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‐Å radiative recombination continuum spectra and the relative 911‐Å and 1356‐Å 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‐Å radiative recombination rate coefficient using recent photoionization cross section information, and the 1356‐Å rate coefficient by fitting HUT 1356/911 ratio data. The derived rate coefficient agrees very closely with our interpretation of independent 7774‐Å data. Both the three‐parameter fit to the 911‐Å radiative recombination continuum and the updated 911 and 1356‐Å 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 cm 3 s −1 for 911‐Å, and 7.3 × 10 −13 cm 3 s −1 for 1356‐Å, both at 1160 K.