Molecular oxygen concentrations and absorption cross sections in the thermosphere derived from extreme ultraviolet occultation profiles

Abstract

A preliminary analysis of some of the Skylab extreme ultraviolet (EUV) occultation data is presented. Radiation at the wavelengths of three strong solar emission lines near 1335, 1216, and 1032 A is absorbed by O2 in the terrestrial atmosphere at altitudes between 90 and 160 km. It was found that the laboratory values for the absorption cross sections at these three wavelengths and the appropriate Jacchia model for the average atmospheric O2 densities needed only small adjustments to be consistent with the mean altitudes of unit optical depth determined from a number of the Skylab occultation profiles. However, the altitudes of unit optical depth, h0, tended to increase with latitude (and magnetic activity) at 1355 and 1032 A, although only a small latitudinal effect was observed at 1216 A. The values of h0 were also about 2 km lower at sunrise than at sunset at 1216 A, possibly a small diurnal effect remaining at 1335 and 1032 A. For future EUV occultation studies it is recommended that an instrument field of view of about 1 × 1 (arc min)² be employed both to provide good altitude resolution and to minimize spatially coherent intensity fluctuations from small features on the solar disk.