A Rotational‐Level Hydrogen Physical Chemistry Model for General Astrophysical Application

Abstract

A physical chemistry model has been developed to predict the non-LTE state of hydrogen-dominated astrophysical objects under electron and photon forcing. The model is composed of five constituents, H2, H, H, H, and H+, and is unique in the application of physical chemistry at the rotational level. The range of behavior is explored for electron and solar deposition. Application of solar forcing includes a prediction of the steady state photoelectron energy distribution, calculated from the interaction of solar photons and photoelectrons with the fine structure. The model is applied to a wide range of physical conditions, including those appropriate to the outer-planet upper atmospheres, from the exobase to the hydrocarbon homopause. Steady state partitioning is found to vary by multiple orders of magnitude in response to variation of neutral diffusion, ambient electron density, and gas kinetic temperature. The model is particularly sensitive to neutral diffusion. The non-LTE H2 X 1Σ (v : J) partitioning exhibited for the range of explored physical conditions and forcing is critical to the prediction of the states of the outer-planet ionospheres. The determination of rotational-level H2 partitioning allows the prediction of discrete and continuum emission features running the entire spectral range for comparison to observations of astrophysical phenomena, including outer-planet aurora and dayglow, comet coma environments, and stellar atmospheres.