A quantitative interpretation is given of the observed quiescent nighttime radiance of nitric oxide in the fundamental vibration-rotation band near 5.3 &mgr;m. The radiance measured in the space shuttle experiment Cryogenic Infrared Radiance Instrumentation for Shuttle (CIRRIS-1A) is known to have two components, one characterized by a thermal population of rotational levels and the other by a highly excited rotational population. The analysis presented here confirms that the thermal population is due to impact excitation of NO by atomic oxygen and attributes the highly excited distribution to the reaction of N(4S) atoms with O2. The measured nighttime emission profile is compared with predictions for several model atmospheres. Both sources of excited NO depend upon the latitude, longitude, local time, and geomagnetic indices. The fraction of vibrationally excited NO produced by the reaction of N(4S) with O2 increases rapidly with altitude from 130 to 200 km and its contribution to cooling, though much less than that from inelastic excitation of NO(&ngr;=0) is, at higher altitudes, comparable to cooling produced by the atomic oxygen fine-structure line at 63 &mgr;m. ¿ 1996 American Geophysical Union