The kinetics of “hot” nitrogen atoms in upper atmosphere neutral chemistry

Abstract

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 O 2 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 O 2, it is found that the departure from a Mazwellian distribution is of little consequence for the odd nitrogen chemistry in the lower thermosphere.