A large (10--30 m s-1) 6--7 day period westward propagating planetary wave was seen in both ground radar and satellite data within the 80--100 km height regime during August and September 1993. Previous analyses of these data argue that it was most likely a manifestation of the (1,-2) Rossby planetary wave commonly referred to as the 5-day wave, perhaps shifted to longer periods by interaction with the mean zonal winds. In this study the global scale wave model (GSWM ) is used to analyze the characteristics of this westward propagating wave. We propose that the observed wave is not a Doppler-shifted 5-day wave but rather an unstable mode, drawing energy from unstable regions in the upper mesosphere, whose realization is global in scale. The concept of atmospheric instability is used to determine a likely source region, and the model is used determine the characteristic period, growth rate, and perturbation fields of the instability-driven wave. ¿ 1997 American Geophysical Union