Abstract
Knowledge of the air density at altitudes above 30 km is needed for such applications as the calculation of space shuttle reentry heating. A method is described for deriving hemispheric (or global) horizontal density fields at 40–60 km directly from radiance maps based on infrared measurements by such instruments as the Nimbus 4 Satellite Infrared Spectrometer and the Selective Chopper Radiometer. Direct regression of air density with the radiance measured in individual channels of these instruments is investigated. From hydrostatic considerations, maximum density-radiance correlation is expected to occur at about 2.5 scale-heights above the level of maximum temperature-radiance correlation; the latter is found near the peak of the transmittance weighting function for each channel. This expectation is substantially verified with the aid of a statistical sample of rocketsonde temperature and density profiles and radiances computed with the appropriate transmittance data. Regression equations are developed for specifying the density with a standard error within 5–7% of the observed density. For the period of a major stratospheric warming in January–February 1973, sample density maps at 50 km are shown, derived from radiance measurements of the NOAA-2 Vertical Temperature Profile Radiometer. These indicate a density increase by more than 50% near the North Pole, from late January to early February.
