Over the past decade, several models have been developed which describe the diurnal behavior of molecular and atomic ions in the altitude range where the atomic and molecular densities are comparable. However, no complete set of diurnal measurements has been available in the past with which to compare these theoretical results, because the altitude regime (130--220 km) lies below the reach of conventional satellites and rocket results give only a transient picture. The Atmosphere Explorer C, D, and E satellites have been taking measurements of ion concentrations together with related aeronomic parameters in the thermosphere for over 4 years. These data have been analyzed to extract the average diurnal variation (i.e., for solar zenith angles less than 105¿) of the ion species NO+, O2+, N2+, and O+ as functions of altitude. The AE data have been used in a number of separate studies which have reduced the uncertainties associated with key reaction rate coefficients. As a test of the overall consistency of th photochemistry of the F1 layer we compare the measured ion concentrations with those calculated by using a model which incorporates our latest understanding of the chemistry and which uses, as input parameters, the measured neutral concentrations and temperature, the solar extreme ultraviolet flux, and electron and ion temperatures. The results show that a fairly simple model, in which the free parameters are minimized, reproduces the measurements to a satisfactory degree.