The auroral ionosphere is a natural emitter of radio waves, and many of these emissions are observable at ground level. Several types of radio emissions have been well documented using a variety of ground-based, stepped-frequency receivers (see reviews by LaBelle  and LaBelle and Weatherwax, ). In particular, auroral roar is a relatively narrowband emission at roughly 2 and 3 times the local electron cyclotron frequency ( ) [Kellogg and Monson, 1979; Kellogg and Monson, 1984; Weatherwax et al., 1993, 1995]. Much effort has been made in characterizing the seasonal, diurnal, and spectral characteristics of auroral roar to aid in determining its generation mechanism [e.g., Weatherwax et al., 1995.
LaBelle et al.,  report two auroral roar events measured in September and October 1994, with a downconverting receiver operated in a semiautomatic mode in Circle Hot Springs, Alaska. These recordings revealed for the first time that auroral roar is composed of multiple narrowband features previously unresolved by stepped-frequency receivers. These initial examples showed the lower bound of the bandwidth of some individual features to be about 30 Hz. Both rising tones and falling tones were observed, and the slopes of the discrete features varied from +1 to -100 kHz s. The discrete features occurred in groups often spaced as close as several hundred hertz. The peak power spectral density of individual features was 1-2 V m Hz [LaBelle et al., 1995].
To increase the number of examples of auroral roar fine structure, we operated the same downconverting receiver at the Northern Studies Centre in Churchill, Manitoba, Canada, during a 3-week campaign in April 1996. One of us (S.G.S.) controlled the center frequency of the downconverter according to information provided by a collocated swept-frequency receiver. The goal of this campaign, based on the two events from 1994, was to collect numerous examples of auroral roar fine structure so we could perform statistical studies of duration and drift of individual features. Presented below are an instrumentation section discussing the radio receivers used in the campaign, a data section showing the auroral roar fine structure captured, a classification scheme for the variety of features observed, and, finally, an interpretation section where the data are interpreted in terms of the two generation mechanisms that have appeared in the literature.