Fine‐structure physical chemistry modeling of Uranus H 2 X quadrupole emission

A new hydrogen physical chemistry model has been developed at the fine‐structure level for application to the giant outer planet thermospheres. The model is applied to Uranus because observations of dayglow H 2 X 1 Σ g + ( v ) quadrupole and H 3 + vibration‐rotation emission made at NASA IRTF and UKIRT provide critical constraints for thermospheric modeling. The observed H 3 + vibration‐rotation emission infers an H 3 + dominant ionosphere, predicted only for non‐LTE H 2 X ( v : J ). Excitation mechanisms explored are solar and non‐solar electron energy deposition. Non‐solar electron forcing is constrained by the EUV H 2 Lyman and Werner band emission measured by Voyager UVS. Analysis indicates that non‐solar electrons are dominant in the energy budget required to predict the observed thermospheric temperature profile. The modeled H 2 X quadrupole emission infers that an additional mechanism is required to excite the H 2 X ( v = 1) population. Non‐thermal H produced in dissociative excitation of H 2 X is a primary candidate.