The thermospheric odd nitrogen photochemistry - Role of non thermal N(4S) atoms

A nonequilibrium model to calculate the Maxwellian 'hot' N(4S) atom population using a Monte Carlo method is developed, and the NO vertical distribution for minimum solar cycle conditions is calculated. It is shown that the energetic photoelectrons produced by solar soft X-rays produce translationally 'hot' N(4S) atoms near 110 km which contribute significantly to the formation of the E-region NO peak observed at this altitude. Consideration of this new source of nitric oxide reconciles the need for an effective N(2D) quantum yield from N2 dissociation exceeding 50 percent derived from odd nitrogen models with the limits put on this value by laboratory and theoretical constraints. The altitude of the modeled NO peak is found to critically depend on the adopted N(2D) yield, and good agreement with the observed peak altitude and density is obtained using the quantum yield derived from theoretical considerations.