NITRIC-OXIDE IN THE LOWER THERMOSPHERE

Satellite measurements of nitric oxide in the lower thermosphere (100- 120 km) show the density to be highly variable. The variation at low l atitudes is correlated with solar activity and the variation at polar latitudes is connected with geomagnetic activity. A study of a one-dim ensional, photochemical model shows that calculations of nitric oxide density in the lower thermosphere are sensitive to uncertainties in th e branching ratios for the production of excited and ground state nitr ogen by two reactions: the dissociative recombination of ionized nitri c oxide and the electron impact dissociation of molecular nitrogen. To a lesser extent, the calculations are also sensitive to uncertainties in the following four reactions: the reaction of excited nitrogen ato ms with molecular oxygen, the photodissociation of nitric oxide, die i on-molecule reaction between ionized molecular nitrogen and atomic oxy gen, and the deactivation of excited atomic nitrogen by atomic oxygen. The large variability in polar nitric oxide is produced by the variat ion in the auroral electron flux. A calculation using die model demons trates that the relationship between the electron flux and the nitric oxide density is non-linear. The Joule heating associated with auroral activity also produces variations in die nitric oxide density. The va riability in low latitude nitric oxide is produced by variations in th e solar soft x-ray flux. A model calculation shows the relationship be tween the x-ray flux and the nitric oxide density to be nearly linear.