Limb scans of nighttime sodium doublet emissions at 589 nm were performed globally from a Sun-synchronous satellite during July 1979. The observations have provided a global description of variation in the emitting layer with latitude and altitude. The limb scan data are consistent with the presence of a 10-km-thick emitting layer near the 90-km peak of the sodium density in the northern mid-latitude mesosphere in summer. At equatorial and southern latitudes, however, a narrower emission layer occurs 5--10 km lower, and the intensity is stronger by a factor of 3 or more. It appears likely that the shape of the Na D emission profile is influenced by the altitude distributions of both sodium and ozone, the reactants which are necessary to produce the emission. The observed variability may be explained by a systematic seasonal mismatch in the altitude of the mesospheric density maxima for sodium and ozone, combined with a known threefold wintertime increase in the sodium density. ¿ American Geophysical Union 1988