Abstract
Thermal equilibrium in planetary atmospheres occurs at altitudes where
the ion, electron, and neutral temperatures are equal. Thermal
equilibrium is postulated to occur in the collision-dominated lower
ionosphere. This postulated altitude is above the lower boundary of all
empirical models of planetary ionospheres. Physics-based model
predictions of the altitude can not be validated due to a lack of
adequate simultaneous observations of temperature profiles. This study
presents temperature profiles from simultaneous observations on
Atmosphere Explorer–C below 140 km and quiet-time neutral observations
from TIMED/GUVI over Millstone Hill. These are compared with profiles
from physics-based models with a discussion of their respective
limitations. We conclude that there does not yet exist a quantitative
understanding of the ion, electron, and neutral thermalization processes
in low-altitude planetary ionospheres. Progress on this topic requires
an adequate database of simultaneous ion, electron, and temperature
profiles in the 110 to 140 km altitude range.