THE IMPACT OF GRAVITY-WAVES ON NITRIC-OXIDE IN THE LOWER THERMOSPHERE

Observations of nitric oxide (NO) by the Solar Mesosphere Explorer (SM E) during equinox indicate a lower-thermosphere equatorial minimum whi ch is at variance with theoretical predictions. To address this discre pancy a zonally averaged model of the thermosphere and upper mesospher e is used to evaluate the influence of a latitude variation in turbule nce. Five numerical simulations were performed with different latitude structures of eddy diffusion (K(T)), ranging from uniform in latitude , peaks at low, mid-, or high-latitude, to a hemispherically asymmetri c distribution. A local increase in eddy diffusion causes the lower th ermosphere to cool and induces a latitude pressure gradient that drive s horizontal and vertical winds. The circulation, turbulent transport and temperature dependent chemistry act to change the distribution of species. Comparison of the model predictions of NO with SME data, and simulated wind and temperature structure with empirical climatology, i ndicates a preference for a midlatitude peak in K(T).