Cascade and radiation trapping effects on atmospheric atomic oxygen emission excited by electron impact

Abstract

The role of cascading in auroral or photoelectron excited atomic oxygen emission is investigated by calculating distorted wave electron impact cross sections for some 24 excited levels in O I. Cascading contributes ∼75% of the effective emission cross section for 1304 Å and 1356 Å. About 98% of the cascade component to these ultraviolet lines is fed through the respective 8446-Å and 7774-Å transitions, with the consequence that the 4368-Å and 3947-Å emissions are very weak. The cascade component to 1304 Å is strongly enhanced in an optically thick medium due to radiation trapping of the lines from the 3s′ ³D, nd³D, and ns³S (n ≥ 4) levels to the ground state. The 1304-Å emission cross section appropriate in the optically thick atmosphere is predicted to be a factor of 2 larger than the emission cross section measured in an optically thin laboratory experiment. Consideration of this effect is necessary to understand the measured atmospheric 1304-Å/1356-Å ratio in terms of laboratory cross sections. Several recent atmospheric measurements on electron excited atomic oxygen 1304-Å, 1356-Å, 4368-Å, 7774-Å, and 1152-Å lines appear to be consistent with our calculated results, which imply that the Stone and Zipf (1974) 1304-Å and 1356-Å laboratory cross sections may be systematically too large by as much as a factor of 2.