Laboratory studies of collisions of energetic H+ and hydrogen with atmospheric constituents

Abstract

Laboratory studies of ionization and excitation of atmospheric gases in collisions with 0.1‐ to 1000‐kev protons and hydrogen atoms are reviewed. Recent measurements of the mixed flux of H+ and H that penetrates the auroral zone atmosphere during a hydrogen aurora indicate that particles with energies less than 10 kev are most important, which implies that energetic hydrogen atoms cause most of the observed ionization and light emission. Both H+ and H produce free electrons by collisional ionization of atmospheric constituents, and stripping of energetic H is an additional source of electrons. Reliable data are available for the cross section for production of electrons in collisions of H+ and H with all important target gases, except atomic oxygen, over the full range of auroral energies. Excitation of atmospheric gases has been widely studied for H+ collisions in the range above 10 kev, but cross sections for H+ below 10 kev and H in any energy range are still largely unknown. Data are available for calculation of expected auroral emissions in the υ′ = 0 bands of the N2+ first negative system and in the υ′ = 0 bands of the N2 second positive system. The ratio of the cross section for electron production by ionization to the cross section for emission of the 391.4‐nm band of the N2+ first negative system is found to be about 20 for energies above 10 kev and then to rise to more than 100 at 1 kev. In the case of the hydrogen emissions, laboratory data are adequate for auroral calculations of Lyman α radiation. Collisional excitation of energetic H to the 2p level, rather than charge transfer excitation involving fast H+, is the major source of Lyman α in the hydrogen aurora. Data on the production of the Balmer α line are lacking at energies below 10 kev. The available Balmer α data suggest a broad peak near 10 kev in the effective auroral cross section that is nearly an order of magnitude smaller than the peak in the Lyman α cross section, which occurs at energies less than or equal to 1 kev. Insufficient laboratory data exist for meaningful calculations of auroral Hβ intensities.