SUSIM UARS also observes occultations of the sun by the Earth's atmosphere. From measurements of the amount of UV light of selected wavelengths that penetrate the atmosphere as a function of tangent altitude, densities of molecular oxygen in the thermosphere and ozone in the mesosphere and stratosphere are recovered. A sophisticated algorithm has been developed to deconvolve both the changing altitude along a given line of sight and the range of lines of sight that are a consequence of the finite size of the sun.
Ozone occultation dwells (measurements) have been made at 255, 275, 292, 309, and 321 nm until December 1999. Molecular oxygen dwells continue to be performed at 144, 147, 160, and 171 nm. These extend from 1991-1999 and 2003-present. The ozone occultation dwells were discontinued because of the difficulty in properly accounting for the strong density variation across the terminator (i.e. the loci of dusk and dawn on the Earth's surface). Molecular oxygen densities by contrast do not vary strongly across the terminator and thus its density can be more reliably estimated.