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 the CRyogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA) experiments in November 1994 and in August 1997. The measured limb radiances were converted to atomic oxygen densities in the altitude regime from 130 km to 175 km using a radiative transfer code and MSIS thermospheric temperatures. 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 densities exhibit considerable horizontal structures which are only in partial agreement with the MSIS predicted relative variations. The latitudinal distribution shows steeper decreases towards the poles than given in MSIS.