The excitation of spectral lines in faint meteor trains

The dissociative recombination of meteoric oxide ions in meteor trains has been examined with a view to determining the spectral characteristics of the associated radiation. Probabilities for the excitation of specific product states have been obtained on a simple statistical basis leading to approximate relative line strengths. For a typical stony meteor the main emitters are found to be five mulitplets of Fe, three of Mg and one of Ca with smaller contributions from several weaker multiplets of Fe and forbidden transitions of Si and Fe. The weighted mean wavelength of these lines is ∼440 nm (or, as seen by the dark-adapted eye, ∼ 490 nm) and, on average, ∼0.25 visible photons arc emitted per recombination. With the possible exception of the sodium D-lines (due to a non-ionic process) recombination lines probably dominate in the emission from trains of meteors fainter than visual magnitude ∼−5. Radiation arising from mutual neturalization of meteoric ions by O⁻ ions is probably due to four of the above mentioned multiplets (two of Mg and two of Fe) and the forbidden oxygen line at 557.7 nm might also be detectable. At ∼90 km the photon output due to neutralization is expected to be an order of magnitude less than that due to recombination. At lower heights neutralization by O₂⁻ becomes more significant, giving rise to many visible multiplets and possibly molecular bands of O₂.