The Need for Improved Knowledge of Ion-Neutral Collision Frequencies in the Lower Thermosphere-Ionosphere*

The terrestrial lower thermosphere-ionosphere (LTI) lies between 100-200 km altitude. At the upper end of this range, ionospheric plasma is driven by magnetospherically-imposed electric fields. Lower down, collisional interaction with the neutral atmosphere becomes increasingly important. The combined effect of collisions and electric fields leads to Joule dissipation of a vast amount of electromagnetic energy incident from the magnetosphere, along with strong heating and upwelling of the neutral atmosphere, generation of neutral winds, rapid chemical changes, and more. Accurate specification of the momentum transfer collision frequencies is central to understanding the interface between Earth’s atmosphere and space. 

 

The key parameter governing the energetics and dynamics of the LTI is the ion-neutral collision frequency, ν_in, of resonant and non-resonant species. However, in situ measurements of this parameter are almost non-existent, in large part because the LTI has been historically inaccessible to satellites due to excessive drag. Indeed, the most commonly-used estimates of ν_in are based on laboratory measurements which are extrapolated from much different regimes of temperature, density, and composition. This talk will describe a concept to determine ν_in systematically via a multi-year satellite mission. Results from such a mission will require validation, for example from state-of-the-art lab-based measurements of ν_in.