A global measurement of lower thermosphere atomic oxygen densities

The first global measurements of the atomic oxygen fine structure emission line O(P-3(1)) --> O(P-3(2)) at a wavelength of 63 mu m were performed by t he CRyogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRIS TA) experiments in November 1994 and in August 1997. The measured limb radi ances were converted to atomic oxygen densities in the altitude regime from 130 km to 175 km using a radiative transfer code and MSIS thermospheric te mperatures. The resultant densities are on the average about 40% below the predictions of the MSIS model for the low solar activity conditions of the CRISTA experiments and at all altitudes covered by the measurement. The den sities exhibit considerable horizontal structures which are only in partial agreement with the MSIS predicted relative variations. The latitudinal dis tribution shows steeper decreases towards the poles than given in MSIS.