An analysis of the measurements taken by the improved stratospheric and mesospheric sounder (ISAMS) in its carbon monoxide wideband channel around 4.6 μm at daytime is presented. The radiances show a good signal to noise ratio up to the lower thermosphere (about 120 km) and have been shown to be mainly due to emission from the weak CO2 4.3 μm isotopic and hot bands. They exhibit a very clear dependence with the solar illumination at tangent heights above about 60 km, where they have been found to be almost exclusively determined by the solar elevation. Below about 50 km they are dominated by the variations of the kinetic temperature. The measurements have been analyzed in the 50--100 km range by using a detailed non local thermodynamic equilibrium (LTE) model of the CO2 states emitting in the 4.3 μm spectral region and the GENLN2 line-by-line radiance code. A large number (up to 32) of CO2 isotopic and hot bands emit significantly in this spectral region. The N2O(&ngr;3=1) and two O3(&ngr;1+&ngr;3) bands also give contributions in the stratosphere and lower mesosphere. The CO(1→0) band is of relative importance only in the lower thermosphere. The absolute radiances as well as the solar zenith angle dependence are well reproduced by the model. The dependence on the solar zenith angle is due to the absorption of solar radiation in the CO2 near-infrared bands. A sensitivity study of the radiances was also conducted. The major conclusions are (1) the inclusion of the excitation of N2(1) from the electronic energy of O(1D) was required to explain the radiances in the lower mesosphere; (2) the value for the rate of the vibrational exchange between CO2(&ngr;1,&ngr;2,1) and N2(1) is very similar to the laboratory measurements and to that used in the analysis of the Spectral Infrared Rocket Experiment (SPIRE) 4.3 μm CO2 atmospheric limb radiances; and (3) the CO2 volume mixing ratio (vmr) in the 70--100 km region is significantly smaller than that measured in rocket experiments and similar to that deduced from the atmospheric trace molecule spectroscopy (ATMOS) absorption measurements. A reanalysis of the SAMS 4.3 μm limb radiance measurements has also been conducted with the same non-LTE model and the GENLN2 line-by-line radiance code. The values for the parameters derived from the ISAMS data also explain the SAMS measurements very satisfactorily. ¿ 1998 American Geophysical Union