Studies of gravity wave–induced fluctuations of the sodium layer using linear and nonlinear models

In this paper we develop two models of the response of the sodium layer to gravity waves. The dynamical and photochemical effects are coupled, and ion reactions are included. First, we use a linear model, in which photochemical and dynamical equations are solved simultaneously and the background atmosphere remains fixed. The other model is a time‐dependent, nonlinear, photochemical‐dynamical two‐dimensional model composed of four modules: a dynamical gravity wave model, a middle atmospheric photochemical model, a sodium layer photochemical model, and an ionospheric model. The main differences between the simulations by the two models occur for sodium evolution near the peak and the bottomside of the sodium layer. Simulations indicate that a stable gravity wave induces large perturbations to the atomic sodium distribution at the bottomside of the sodium layer and that the perturbations are in phase with those of temperature. At the topside of the layer, sodium and temperature perturbations are out of phase. The response of the sodium layer to gravity waves is largest around 86 km, in the bottomside of the sodium layer, where the vertical gradients of sodium are larger. The background distribution of sodium evolves in the presence of the wave; the horizontal averaged sodium layer extends to lower altitudes. The sodium layer perturbations persist for a long time after the dynamical gravity wave disappears. For a large amplitude gravity wave the nonlinear simulation of the sodium layer evolution is reasonable, while the linear simulation gives an unphysical solution at the bottom side of the sodium layer.