We have analyzed daytime composition measurements in the thermosphere from Atmosphere Explorer-C by use of a theory of the ion chemistry. We compare predicted and measured N2 + densities in order to infer global variations of the solar ionizing flux. Our results indicate that the solar flux of Hinteregger measured on Atmosphere Explorer-C provides an adequate basis for the calculation of solar ultraviolet photo-ionization rates. The role of metastable species is examined, and theoretical O+(2D) densities are presented.