Comparison of empirical and theoretical models of species in the lower thermosphere

Abstract Sophisticated one-dimensional photochemical codes and models for calculating equilibrium concentrations of minor species in the lower thermosphere and upper mesosphere are now available. However, large-scale horizontal and vertical motions cause major seasonal/latitudinal as well as diurnal variations of the distributions of long-lived major and minor species. The UCL/Sheffield coupled global 3-dimensional thermosphere / ionosphere model includes a self-consistent treatment of odd nitrogen species in ionic and neutral form, and also treats self-consistently the global background thermospheric circulation and temperature structure. However, as yet, the 3-D models are not as sophisticated as 2-D models. Thus the UCL 2-D model, which includes a more sophisticated treatment of chemical and radiation terms, is used in combination with the 3-D model. The 2-D model simulates adequately mean zonal structure and seasonal / latitudinal variations, but fails to take account of diurnal variations. Solar Mesospheric Explorer data provides one detailed comparison data set for examining the variability of nitric oxide during the solar cycle, the response to geomagnetic activity, and seasonal/ latitudinal variations. This study supports previous studies, estimating that the solar cycle variation of mean equatorial nitric oxide density, around the altitude of the peak density in the lower thermosphere (approx 110 km) is between a factor of 3 and 4. In addition to predicted seasonal / latitudinal variations, turbulent energy dissipation of tidal, gravity and planetary waves also causes important compositional changes within the lower thermosphere. These may amount to enhancements or depletions of a factor of 3 with nitric oxide, and rather less with atomic oxygen.