Hot neutral H in the heliosphere: Elastic H-H, H-P collisions

Williams et al. (1997) have suggested that a population of hot hydrogen atoms is created in the heliosphere through elastic H-H collisions between energetic 'solar' atoms (neutralized solar wind) and interstellar atoms. They used a BGK-like approximation (Bhatnagar et al., 1954) for the Boltzmann collision term and the collision cross sections suggested by Dalgarno (1960). We show that both assumptions result in a significant overestimation of the the H-H collision effect. On the basis of calculated momentum transfer cross-sections for elastic H-H collisions, we argue that elastic H-H and H-p collisions cannot produce hot H atoms in the heliosphere.