Diffusion model for the phase delay between thermospheric density and temperature

Considering a two-dimensional time-dependent model in which the thermosphere dynamics is excited by the UV heat input within the thermosphere, it is shown that the wind-induced variations in the diurnal component of atomic oxygen dominate over its temperature-induced variations up to 200 km. The assumption of diffusive equilibrium is therefore in general not valid for O within the lower thermosphere. The effect of the diurnal wind circulation is to redistribute O so that (a) the diurnal variations in the [O]/[N2] and [O]/[O2] ratios are damped by about 20%, thus contributing to the maintenance of the nighttime F2 region, and (b) the maximum in the diurnal variation of O is shifted by one to two hours away from the temperature maximum toward noon, thus contributing significantly to the temperature-density time lag at thermospheric heights above 200 km, where O becomes the major constituent.