A study of the effect of a thermospheric population of nonthermal O(1D) atoms on the 6300 ¿ emission is undertaken based on a comparison between daytime observations from space and theoretical simulations. Vertical temperature profiles deduced from 6300 ¿ airglow emission measurements using the Fabry-Perot interferometer (FPI) instrument onboard the Dynamics Explorer 2 (DE 2) satellite are compared to the MSIS-90 model. Metastable O(1D) temperatures about 150 K larger than the MSIS neutral kinetic temperature are deduced from the 6300 ¿ line profiles observed during daytime, when the satellite altitude is higher than 400 km. We propose a theoretical explanation for this difference, based on the presence of a nonthermal O(1D) population in the line-of-sight of the instrument. Monte Carlo simulations of the nonthermal O(1D) energy distribution function are used to calculate the red line emission and to simulate the FPI behavior, including the line-of-sight integration. The comparison between the simulated data and the FPI ones as well as sensitivity tests allow us to conclude that the presence of nonthermal atoms in the instrument field of view is the most likely explanation of the observed discrepancy. ¿ 2001 American Geophysical Union