Hydrogen Dynamics in a Coupled Thermosphere‐Lower Exosphere Model With Charge Exchange

Abstract Hydrogen transport in the exosphere impacts its distribution in the thermosphere, and vice versa, necessitating that the two regions be modeled together to fully capture their coupled dynamics. However, upper atmospheric model development is complicated by the transition of the highly collisional thermosphere into the exosphere, where collisions between atoms are rare. This paper extends work examining the neutral transport and circulation of hydrogen using a two‐way coupled thermosphere‐exosphere model. Neutral interactions with H+ and O+ are introduced into the model exosphere to examine the influence of charge exchange on hydrogen dynamics from the mesopause to ∼20,000 km altitude, that is, the lower exosphere. The resultant model, ExoTherM, shows chemical loss of hydrogen can alter the hydrogen escape flux computed at the exobase. The model reaches an equilibrium without a prescribed balance between thermal and non‐thermal escape methods, resulting in a significantly reduced thermosphere density during solar minimum compared to the model without charge exchange. The three‐cell framework of hydrogen transport and circulation derived from the neutrals‐only exosphere model is retained. Finally, charge exchange is shown to increase the hydrogen velocity distribution width in the lower exosphere, influencing hydrogen's motion in an altitude‐dependent manner. This work is intentionally limited to an upper boundary of ∼20,000 km and is not representative of the entire hydrogen exosphere. Within this scope, this study emphasizes the impact of ion‐neutral chemistry not only on upward hydrogen flux but also on neutral hydrogen dynamics throughout the upper atmosphere due to the coupled nature of the thermosphere‐lower exosphere system.