SUBTHERMAL NITRIC-OXIDE SPIN-ORBIT DISTRIBUTIONS IN THE THERMOSPHERE

The two NO(X2PI, upsilon=1, OMEGA=1/2,3/2) spin-orbit populations in t he Earth's thermosphere have been found to depart by more than a facto r of 2 from the ratio expected from thermal equilibrium. The effective temperature describing the observed population distribution is as muc h as 700 K lower than the local kinetic temperature. Absolute NO(upsil on=1, J, OMEGA) column densities were derived from high-resolution (1 cm-1) infrared earthlimb spectra for tangent altitudes up to 200 km, o btained in the CIRRIS 1A Space Shuttle experiment. Nonlinear least-squ ares synthetic spectral fitting was used to analyze the NO DELTAupsilo n=1 fundamental band emissions near 5.3 mum. The spin-orbit distributi on represents and third degree of freedom, along with vibration and ro tation, that is not in equilibrium with the local kinetic temperature. These observations may significantly impact the interpretation of ban d-integrated measurements of NO in the upper atmosphere, for which equ ilibrium sublevel distributions have been assumed. The subthermal dist ribution is most likely produced in the collisional uppumping of NO(up silon=0) by O atoms, which is the major source of NO(upsilon=1) in the thermosphere. This inference suggests that the present effect is rela ted to the subthermal spin-orbit distributions observed in laboratory studies of NO2 photodissociation.