Observation of the O-atom Rydberg Series Produced by Radiative Recombination in the Equatorial Ionospheric Nightglow

Radiative recombination [O⁺ + e -> O*] generates two manifolds of lines in the ionosphere - a quintet series and a triplet series of Rydberg transitions. The final radiative steps correspond to the VUV lines, 135.6 and 130.4 nm, respectively. The precursors of these two lines are the 844.6 and 777.4 nm transitions, but there are many other lines throughout the near UV into the infrared. Most have not previously been reported in the nightglow, and there are few observations of the quintet sequence from the laboratory. Sky spectra from the 10-meter Keck telescopes on Mauna Kea are proving to be an important aeronomic tool for many emission studies, and here we are able to determine accurate wavelengths for numerous O-atom Rydberg lines, some not previously detected. In addition, standard star intensity calibration of spectra taken with the ESI echelle spectrograph on the Keck II telescope enable us to determine absolute intensities of the Rydberg sequences during solar maximum, in 2000. The quintet manifold produces the greatest number of lines; in the nd-3p sequence the data extend to n = 11. Comparison is made with the cross section calculations of Escalante and Victor [1992], which include cascading contributions. Excellent agreement is found for the quintet sequences, in terms of the fall-off of cross sections with increasing values of n. For the triplets, the experimental data approach the optically thick calculations of Escalante and Victor, as expected. V. Escalante and G. A. Victor, Planet. Space Sci. 40, 1705 (1992)