Model update for mesospheric/thermospheric nitric oxide

In reviewing the large deficit in model predictions of nitric oxide (NO) ab undance near the mesosphere/lower thermosphere [NO] peak region, this paper shows that the deficit is now largely reduced using an updated model. Prio r model/data comparison study (Swaminathan et al., 1998) which included com prehensive chemistry, contemporaneously measured solar soft x ray flux, and time-dependence of the suprathermal N(S-4) atom source, pointed to a large net deficit based on current NO chemistry. Although the suprathermal sourc e has been invoked for many decades, its importance has been recently dises tablished (Balalakrishnan et al. 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 bas ed 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 densit y within 30% of the data after accounting for the ever-present auroral tran sport contribution derived by Earth et al. (1999) from recent SNOE measurem ents. This result dramatically impacts the thermospheric nitric oxide abund ance problem that has existed for many decades. Further search for minor NO chemical sources is only appropriate while also quantifying the role of mu ltidimensional atmospheric dynamics in detailed model/data comparisons. (C) 2001 Elsevier Science Ltd. All rights reserved.