Kinetic Modeling of Thermal and Non-Thermal Hydrogen in Earth's Exosphere

The terrestrial hydrogen (H) exosphere extends over several tens of Earth radii and exhibits substantial seasonal and solar-cycle variability. It includes thermal H supplied from the upper thermosphere and non-thermal H mainly produced through charge exchange with magnetospheric ions. We use the KInetic-based Terrestrial Exospheric (KITE) model to simulate three-dimensional H distributions for equinox, summer, and winter under representative 2008 solar-minimum and 2014 solar-maximum conditions. Thermal H is generated using spatially varying NRLMSIS~2.1 density and temperature at 500~km as lower-boundary conditions, while resonant charge exchange with a synthetic plasmaspheric H+ population produces the non-thermal component. The modeled thermal exosphere is more extended during solar maximum, with the median density enhancement relative to solar minimum increasing from approximately 73% at 6 RE to 88% at $10RE. At 10 RE, the non-thermal population accounts for approximately 26% of the total H density during solar minimum, compared with 7.5% during solar maximum. Including charge exchange improves agreement with H densities derived from NASA's TWINS observations, although the modeled densities remain lower, particularly during solar minimum. Synthetic wide-field Ly-alpha images show that the non-thermal intensity reaches approximately 31-44% of the thermal intensity in the 2008 equinox case, compared with 8.8-11.1% in 2014. These results demonstrate how kinetic forward modeling can connect three-dimensional thermal and non-thermal H distributions with wide-field Ly-α observations relevant to the Carruthers Geocorona Observatory.