Lower Thermospheric Composition Inferred from Total Density Measurements and the OGO-6 450-km Model.

It is shown that the thermospheric composition can be determined from the total density if the composition at another height is known. The method is based on the assumption that the composition at the two heights is related via diffusive equilibrium however, a knowledge of the temperature profile linking the two altitudes is not required. The method has been applied using lower thermospheric density data at 175 km averaged over a 2 week period of low geomagnetic activity and the composition at 450 km as specified by the OGO-6 model. Assuming that the OGO-6 model predictions were appropriate for the conditions of the density measurements, the uncertainty in the inferred composition is of the order of the uncertainty in the density. Although the observed average density at 175 km was consistent with the CIRA-1972 model, the inferred composition was substantially different, the ON2 ratio being similar to the predictions of the low-altitude OGO-6 model. The inferred ON2 ratio varied from about 0.8 at low latitudes to 0.4 at high auroral latitudes. These ratios, while lower than those suggested by Von Zahn and Moe, are consistent with some recent in situ measurements. Author