A theoretical method is proposed to obtain thermal structural information about the meanopause using multiple rotational lines from ground-based observations of a single vibration-rotation nightglow Meinel band of the electronic ground state of OH. The method determines the mean temperature of the emission region, the temperature difference across the region, and the magnitude of the temperature gradient for physically realistic assumptions about emission region thickness. It is applied to the OH (6, 2) Minel band spectra of the nightglow obtained during periods of low geomagnetic activity during January 1986 at Longyearbyen, Spitzbergen. The results of our analyses indicate that during the periods of observation the emission region of the nightglow OH Meinel bands is significantly nonisothermal, with temperature gradient magnitudes of about 10 K km-1 (for an emission region thickness of 8 km). Such large temperature gradients are consistent with other independent measurements of a highly structured mesopause reported in the literature. Large-amplitude gravity waves propagating through the mesopause are probably responsible for the large temperature gradients. ¿American Geophysical Union 1990