Observations of OI 7774 emission excited by conjugate photoelectrons

Abstract

Observations and computer calculations of OI 7774 airglow emissions excited by conjugate photoelectrons have been carried out. The observations were made at McDonald Observatory, Texas using a 2m grille spectrometer from December 1972 to June 1973. The zenithal emission intensity during conjugate photoelectron precipitation was fairly constant at 2–4 R until conjugate sunset, after which it diminished steadily and ceased near a conjugate solar zenith angle ( χ c ) of 105 ± 3°. A predawn enhancement in both OI 7774 and [OI] 6300 was observed to commence near χ c ∼ 102°. The computations utilize the two-stream technique of Nagy and Banks (1970) to obtain the escaping photoelectron flux and the local excitation rates of the oxygen emissions. Good agreement with the observations is obtained for the dependence of the emission rate on conjugate solar zenith angle. A lack of agreement in absolute intensity may not be due entirely to uncertainties in the excitation cross section. The discrepancy may indicate significant magnetospheric scattering of photoelectrons with energy greater than 15 eV.