Impact of Recent Aeronomically Relevant Laboratory Data to Our Understanding of Thermospheric Nitric Oxide (NO)

The collisionally-dominated mesosphere and lower thermosphere (MLT) remains one of the most poorly sampled regions of the upper atmosphere. Central to the MLT is NO which, although considered a minor species, plays a central role in determining the MLT temperature and ion composition. Because of the relative paucity of observations, our understanding of the detailed processes leading to NO formation and loss in the MLT relies on models somewhat more than elsewhere. Yet models are only as accurate as our knowledge of the physical and chemical processes that they contain. Many laboratory and quantum-chemical results relevant to NO production and loss have appeared in the past decade, some of which have modified the values of basic aeronomical parameters while others have revealed previously ignored processes to be important. This talk will specifically address the impact of the following recent results on the NO density and photoelectron spectrum:

The bulk of evidence supports the existence of an NO product channel from the reaction of vibrationally excited N₂(A) with atomic oxygen.

The most recent determinations of the electron impact cross sections for N₂ excitation, dissociation, and ionization are between 20-50% smaller than previous results.

The recent completion of a high-resolution determination of the optically-allowed N₂ oscillator strengths below the ionization threshold enables a more accurate determination of the N(⁴S,²D,²P) neutral dissociative yield. Additionally, many new laboratory and theoretical results pertaining to the chemical production and loss of odd nitrogen have appeared. Although room temperature rates for most of these reactions are in general agreement, the branching ratios and temperature dependence for several of the most important remain poorly known. The result is that MLT NO density measurements can be recovered with rather different model parameters.