Ionosphere‐exosphere coupling through charge exchange and momentum transfer in hydrogen‐proton collisions

Abstract

We examine the implications of a traditional assumption of exospheric physics, that collisions of hydrogen atoms and protons preferentially result in charge exchange with negligible momentum transfer. Initially adopted as a necessary convenience to accommodate limited computer resources in exosphere model calculations, this approximation results in a direct transformation of the proton velocity distribution into a hot component of neutral hydrogen. It leads to significant enhancements in both escape and in the formation of extended planetary coronae. With expanding computational facilities, the need for the approximation has passed. As the first step toward its replacement with a realistic, quantum mechanical model of the H‐H+ collision process, we calculate differential and cumulative cross sections for quantum elastic scattering of indistinguishable nuclei for a fine grid of encounter energies and scattering angles. These data are used to study the nature of ionosphere‐exosphere coupling through H‐H+ collisions, and to demonstrate that the distribution of velocities of scattered H produced in the traditional exospheric charge exchange approximation, as well as that arising from an alternative, fluid dynamics approach, leads to unacceptable abundances of coronal atoms in long‐term, highly elliptic trajectories.