Fine structure of auroral roar emissions

In September 1994, a narrowband downconverting receiver was installed at Circle Hot Springs, Alaska, to complement ongoing stepped frequency receiver observations of auroral roar, a relatively narrowband emission at frequencies roughly 2 and 3 times the ionospheric electron gyrofrequency (fce). By good fortune, auroral roar events were captured at 0957 UT (0057 LT) on September 6, 1994, and at 1015 UT (0115 LT) on October 23, 1994. The narrowband recording with a center frequency of 2.853 MHz and bandwidth of 10 kHz revealed that these auroral roar events were composed of multiple narrowband features which drift in frequency in a complicated pattern. Both rising tones and falling tones were observed, and the slopes of the discrete features varied from nearly flat to 100 kHz/s. When the downconverted signals are played through a loudspeaker, they resembled structured VLF emissions such as chorus. These data show that auroral roar, like auroral kilometric radiation, is at least sometimes characterized by fine frequency structures previously not resolved by stepped frequency analyzers. The existence of such fine structures has significance for theories of the generation mechanism of auroral roar, which remains unexplained.