Analysis of the oxygen nightglow measured by the Hopkins Ultraviolet Telescope: Implications for ionospheric partial radiative recombination rate coefficients

The Hopkins Ultraviolet Telescope (HUT) recorded nighttime ultraviolet O+ spectra in the tropical ionosphere during the December 1990 Astro 1 space shuttle mission. We reexamine the HUT 911-A radiative recombination continuum spectra and the relative 911-A and 1356-A intensities in order to address questions raised in a previous study. First, we find that the radiative recombination of O+ to the ground state of O accounts for nearly all of the continuum emission. The need for a second component beyond the ionization threshold is eliminated by allowing for an instrumental wavelength shift. Second, we find that certain low-count spectral features in some of the data are not due to a non-Maxwellian energy distribution because of the fast electron thermalization in the nighttime F region. Instead, we show that these features are consistent with random fluctuations alone. Third, the best known O+ – O− mutual neutralization chemistry gives good agreement between theoretical and measured 1356/911 ratios, thereby demonstrating the consistency among the various determinations of the O− photodetachment cross section. We update the 911-A radiative recombination rate coefficient using recent photoionization cross section information, and the 1356-A rate coefficient by fitting HUT 1356/911 ratio data. The derived rate coefficient agrees very closely with our interpretation of independent 7774-A data. Both the three-parameter fit to the 911-A radiative recombination continuum and the updated 911 and 1356-A radiative recombination rate coefficients improve the agreement between the model and measured 1356/911 ratios. The updated radiative recombination rate coefficients are 3.5 × 10−13 cm3 s−1 for 911-A, and 7.3 × 10−13 cm3 s−1 for 1356-A, both at 1160 K.