What Is the Altitude of Thermal Equilibrium?

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 ionosphere. This postulated altitude is above the lower boundary of all empirical models of planetary ionospheres. Physics‐based model predictions of the altitude cannot 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 Thermosphere Ionosphere Mesosphere Energy and Dynamics/Global UltraViolet Imager 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 neutral temperature profiles in the 110–140 km altitude range. Plain Language Summary: Solar radiation ionizes and heats the neutral atmosphere in the process of creating the ionosphere. Heating begins at the altitude where energy carried by photoelectrons produced by solar extreme ultra violet radiation is not absorbed locally. This is the altitude where the ionosphere and neutral atmosphere are no longer in thermal equilibrium. There are numerous ionospheric models predicting the neutral, ion, and electron temperatures (Tn, Ti, and Te) as a function of altitude. These models predict that, in the absence of other heating mechanisms thermal equilibrium where Ti = Ti = Tn occurs below ∼120 km. Temperatures in this altitude range are used as upper boundary conditions for models attempting to describe energy transfer from the upper atmosphere to the ionosphere. Our study shows that ionospheric models of neutral and plasma temperatures cannot be validated in the region of thermal equilibrium because existing observations are sparse and do not have the precision and/or required spatial and temporal resolution. Progress in understanding energy transfer from the atmosphere to the ionosphere requires validated models of the altitude of thermal equilibrium. Key Points: Thermal equilibrium is postulated to be attained at the lowest altitude heated by photoelectrons but has not been observedQuantitative understanding of thermal equilibrium requires observations that validate modelsThere is an ongoing community need for simultaneous neutral, ion, and electron temperature profiles below 140 km altitude [ABSTRACT FROM AUTHOR]