A study is presented that discusses the information content of a zenith viewing brightness ratio for optically thin O and N2 emissions arising from auroral electron impact excitation. The discussion focuses on the altitude dependence of the ratio and distinguishes between a topside portion and the rest which lies below. The topside is defined as the region in which the auroral electron flux is essentially constant with altitude. The lower boundary to the topside is a function of the average energy ⟨E⟩ of the precipitating electrons. The profile of the brightness ratio is a function of ⟨E⟩ and the ratio of O to N2 column densities (designated as O/N2). On the topside, the ⟨E⟩ dependence disappears leaving a unique relationship between the brightness ratio and O/N2. Thus profiles of both quantities have identical shapes on the topside. The shape depends on the exospheric temperature Texo. Thus a model-based brightness ratio being used to compare with measurements on the topside must take this into account. The temperature can be quantified from measurements having good counting statistics or in their absence from an atmospheric model such as the mass spectrometer/incoherent scatter (MSIS) model. Without knowledge of ⟨E⟩, data/model comparisons must be restricted to the topside in order to uniquely specify O/N2. With such knowledge, however, data/model comparisons may be extended into the region below the topside. The chief advantages are better counting statistics and the decreasing dependence of the brightness ratio on Texo with decreasing altitude. Derived values of O/N2 may be related to O density profiles throughout the lower thermosphere with the use of a model atmosphere. MSIS-83 is used in this work following the preference of J. H. Hecht and colleagues in analyzing ground-based optical data. The findings discussed above are used to analyze zenith viewing OI 844.6/N2+391.4 ratio data from the Atmospheric Response in Aurora (ARIA) flights I and IV. The ARIA I data support an O concentration that is a factor of ~1.1 above MSIS-83 (similar to the findings of Hecht et al. [1995>), while the ARIA IV data lead to a factor of ~0.75 referenced to the ARIA IV MSIS atmosphere. Both atmospheres possess similar O/N2 values below 120 km. Within relative calibration errors between the experiments, one may conclude that the ARIA IV O concentrations throughout the lower thermosphere (120 km--200 km) were less than ARIA I by a factor of ~0.7.¿ 1997 American Geophysical Union