Understanding the transport of atomic oxygen within the thermosphere, using a numerical global thermospheric model

Abstract

A 2-dimensional version of the UCL global thermospheric model has been adapted by including detailed oxygen and nitrogen chemistry, with appropriate completion of the energy equation, and adding the thermal infra-red cooling by [O] and [NO]. This solution includes solar and auroral production of odd nitrogen species. A series of experiments and sensitivity studies has been carried out with this model to investigate the interactions between atmospheric dynamics and minor species transport and density, which occur as a result of varying the amount of turbulent mixing in the upper mesosphere and lower thermosphere. First results, for an equinox situation, indicate that increased turbulent transport near and above the mesopause results in a strong increase in mesospheric nitric oxide, a rather smaller increase in mesospheric atomic oxygen, and a sharp reduction in mesopause/lower thermosphere temperature. The cooling results from additional radiative cooling by nitric oxide, transported downwards from the lower thermosphere. Neutral winds of the lower thermosphere are strongly modified, with zonal wind jets of up to 100 m s −1 at middle latitudes. The numerical experiments which have been conducted indicate that relatively modest variations of turbulence within the upper mesosphere and lower thermosphere have quite dramatic effects on the dynamics and on the energy budget of the lower thermosphere. The changes in wind and thermal structure also indicate that the propagation of tidal and gravity wave perturbations may be strongly modified as a result of changes of turbulence. Energy extracted from propagating tides and gravity waves has not been included in the present simulations, and may partly mitigate the strong radiative mesopause cooling. The results of the simulations stress the necessity for combined measurements of chemistry, temperature structure and dynamics if the observations of minor species concentrations are to be correctly interpreted.