Plasma-Neutral Interactions in the Lower Thermosphere-Ionosphere: The need for in situ measurements to address outstanding questions

The lower thermosphere-ionosphere (LTI) is a key transition region between the Earth’s neutral atmosphere and plasma-dominated space. Interactions between ions and neutrals maximize within the LTI and in particular at altitudes from 100 to 200 km, which is the least visited region of the near-Earth environment due to enhanced atmospheric drag. The lack of in situ co-temporal and co-spatial measurements of all relevant parameters and their elusiveness to most remote-sensing methods means that the complex interactions between neutral and charged constituents in the LTI remain poorly characterized to this date. This lack of measurements, together with the ambiguity in the quantification of key processes in the 100 to 200 km altitude range, affect current modeling efforts to expand atmospheric models upward to include the LTI and limit current space weather prediction capabilities. In this talk, fundamental science themes in ionosphere-thermosphere physics and related societal and operational needs are outlined; past proposed implementation schemes to sample this transition region are reviewed; and recent efforts by ESA and NASA to highlight outstanding science questions in the LTI and the need for in situ measurements to address them are presented.