A technique for recovering the vertical number density profile of atmospheric gases from planetary occultation data

Abstract

The occultation technique of determining the properties of the atmosphere using absorption spectroscopy is examined. The intensity of a star, in certain atmospheric absorption bands, is monitored by a satellite tracking the star during occultation by the Earth's atmosphere. The intensity data in certain wavelength intervals, where absorption is attributed to a single species, are related to the tangential column number density of the absorbing species through Beer's law. The equation for the tangential column number density is the Abel integral equation which is inverted to obtain the number density profile of the absorbing species at the occultation tangent ray point. Two numerical schemes for inverting the Abel integral equation for signals of low intensity with statistical noise superimposed are presented; one for determining the number density profile of atmospheric species that decrease exponentially with height, and the second for determining the profile of constituents having a more complex vertical structure, such as ozone. The accuracy of retrieving the number density distribution from planetary occultation data is examined. A theoretical analysis of the errors in determining the number density from occultation data of very low signal intensity is also presented. The errors in retrieving the number density profile are related to the intensity of the source, the number of data points per scan, and the degree of data smoothing required before inversion. As a specific example, calculations are made of the errors in retrieving the molecular oxygen and ozone number density profiles from occultation intensity data in the Schumann-Runge continuum of molecular oxygen at 1450 A and the Hartley continuum of ozone at 2450 A.