospheric sounder

AUTHORS:
  Zaragoza G., M. Lopez-Puertas, A. Lambert, J. J. Remedios, 
  and F. W. Taylor

REFERENCE:
  J. Geophys. Res., 103, 31293-31308, 1998 

ABSTRACT:

A detailed analysis of the H2O 6.9 mu 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 (non-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 nu_2 fundamental band of
H2O) 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 H2O in the mesosphere. A value between 1.0 and 3.0E-12 
cm3/s, close to the lower limit of the measurements, has been deduced for
the rate coefficient of the vibrational exchange between the nu_2
vibrational mode of H2O and O2(1). Also, evidence of the excitation of
O2(1), with a quantum yield between 2 and 6, after photolysis of O3 in
the Hartley band has been found in the measurements.