Direct in situ measurements of thermospheric temperature

The Open Source Neutral Mass Spectrometer on the Atmosphere Explorer (AE) satellites, intended primarily to measure neutral constituent densities, has provided direct in situ measurements of the neutral gas temperature by means of the ‘fly-through’ mode of operation. Evaluation of this new technique is reported here and is facilitated by comparison of the derived neutral temperature (Tn) with ion temperatures (Ti) obtained simultaneously from the on-board retarding potential analyzer for altitudes and conditions where the two temperatures should be equal. A statistical analysis that compared two sets of these data from the AE-D satellite indicates an average difference of 2(±3.6)% between concurrently observed values of Tn and Ti. This ∼14°K average excess of ion over fly-through neutral temperatures is well within the expected absolute errors of the individual experiments. Consistency between localized pairs of Tn and Ti is further demonstrated through profiles depicting their altitude distributions between 150 and 225 km. The overall magnitude of temperatures calculated from the Jacchia (1971) model results in a better representation of the observations than do the relatively higher temperatures predicted for this region by the MSIS model. Reasonable agreement is also found between observed temperatures and those neutral temperatures derived from altitude distributions of N2 particle densities.