Model update for mesospheric/thermospheric nitric oxide

Abstract

In reviewing the large deficit in model predictions of nitric oxide (NO) abundance near the mesosphere/lower thermosphere [NO] peak region, this paper shows that the deficit is now largely reduced using an updated model. Prior model/data comparison study (Swaminathan, 1998) which included comprehensive chemistry, contemporaneously measured solar soft x ray flux, and time-dependence of the suprathermal N(⁴S) atom source, pointed to a large net deficit based on current NO chemistry. Although the suprathermal source has been invoked for many decades, its importance has been recently disestablished (Balakrishnan 1998). The present update to the model has no suprathermal N(⁴S) source, a recently measured higher NO⁺ recombination branching ratio and extended empirical solar x ray wavelength coverage based on combined YOHKOH and SNOE solar x ray flux data; and it is found that the large deficit is no longer present. The model predicts [NO] peak density within 30% of the data after accounting for the ever-present auroral transport contribution derived by Barth (1999) from recent SNOE measurements. This result dramatically impacts the thermospheric nitric oxide abundance problem that has existed for many decades. Further search for minor NO chemical sources is only appropriate while also quantifying the role of multidimensional atmospheric dynamics in detailed model/data comparisons.