Conclusion

LaBelle et al., [1995] described initial observations of the fine structure of auroral roar emissions. Further observations described here reveal a greater variety of fine structures than suggested by the earlier observations: features that drift upward in frequency, drift downward in frequency, and drift both up and down and features with zero frequency drift and others with frequency drifts approaching 1 MHz s$^{-1}$. There is evidence for an asymmetry in that the largest frequency drifts are predominantly negative (frequency decreasing with time). Durations of the events range from the instrumental limit of ~10 ms to several seconds. When multiple structures are observed simultaneously, the frequency spacing between these is in the range 100-500 Hz. Significantly, the bandwidth of the features is in some cases less than 6 Hz ( $f/ \delta f \sim 5 \times 10^5$); if the source size is inferred from the $f=2$$f_{ce}$ condition in a dipole field, its vertical extent must be smaller than the wavelength of the electromagnetic waves.

No theory addresses the generation of auroral roar fine structure. For similar fine structure in AKR, Calvert [1982] suggests a laser-feedback mechanism in which the walls of a density cavity feed a portion of the electromagnetic energy back into the region where electron energy is converted to waves via the cyclotron maser instability. For the X or O modes at ionospheric altitudes this idea seems implausible for a cavity with vertical (field-aligned) walls, because it requires too large a cavity. If the walls are $\sim 15^\circ$ from vertical, a laser-feedback mechanism with the X or O mode can function in principle, but it is unlikely that the cyclotron maser instability is the source of the energy because its growth rate is too low. Finally, it is significant that the timescales of the fine structure features often greatly exceed the inverse electron-neutral collision frequency, implying that features of individual batches of electrons would be isotropized by collisions on a timescale shorter than the observed features. In any case, only thermal electrons (of the order of 0.1 eV) have low enough parallel drift speeds to match the observed frequency drifts and frequency ranges of the wave features. It remains a theoretical challenge to explain these fine structure features of auroral roar emissions.

The authors acknowledge helpful discussions with A. T. Weatherwax and R. A. Treumann. S. G. Shepherd thanks the staff at the Churchill Northern Studies Centre for making his stay as comfortable as possible. This research was supported by National Science Foundation grant ATM-9316126 to Dartmouth College.

The Editor thanks T. J. Rosenberg and T. Oguti for their assistance in evaluating this paper.