A dramatic front of airglow radiance and wave structure reported by Taylor et al. <1995> was attributed, in a previous paper of ours <Dewan and Picard, 1998>, to a manifestation of an internal undular bore in the mesosphere. In the current paper we address the question of what physical process could be responsible for generating such a bore at that altitude. While it is relatively easy to find sources of internal tropospheric bores, including the dramatic morning glory, the same cannot be said for mesospheric bores. It will be argued here that a likely candidate for the generator of such bores is the interaction of gravity waves with the mean flow at a critical layer. This interaction could take place within an already existing inversion layer, the latter playing the role of the channel in which the bore propagates. As Huang et al. <1998> have shown, a similar wave/critical-level interaction may be responsible for the inversion layer in question. Hence we are proposing that the physical process producing the channel is the same as the one responsible for subsequently generating the bore. ¿ 2001 American Geophysical Union