We present the essential dynamic model of the mesospheric photochemical system (PCS) and suggest a step-by-step procedure for elaborating such a model of an arbitrary atmospheric PCS. The model demonstrates the same possibilities of nonlinear dynamic behavior and qualitatively the same dynamic characteristics as the corresponding original model, but is much simpler than the latter. We show the adequacy of the essential model compared with the original one in bifurcation diagrams, equilibrium states, and such new characteristics as correlation dimension and minimum embedding dimension of a chaotic attractor. The model can be used both for identifying and studying the mechanisms of the nonlinear dynamic behavior of the mesospheric PCS, as well as for solving a number of problems aimed at revealing nonlinear photochemical phenomena in the actual mesosphere. ¿ 1998 American Geophysical Union