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 su ggested that the process is responsible for the highly rotationally ex cited nitric oxide detected by the space shuttle experiment CIRRIS 1A. The influence of translationally hot atoms on the chemical compositio n and energetics of planetary thermospheres has been investigated by L ogan and McElroy [1976], Solomon [1983], Fox and Dalgarno [1983], Nagy et al. [1990], and Gerard et al. [1991, 1993]. Shematovich et al. [19 91], using a non-equilibrium kinetic model, have calculated the steady -state translational energy distribution of the ground state (4S) nitr ogen atoms in the daylit atmosphere of the Earth at 140 km altitude. T he calculated distribution shows a significant overpopulation of highe r energy atoms over a Maxwell-Boltzmann distribution at the local tran slational 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 th ermal 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 lat itudes this additional mechanism contributes 6 to 30 percent of the ni tric oxide,produced in the lower thermosphere.