Hydrogen collisions in planetary atmospheres, ionospheres, and magnetospheres

Abstract

Hydrogen is the most abundant element in the universe. Molecular hydrogen is the dominant chemical species in the atmospheres of the giant planets. Because of their low masses, neutral and ionized hydrogen atoms are the dominant species in the high atmospheres of many planets. Finally, protons are the principal heavy component of the solar wind. Here we present a critical evaluation of the current state of understanding of the chemical reaction rates and collision cross sections for several important hydrogen collision processes in planetary atmospheres, ionospheres, and magnetospheres. Accurate ab initio quantum theory will play an important role. The collision processes are grouped as follows: (a) H ++H charge transfer, (b) H ++H 2( v) charge transfer and vibrational relaxation, and (c) H 2 (v, J)+H 2 vibrational, rotational, and ortho–para relaxation. In each case we provide explicit representations as tabulations or compact formulas. Particularly important conclusions are that H ++H 2( v) collisions are more likely to result in vibrational relaxation than charge transfer and H 2 ortho–para conversion is at least an order-of-magnitude faster than previously assumed.