Nadir-viewing far ultraviolet observations obtained on the S3-4 satellite show that during the daytime the N2 Lyman-Birge-Hopfield (LBH) emission exhibits a change in vibrational distribution and becomes substantially brighter than predicted from photoelectron theory when the satellite is near perigee. At night, the intensity depends on altitude and no emission is detected when the vehicle is above 230 km. The altitude variation of the signal is proportional to the cube of the N2 density or the product of the square of the N2 density and the O density. We conclude that there is an LBH emission which is excited by the interaction of the vehicle with the ambient atmosphere, and that it is this emission which was previously interpreted as nightglow. The vibrational distribution is sharply peaked at v'=0. The altitude variation of the emission suggests a three stage excitation process in which N2is adsorbed, collisionally excited, and finally electronically excited and desorbed by ambient N2 collisions, but physically reasonable reaction rates fail to account for the observed intensities. ¿American Geophysical Union 1987