Spectral studies of 42 MHz CW auroral backscatter indicate the existence of two general categories of spectra, discrete and diffuse, that correspond to radio echoes from auroral irregularities produced by different mechanisms. The discrete spectra are of two types, narrow and broad, probably caused by longitudinal plasma waves. The narrow discrete spectra are almost certainly due to two-stream generated ion-acoustic plasma waves. The diffuse spectra constitute about 70–75% of the total observations and appear to be due to a random assembly of electron irregularities, possibly caused by plasma turbulence, whose mean drift motion changes from being strongly southward to weakly northward as time proceeds from the midnight to dawn sector. These echoes disappear when the mean velocity crosses zero, which implies that the same parameter may be responsible for both the mean drift and generation of the irregularities. It is argued that this parameter is probably the east–west electric field and that the motions observed are E×B Hall-drift motions.