THE SPECTRUM OF THE TROPICAL OXYGEN NIGHTGLOW OBSERVED AT 3A RESOLUTION WITH THE HOPKINS ULTRAVIOLET TELESCOPE

Ultraviolet spectra of the tropical oxygen nightglow in the range of 830 to 1850 angstrom (in first order) at 3 angstrom resolution were obtained with the Hopkins Ultraviolet Telescope during the Astro-1 space shuttle mission in December 1990. We present here the data obtained on a setting celestial target as the zenith angle of the line-of-sight varied from 77 to 95-degrees. The dominant features in the spectrum (other than geocoronal hydrogen) are O I-lambda-lambda-1304 and 1356 and the radiative recombination continuum near 911 angstrom. The continuum is resolved and found to be consistent with an electron temperature in the range 1000-1250 K. The observed ratio of the brightness of O I-lambda-1356 to the continuum suggests that O+ - O- mutual neutralization contributes about 40% to the 1356 angstrom emission. The dependence of the optically thin emissions on zenith angle is consistent with a simple ionospheric model. Weak O I-lambda-989 emission is also detected, but there is no evidence for any similarly produced atomic nitrogen emissions.