The kinetics of "hot" nitrogen atoms in upper atmosphere neutral chemistry

The fate of suprathermal ground state nitrogen atoms produced by auroral electron bombardment of the thermosphere is investigated using a kinetic theory approach. The processes considered are thermaliiation, reactive collisions and transport. At neutral gas temperatures and densities encountered at altitudes below 150 km, thermalication is virtually complete in a fraction of a second, and N(⁴S) atoms acquire a Maxwellian distribution. Transport is unimportant during thermalication due to the short time constant and small scattering length. The reactive collision rate of N(⁴S) with O₂ to produce NO molecules is very sensitive to the relative velocity of the reactants, and therefore to the form of the non-thermal velocity distribution fonction. Adopting a hard-sphere collision model to describe elastic collisions, and a best estimate for the reactive cross section with O₂, it is found that the departure from a Mazwellian distribution is of little consequence for the odd nitrogen chemistry in the lower thermosphere.