Downward flux of atmospheric 63-µm emission from atomic oxygen at balloon altitudes

Abstract

The 63-µm emission line from the ground electronic state fine-structure transition (3P1–3P2) of atomic oxygen, first suggested as a major source of thermospheric cooling by Bates1 and subsequently discussed theoretically2,3, has been measured over a range of thermospheric altitudes4–6. The rocket measurements showed that the downward intensity remained essentially constant between 85 and 100 km, as expected for an optically-thick emitting region. As a result, OI emission is now thought to be less important as a source of atmospheric cooling than upward radiation from the 5.3 µm band of NO (ref. 7). Nevertheless, measurements of the intensity distribution of OI emission in the lower thermosphere should help to discriminate between theoretical models8 and, in particular, address the appropriateness of ‘local thermodynamic equilibrium’ at these altitudes. We report here high spectral resolution measurements of OI emission at 30 km. The downward OI flux is measured to be (2.4 ± 0.5) × 10−5 W m−2 sr−1, somewhat larger than expected on the basis of previous rocket measurements or theoretical predictions. Furthermore, this value is found to be independent of zenith angle.