Numerical simulations of the seasonal/latitudinal variations of atomic oxygen and nitric oxide in the lower thermosphere and mesosphere

Abstract

A 2-Dimensional zonally-averaged thermospheric model and the global UCL thermospheric model have been used to investigate the seasonal, solar activity and geomagnetic variation of atomic oxygen and nitric oxide. The 2-Dimensional model includes detailed oxygen and nitrogen chemistry, with appropriate completion of the energy equation, by adding the thermal infrared cooling by [O] and [NO]. This solution includes solar and auroral production of odd nitrogen compounds and metastable species. This model has been used for three investigations: firstly, to study the interactions between atmospheric dynamics and minor species transport and density, secondly, to examine the seasonal variations of atomic oxygen and nitric oxide within the upper mesosphere and thermosphere and their response to solar and geomagnetic activity variations; thirdly, to study the factor of 7 – 8 peak nitric oxide density increase as solar F 10.7 cm flux increases from 70 to 240 reported from the Solar Mesospheric Explorer. Auroral production of [NO] is shown to be the dominant source at high latitudes, generating peak [NO] densities a factor of 10 greater than typical number densities at low latitudes. At low latitudes, the predicted variation of the peak [NO] density, near 110 km, with the solar F 10.7 cm flux is rather smaller than is observed. This is most likely due to an overestimate of the soft X-ray flux at low solar activity, for times of extremely low sunspot number, as occurred in June 1986. As observed on pressure levels, the variation of [0] density is small. The global circulation during solstice and periods of elevated geomagnetic activity causes depletion of [0] in regions of upwelling, and enhancements in regions of downwelling.