HIGHLY ROTATIONALLY EXCITED NITRIC-OXIDE IN THE TERRESTRIAL THERMOSPHERE

Reaction of fast non-thermal N(4S) atoms with O2 molecules is shown to produce NO with large rotational and vibrational excitation. It is suggested that the process is responsible for the highly rotationally excited nitric oxide detected by the space shuttle experiment CIRRIS 1A. The influence of translationally hot atoms on the chemical composition and energetics of planetary thermospheres has been investigated by Logan and McElroy [1976], Solomon [1983], Fox and Dalgarno [1983], Nagy et al. [1990], and Gerard et al. [1991, 1993]. Shematovich et al. [1991], using a non-equilibrium kinetic model, have calculated the steady-state translational energy distribution of the ground state (4S) nitrogen atoms in the daylit atmosphere of the Earth at 140 km altitude. The calculated distribution shows a significant overpopulation of higher energy atoms over a Maxwell-Boltzmann distribution at the local translational temperature of 555K, of one and three orders of magnitude at energies of 0.5eV and 0.75eV, respectively, and the rate coefficient k(ne) for the process N(4S) + O2 --> NO + O + 1.385eV (1) is enhanced by several orders of magnitude over the thermal rate coefficient k(e). At an altitude of 110 km, where the temperature is about 275K, the thermal rate coefficient obtained from the standard expression k(e) = 4.4 x 10(-12)exp[-3220/T]cm3sec-1 [Rees, 1989] is 3.6 x 10(-17)cm3sec-1, whereas the nonequilibrium value is 2.0 x 10(-11)cm3s-1 [Shematovich et al. 1991]. The reaction of the hot N(4S) atoms with O2 provides an additional mechanism for the production of nitric oxide. Gerard et al. [1991] calculated that for solar minimum conditions at equatorial latitudes this additional mechanism contributes 6 to 30 percent of the nitric oxide,produced in the lower thermosphere.