A review of thirteen years of video recordings shows that auroras of all types frequently exhibit intensity enhancements in one or more thin horizontal layers having vertical extents that are only a small fraction of the local atmospheric scale height. These layers appear to be stable atmospheric layers that interact more effectively with precipitating auroral electrons than does the bulk atmosphere. The spectrum of the enhanced aurora is grossly similar to that of the background aurora, leading us to suggest that the enhancement is the result of a localized plasma instability that energizes ambient electrons at the expense of the kinetic energy of the precipitating electrons. This is phenomenologically similar to a beam-plasma discharge. But, whatever the process, it is at least competitive with direct collisional interactions in the enhanced aurora and may be important in normal aurora as well.